Javascript Read From File Store in Array

Indexed collections

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This chapter introduces collections of data which are ordered by an index value. This includes arrays and array-like constructs such every bit Assortment objects and TypedArray objects.

Array object

An array is an ordered list of values that you refer to with a name and an alphabetize.

For instance, consider an array called emp, which contains employees' names indexed by their numerical employee number. So emp[0] would be employee number cypher, emp[i] employee number ane, and and then on.

JavaScript does non have an explicit array data blazon. However, you lot can use the predefined Array object and its methods to work with arrays in your applications. The Array object has methods for manipulating arrays in various ways, such as joining, reversing, and sorting them. It has a belongings for determining the array length and other backdrop for utilise with regular expressions.

Creating an array

The following statements create equivalent arrays:

                                  let                  arr                  =                  new                  Array                  (element0,                  element1,                  ...                  ,                  elementN)                  let                  arr                  =                  Array                  (element0,                  element1,                  ...                  ,                  elementN)                  let                  arr                  =                  [element0,                  element1,                  ...                  ,                  elementN]                              

element0, element1, ..., elementN is a listing of values for the array's elements. When these values are specified, the array is initialized with them as the array'due south elements. The assortment'southward length holding is set to the number of arguments.

The bracket syntax is called an "array literal" or "array initializer." It'south shorter than other forms of assortment cosmos, and so is mostly preferred. See Assortment literals for details.

To create an array with non-zero length, but without any items, either of the post-obit can be used:

                                  // This...                  let                  arr                  =                  new                  Array                  (arrayLength)                  // ...results in the same array as this                  permit                  arr                  =                  Array                  (arrayLength)                  // This has exactly the aforementioned outcome                  let                  arr                  =                  [                  ]                  arr.length                  =                  arrayLength                              

Notation: In the higher up code, arrayLength must exist a Number. Otherwise, an array with a single element (the provided value) volition be created. Calling arr.length will render arrayLength, only the assortment doesn't comprise any elements. A for...in loop will not find any holding on the assortment.

In add-on to a newly defined variable equally shown above, arrays tin also be assigned equally a holding of a new or an existing object:

                                  allow                  obj                  =                  {                  }                  // ...                  obj.prop                  =                  [element0,                  element1,                  ...                  ,                  elementN]                  // OR                  let                  obj                  =                  {                  prop                  :                  [element0,                  element1,                  ...                  .                  ,                  elementN]                  }                              

If you wish to initialize an array with a single element, and the element happens to be a Number, yous must utilise the subclass syntax. When a single Number value is passed to the Array() constructor or function, it is interpreted every bit an arrayLength, not equally a single element.

                                  allow                  arr                  =                  [                  42                  ]                  // Creates an array with only one element:                  // the number 42.                  let                  arr                  =                  Assortment                  (                  42                  )                  // Creates an array with no elements                  // and arr.length set to 42.                  //                  // This is equivalent to:                  let                  arr                  =                  [                  ]                  arr.length                  =                  42                              

Calling Array(N) results in a RangeError, if N is a non-whole number whose fractional portion is non-zero. The following instance illustrates this behavior.

                                  let                  arr                  =                  Array                  (                  9.3                  )                  // RangeError: Invalid array length                              

If your code needs to create arrays with single elements of an capricious data type, information technology is safer to use assortment literals. Alternatively, create an empty array starting time earlier adding the unmarried element to information technology.

In ES2015, you can use the Array.of static method to create arrays with single element.

                                  let                  wisenArray                  =                  Assortment.                  of                  (                  9.3                  )                  // wisenArray contains but one element ix.3                              

Referring to array elements

Because elements are also properties, you can admission the using property accessors. Suppose you lot define the following array:

                                  let                  myArray                  =                  [                  'Current of air'                  ,                  'Rain'                  ,                  'Burn'                  ]                              

You tin refer to the beginning element of the array equally myArray[0], the second element of the array every bit myArray[1], etc… The index of the elements begins with zero.

Notation: Yous can also utilise belongings accessors to access other properties of the assortment, similar with an object.

                                      let                    arr                    =                    [                    '1'                    ,                    '2'                    ,                    'three'                    ]                    arr[                    2                    ]                    // three                    arr[                    'length'                    ]                    // 3                                  

Populating an array

You tin populate an array past assigning values to its elements. For example:

                                  let                  emp                  =                  [                  ]                  emp[                  0                  ]                  =                  'Casey Jones'                  emp[                  one                  ]                  =                  'Phil Lesh'                  emp[                  2                  ]                  =                  'Baronial West'                              

Notation: If you supply a non-integer value to the assortment operator in the code above, a property will be created in the object representing the array, instead of an assortment element.

                                      let                    arr                    =                    [                    ]                    arr[                    3.4                    ]                    =                    'Oranges'                    console.                    log                    (arr.length)                    // 0                    console.                    log                    (arr.                    hasOwnProperty                    (                    3.four                    )                    )                    // true                                  

You lot tin can also populate an array when you lot create it:

                                  allow                  myArray                  =                  new                  Array                  (                  'Hello'                  ,                  myVar,                  iii.14159                  )                  // OR                  let                  myArray                  =                  [                  'Mango'                  ,                  'Apple'                  ,                  'Orange'                  ]                              

Understanding length

At the implementation level, JavaScript's arrays actually store their elements as standard object backdrop, using the array index as the property name.

The length property is special. It always returns the index of the terminal element plus i. (In the example below, 'Dusty' is indexed at xxx, so cats.length returns 30 + 1).

Remember, JavaScript Assortment indexes are 0-based: they start at 0, non 1. This means that the length property will exist one more than the highest index stored in the array:

                                  let                  cats                  =                  [                  ]                  cats[                  30                  ]                  =                  [                  'Dusty'                  ]                  console.                  log                  (cats.length)                  // 31                              

You tin can as well assign to the length property.

Writing a value that is shorter than the number of stored items truncates the assortment. Writing 0 empties it entirely:

                                  permit                  cats                  =                  [                  'Dusty'                  ,                  'Misty'                  ,                  'Twiggy'                  ]                  panel.                  log                  (cats.length)                  // three                  cats.length                  =                  2                  console.                  log                  (cats)                  // logs "Dusty, Misty" - Twiggy has been removed                  cats.length                  =                  0                  console.                  log                  (cats)                  // logs []; the cats array is empty                  cats.length                  =                  three                  console.                  log                  (cats)                  // logs [ <3 empty items> ]                              

Iterating over arrays

A mutual operation is to iterate over the values of an array, processing each one in some fashion. The simplest manner to do this is as follows:

                                  let                  colors                  =                  [                  'scarlet'                  ,                  'dark-green'                  ,                  'blue'                  ]                  for                  (                  let                  i                  =                  0                  ;                  i                  <                  colors.length;                  i++                  )                  {                  console.                  log                  (colors[i]                  )                  }                              

If you know that none of the elements in your array evaluate to false in a boolean context—if your array consists only of DOM nodes, for instance—you tin can use a more efficient idiom:

                                  permit                  divs                  =                  certificate.                  getElementsByTagName                  (                  'div'                  )                  for                  (                  let                  i                  =                  0                  ,                  div;                  div                  =                  divs[i]                  ;                  i++                  )                  {                  /* Process div in some way */                  }                              

This avoids the overhead of checking the length of the array, and ensures that the div variable is reassigned to the current item each time around the loop for added convenience.

The forEach() method provides another manner of iterating over an assortment:

                                  let                  colors                  =                  [                  'red'                  ,                  'dark-green'                  ,                  'blue'                  ]                  colors.                  forEach                  (                  function                  (                  color                  )                  {                  console.                  log                  (color)                  }                  )                  // red                  // green                  // blue                              

Alternatively, you can shorten the lawmaking for the forEach parameter with ES2015 Pointer Functions:

                                  let                  colors                  =                  [                  'crimson'                  ,                  'green'                  ,                  'blue'                  ]                  colors.                  forEach                  (                  color                  =>                  console.                  log                  (color)                  )                  // red                  // green                  // blue                              

The function passed to forEach is executed once for every detail in the array, with the array item passed every bit the argument to the function. Unassigned values are not iterated in a forEach loop.

Notation that the elements of an array that are omitted when the array is defined are non listed when iterating past forEach, merely are listed when undefined has been manually assigned to the element:

                                  let                  array                  =                  [                  'starting time'                  ,                  'second'                  ,                  ,                  'fourth'                  ]                  array.                  forEach                  (                  function                  (                  element                  )                  {                  panel.                  log                  (element)                  }                  )                  // first                  // second                  // fourth                  if                  (array[                  ii                  ]                  ===                  undefined                  )                  {                  console.                  log                  (                  'array[2] is undefined'                  )                  // true                  }                  array                  =                  [                  'first'                  ,                  '2d'                  ,                  undefined                  ,                  'quaternary'                  ]                  array.                  forEach                  (                  function                  (                  element                  )                  {                  console.                  log                  (element)                  }                  )                  // starting time                  // second                  // undefined                  // fourth                              

Since JavaScript elements are saved as standard object properties, it is not advisable to iterate through JavaScript arrays using for...in loops, because normal elements and all enumerable properties volition be listed.

Array methods

The Array object has the following methods:

concat() joins two or more arrays and returns a new array.

                                  allow                  myArray                  =                  new                  Array                  (                  '1'                  ,                  'two'                  ,                  'three'                  )                  myArray                  =                  myArray.                  concat                  (                  'a'                  ,                  'b'                  ,                  'c'                  )                  // myArray is now ["1", "2", "3", "a", "b", "c"]                              

join(delimiter = ',') joins all elements of an array into a string.

                                  permit                  myArray                  =                  new                  Array                  (                  'Air current'                  ,                  'Pelting'                  ,                  'Fire'                  )                  allow                  listing                  =                  myArray.                  bring together                  (                  ' - '                  )                  // listing is "Wind - Rain - Fire"                              

push() adds one or more elements to the cease of an array and returns the resulting length of the array.

                                  allow                  myArray                  =                  new                  Array                  (                  '1'                  ,                  '2'                  )                  myArray.                  button                  (                  '3'                  )                  // myArray is now ["ane", "2", "3"]                              

pop() removes the concluding element from an array and returns that element.

                                  let                  myArray                  =                  new                  Assortment                  (                  'one'                  ,                  '2'                  ,                  '3'                  )                  allow                  final                  =                  myArray.                  pop                  (                  )                  // myArray is now ["i", "two"], last = "3"                              

shift() removes the offset element from an array and returns that element.

                                  let                  myArray                  =                  new                  Array                  (                  '1'                  ,                  '2'                  ,                  '3'                  )                  let                  first                  =                  myArray.                  shift                  (                  )                  // myArray is now ["ii", "3"], commencement is "one"                              

unshift() adds one or more elements to the forepart of an array and returns the new length of the assortment.

                                  let                  myArray                  =                  new                  Assortment                  (                  '1'                  ,                  'ii'                  ,                  '3'                  )                  myArray.                  unshift                  (                  '4'                  ,                  '5'                  )                  // myArray becomes ["four", "5", "one", "2", "3"]                              

slice(start_index, up_to_index) extracts a department of an array and returns a new array.

                                  allow                  myArray                  =                  new                  Array                  (                  'a'                  ,                  'b'                  ,                  'c'                  ,                  'd'                  ,                  'east'                  )                  myArray                  =                  myArray.                  piece                  (                  1                  ,                  4                  )                  // starts at index ane and extracts all elements                  // until alphabetize 3, returning [ "b", "c", "d"]                              

splice(index, count_to_remove, addElement1, addElement2, ...) removes elements from an assortment and (optionally) replaces them. It returns the items which were removed from the assortment.

                                  allow                  myArray                  =                  new                  Array                  (                  '1'                  ,                  '2'                  ,                  '3'                  ,                  '4'                  ,                  '5'                  )                  myArray.                  splice                  (                  i                  ,                  3                  ,                  'a'                  ,                  'b'                  ,                  'c'                  ,                  'd'                  )                  // myArray is now ["1", "a", "b", "c", "d", "5"]                  // This code started at index i (or where the "ii" was),                  // removed three elements there, and and then inserted all consecutive                  // elements in its place.                              

reverse() transposes the elements of an assortment, in identify: the first array element becomes the final and the final becomes the beginning. It returns a reference to the assortment.

                                  let                  myArray                  =                  new                  Assortment                  (                  'one'                  ,                  'ii'                  ,                  '3'                  )                  myArray.                  reverse                  (                  )                  // transposes the assortment so that myArray = ["3", "2", "1"]                              

sort() sorts the elements of an array in place, and returns a reference to the array.

                                  let                  myArray                  =                  new                  Array                  (                  'Air current'                  ,                  'Rain'                  ,                  'Fire'                  )                  myArray.                  sort                  (                  )                  // sorts the array and then that myArray = ["Burn", "Rain", "Wind"]                              

sort() can too accept a callback function to determine how array elements are compared.

The sort method (and others below) that take a callback are known as iterative methods, because they iterate over the entire array in some mode. Each one takes an optional 2d argument called thisObject. If provided, thisObject becomes the value of the this keyword inside the trunk of the callback office. If not provided, as with other cases where a office is invoked outside of an explicit object context, this volition refer to the global object (window) when using arrow role as callback, or undefined when using normal function as callback.

The callback function is called with two arguments, that are assortment'southward elements.

The function below compares two values and returns one of three values:

For instance, the following will sort by the last letter of the alphabet of a string:

                                  let                  sortFn                  =                  function                  (                  a,                    b                  )                  {                  if                  (a[a.length                  -                  one                  ]                  <                  b[b.length                  -                  1                  ]                  )                  return                  -                  1                  ;                  if                  (a[a.length                  -                  ane                  ]                  >                  b[b.length                  -                  1                  ]                  )                  render                  one                  ;                  if                  (a[a.length                  -                  1                  ]                  ==                  b[b.length                  -                  1                  ]                  )                  render                  0                  ;                  }                  myArray.                  sort                  (sortFn)                  // sorts the array so that myArray = ["Wind","Burn down","Rain"]                              

• if a is less than b by the sorting system, return -1 (or any negative number)
• if a is greater than b past the sorting system, return ane (or whatever positive number)
• if a and b are considered equivalent, render 0.

indexOf(searchElement[, fromIndex]) searches the assortment for searchElement and returns the index of the first match.

                                  let                  a                  =                  [                  'a'                  ,                  'b'                  ,                  'a'                  ,                  'b'                  ,                  'a'                  ]                  console.                  log                  (a.                  indexOf                  (                  'b'                  )                  )                  // logs 1                  // Now endeavour once more, starting from afterwards the terminal friction match                  console.                  log                  (a.                  indexOf                  (                  'b'                  ,                  2                  )                  )                  // logs three                  console.                  log                  (a.                  indexOf                  (                  'z'                  )                  )                  // logs -i, because 'z' was not found                              

lastIndexOf(searchElement[, fromIndex]) works similar indexOf, but starts at the stop and searches backwards.

                                  let                  a                  =                  [                  'a'                  ,                  'b'                  ,                  'c'                  ,                  'd'                  ,                  'a'                  ,                  'b'                  ]                  console.                  log                  (a.                  lastIndexOf                  (                  'b'                  )                  )                  // logs v                  // Now try again, starting from earlier the concluding match                  console.                  log                  (a.                  lastIndexOf                  (                  'b'                  ,                  4                  )                  )                  // logs one                  console.                  log                  (a.                  lastIndexOf                  (                  'z'                  )                  )                  // logs -1                              

forEach(callback[, thisObject]) executes callback on every assortment item and returns undefined.

                                  let                  a                  =                  [                  'a'                  ,                  'b'                  ,                  'c'                  ]                  a.                  forEach                  (                  role                  (                  element                  )                  {                  console.                  log                  (element)                  }                  )                  // logs each item in plow                              

map(callback[, thisObject]) returns a new array of the render value from executing callback on every array item.

                                  let                  a1                  =                  [                  'a'                  ,                  'b'                  ,                  'c'                  ]                  let                  a2                  =                  a1.                  map                  (                  office                  (                  item                  )                  {                  return                  item.                  toUpperCase                  (                  )                  }                  )                  console.                  log                  (a2)                  // logs ['A', 'B', 'C']                              

filter(callback[, thisObject]) returns a new assortment containing the items for which callback returned true.

                                  let                  a1                  =                  [                  'a'                  ,                  10                  ,                  'b'                  ,                  xx                  ,                  'c'                  ,                  30                  ]                  permit                  a2                  =                  a1.                  filter                  (                  role                  (                  item                  )                  {                  return                  typeof                  item                  ===                  'number'                  ;                  }                  )                  console.                  log                  (a2)                  // logs [ten, 20, 30]                              

every(callback[, thisObject]) returns true if callback returns truthful for every item in the assortment.

                                  function                  isNumber                  (                  value                  )                  {                  render                  typeof                  value                  ===                  'number'                  }                  allow                  a1                  =                  [                  one                  ,                  ii                  ,                  three                  ]                  panel.                  log                  (a1.                  every                  (isNumber)                  )                  // logs truthful                  let                  a2                  =                  [                  one                  ,                  'two'                  ,                  3                  ]                  panel.                  log                  (a2.                  every                  (isNumber)                  )                  // logs imitation                              

some(callback[, thisObject]) returns true if callback returns truthful for at least ane item in the assortment.

                                  function                  isNumber                  (                  value                  )                  {                  return                  typeof                  value                  ===                  'number'                  }                  let                  a1                  =                  [                  1                  ,                  ii                  ,                  3                  ]                  panel.                  log                  (a1.                  some                  (isNumber)                  )                  // logs true                  let                  a2                  =                  [                  1                  ,                  '2'                  ,                  3                  ]                  console.                  log                  (a2.                  some                  (isNumber)                  )                  // logs true                  let                  a3                  =                  [                  '1'                  ,                  '2'                  ,                  '3'                  ]                  console.                  log                  (a3.                  some                  (isNumber)                  )                  // logs imitation                              

reduce(callback[, initialValue]) applies callback(accumulator, currentValue[, currentIndex[, array]]) for each value in the array for the purpose of reducing the listing of items downwardly to a single value. The reduce role returns the final value returned by callback function.

If initialValue is specified, then callback is chosen with initialValue as the first parameter value and the value of the first item in the array every bit the 2d parameter value.

If initialValue is not specified, then callback's first two parameter values volition be the first and second elements of the array. On every subsequent telephone call, the first parameter's value will be whatever callback returned on the previous call, and the second parameter's value will be the next value in the array.

If callback needs access to the index of the item being processed, or access to the entire assortment, they are available as optional parameters.

                                  let                  a                  =                  [                  ten                  ,                  20                  ,                  30                  ]                  let                  total                  =                  a.                  reduce                  (                  office                  (                  accumulator,                    currentValue                  )                  {                  return                  accumulator                  +                  currentValue                  }                  ,                  0                  )                  panel.                  log                  (total)                  // Prints 60                              

reduceRight(callback[, initialValue]) works similar reduce(), but starts with the last element.

reduce and reduceRight are the least obvious of the iterative assortment methods. They should be used for algorithms that combine two values recursively in social club to reduce a sequence down to a single value.

Multi-dimensional arrays

Arrays can be nested, meaning that an array tin can contain another array every bit an element. Using this characteristic of JavaScript arrays, multi-dimensional arrays tin can be created.

The following code creates a two-dimensional array.

                                  let                  a                  =                  new                  Array                  (                  4                  )                  for                  (                  let                  i                  =                  0                  ;                  i                  <                  4                  ;                  i++                  )                  {                  a[i]                  =                  new                  Assortment                  (                  4                  )                  for                  (                  let                  j                  =                  0                  ;                  j                  <                  iv                  ;                  j++                  )                  {                  a[i]                  [j]                  =                  '['                  +                  i                  +                  ', '                  +                  j                  +                  ']'                  }                  }                              

This example creates an assortment with the post-obit rows:

Row 0: [0, 0] [0, 1] [0, 2] [0, iii] Row 1: [1, 0] [ane, 1] [1, ii] [1, iii] Row 2: [2, 0] [2, 1] [2, 2] [2, 3] Row 3: [iii, 0] [iii, i] [3, two] [3, 3]            

Using arrays to store other properties

Arrays can also exist used like objects, to store related information.

                                  const                  arr                  =                  [                  ane                  ,                  2                  ,                  3                  ]                  ;                  arr.property                  =                  "value"                  ;                  console.                  log                  (arr.holding)                  ;                  // Logs "value"                              

Arrays and regular expressions

When an array is the result of a lucifer between a regular expression and a cord, the array returns properties and elements that provide information about the match. An array is the render value of RegExp.exec(), String.match(), and String.split(). For information on using arrays with regular expressions, see Regular Expressions.

Working with array-like objects

Some JavaScript objects, such every bit the NodeList returned by certificate.getElementsByTagName() or the arguments object made available within the body of a function, look and behave like arrays on the surface but do non share all of their methods. The arguments object provides a length aspect but does not implement the forEach() method, for example.

Assortment methods cannot be called directly on array-like objects.

                                  role                  printArguments                  (                  )                  {                  arguments.                  forEach                  (                  function                  (                  item                  )                  {                  // TypeError: arguments.forEach is non a function                  panel.                  log                  (item)                  ;                  }                  )                  ;                  }                              

But you can telephone call them indirectly using Office.prototype.call().

                                  function                  printArguments                  (                  )                  {                  Assortment                  .prototype.                  forEach                  .                  phone call                  (arguments,                  function                  (                  detail                  )                  {                  console.                  log                  (particular)                  ;                  }                  )                  ;                  }                              

Array prototype methods can be used on strings too, since they provide sequential access to their characters in a like mode to arrays:

                                  Array                  .image.                  forEach                  .                  telephone call                  (                  'a string'                  ,                  function                  (                  chr                  )                  {                  panel.                  log                  (chr)                  }                  )                              

Typed Arrays

JavaScript typed arrays are array-like objects and provide a mechanism for accessing raw binary data. As you already know, Array objects grow and shrink dynamically and can have any JavaScript value. JavaScript engines perform optimizations so that these arrays are fast. However, as web applications become more and more powerful, adding features such as audio and video manipulation, access to raw data using WebSockets, and then forth, it has become articulate that at that place are times when information technology would be helpful for JavaScript code to be able to rapidly and easily manipulate raw binary data in typed arrays.

Buffers and views: typed array architecture

To achieve maximum flexibility and efficiency, JavaScript typed arrays split the implementation into buffers and views. A buffer (implemented by the ArrayBuffer object) is an object representing a chunk of data; it has no format to speak of, and offers no mechanism for accessing its contents. In guild to admission the memory contained in a buffer, yous need to use a view. A view provides a context — that is, a information blazon, starting offset, and number of elements — that turns the data into an actual typed array.

ArrayBuffer

The ArrayBuffer is a data blazon that is used to stand for a generic, fixed-length binary data buffer. You can't directly dispense the contents of an ArrayBuffer; instead, you create a typed array view or a DataView which represents the buffer in a specific format, and use that to read and write the contents of the buffer.

Typed array views

Javascript Read From File Store in Array

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Indexed_collections

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