Indexed collections - JavaScript 编辑

• « Previous
• Next »

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

Array object

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

For example, consider an array called emp, which contains employees' names indexed by their numerical employee number. So emp[0] would be employee number zero, emp[1] employee number one, and so on.

JavaScript does not have an explicit array data type. However, you 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 property for determining the array length and other properties for use 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 list of values for the array's elements. When these values are specified, the array is initialized with them as the array's elements. The array's length property is set to the number of arguments.

The bracket syntax is called an "array literal" or "array initializer." It's shorter than other forms of array creation, and so is generally preferred. See Array literals for details.

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

// This...
let arr = new Array(arrayLength)

// ...results in the same array as this
let arr = Array(arrayLength)

// This has exactly the same effect
let arr = []
arr.length = arrayLength

Note: In the above code, arrayLength must be a Number. Otherwise, an array with a single element (the provided value) will be created. Calling arr.length will return arrayLength, but the array doesn't contain any elements. A for...in loop will not find any property on the array.

In addition to a newly defined variable as shown above, arrays can also be assigned as a property of a new or an existing object:

let 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, you must use the bracket syntax. When a single Number value is passed to the Array() constructor or function, it is interpreted as an arrayLength, not as a single element.

let arr = [42]       // Creates an array with only one element:
                     // the number 42.

let arr = Array(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 example illustrates this behavior.

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

If your code needs to create arrays with single elements of an arbitrary data type, it is safer to use array literals. Alternatively, create an empty array first before adding the single element to it.

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

let wisenArray = Array.of(9.3)   // wisenArray contains only one element 9.3

Referring to array elements

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

let myArray = ['Wind', 'Rain', 'Fire']

You can refer to the first element of the array as myArray[0], the second element of the array as myArray[1], etc… The index of the elements begins with zero.

Note: You can also use property accessors to access other properties of the array, like with an object.

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

Populating an array

You can populate an array by assigning values to its elements. For example:

let emp = []
emp[0] = 'Casey Jones'
emp[1] = 'Phil Lesh'
emp[2] = 'August West'

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

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

You can also populate an array when you create it:

let myArray = new Array('Hello', myVar, 3.14159)
// OR
let myArray = ['Mango', 'Apple', 'Orange']

Understanding length

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

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

Remember, JavaScript Array indexes are 0-based: they start at 0, not 1. This means that the length property will be one more than the highest index stored in the array:

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

You can also assign to the length property.

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

let cats = ['Dusty', 'Misty', 'Twiggy']
console.log(cats.length)  // 3

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 = 3
console.log(cats)  // logs [ <3 empty items> ]

Iterating over arrays

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

let colors = ['red', '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 example—you can use a more efficient idiom:

let divs = document.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 way of iterating over an array:

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

Alternatively, you can shorten the code for the forEach parameter with ES2015 Arrow Functions:

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

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

Note that the elements of an array that are omitted when the array is defined are not listed when iterating by forEach, but are listed when undefined has been manually assigned to the element:

let array = ['first', 'second', , 'fourth']

array.forEach(function(element) {
  console.log(element)
})
// first
// second
// fourth

if (array[2] === undefined) {
  console.log('array[2] is undefined')  // true
}

array = ['first', 'second', undefined, 'fourth']

array.forEach(function(element) {
  console.log(element)
})
// first
// 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 will be listed.

Array methods

The Array object has the following methods:

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

let myArray = new Array('1', '2', '3')
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.

let myArray = new Array('Wind', 'Rain', 'Fire')
let list = myArray.join(' - ') // list is "Wind - Rain - Fire"

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

let myArray = new Array('1', '2')
myArray.push('3')  // myArray is now ["1", "2", "3"]

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

let myArray = new Array('1', '2', '3')
let last = myArray.pop()
// myArray is now ["1", "2"], last = "3"

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

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

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

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

slice(start_index, upto_index) extracts a section of an array and returns a new array.

let myArray = new Array('a', 'b', 'c', 'd', 'e')
myArray = myArray.slice(1, 4)  // starts at index 1 and extracts all elements
                               // until index 3, returning [ "b", "c", "d"]

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

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

reverse() transposes the elements of an array, in place: the first array element becomes the last and the last becomes the first. It returns a reference to the array.

let myArray = new Array('1', '2', '3')
myArray.reverse()
// transposes the array 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('Wind', 'Rain', 'Fire')
myArray.sort()
// sorts the array so that myArray = ["Fire", "Rain", "Wind"]

sort() can also take 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 fashion. Each one takes an optional second argument called thisObject. If provided, thisObject becomes the value of the this keyword inside the body of the callback function. If not provided, as with other cases where a function is invoked outside of an explicit object context, this will refer to the global object (window) when using arrow function as callback, or undefined when using normal function as callback.

The callback function is called with two arguments, that are array's elements.

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

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

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

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

indexOf(searchElement[, fromIndex]) searches the array 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 try again, starting from after the last match
console.log(a.indexOf('b', 2))  // logs 3
console.log(a.indexOf('z'))     // logs -1, because 'z' was not found

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

let a = ['a', 'b', 'c', 'd', 'a', 'b']
console.log(a.lastIndexOf('b'))     // logs 5

// Now try again, starting from before the last match
console.log(a.lastIndexOf('b', 4))  // logs 1
console.log(a.lastIndexOf('z'))     // logs -1

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

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

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

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

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

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

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

function isNumber(value) {
  return typeof value === 'number'
}
let a1 = [1, 2, 3]
console.log(a1.every(isNumber))  // logs true
let a2 = [1, '2', 3]
console.log(a2.every(isNumber))  // logs false

some(callback[, thisObject]) returns true if callback returns true for at least one item in the array.

function isNumber(value) {
  return typeof value === 'number'
}
let a1 = [1, 2, 3]
console.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 false

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

If initialValue is specified, then callback is called with initialValue as the first parameter value and the value of the first item in the array as the second parameter value. 

If initialValue is not specified, then callback's first two parameter values will be the first and second elements of the array.  On every subsequent 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, on access to the entire array, they are available as optional parameters.

let a = [10, 20, 30]
let total = a.reduce(function(accumulator, currentValue) { return accumulator + currentValue }, 0)
console.log(total) // Prints 60

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

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

Multi-dimensional arrays

Arrays can be nested, meaning that an array can contain another array as an element. Using this characteristic of JavaScript arrays, multi-dimensional arrays 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 Array(4)
  for (let j = 0; j < 4; j++) {
    a[i][j] = '[' + i + ', ' + j + ']'
  }
}

This example creates an array with the following rows:

Row 0: [0, 0] [0, 1] [0, 2] [0, 3]
Row 1: [1, 0] [1, 1] [1, 2] [1, 3]
Row 2: [2, 0] [2, 1] [2, 2] [2, 3]
Row 3: [3, 0] [3, 1] [3, 2] [3, 3]

Using arrays to store other properties

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

const arr = [1, 2, 3];
arr.property = "value";
console.log(arr.property);  // Logs "value"

Arrays and regular expressions

When an array is the result of a match between a regular expression and a string, the array returns properties and elements that provide information about the match. An array is the return 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 as the NodeList returned by document.getElementsByTagName() or the arguments object made available within the body of a function, look and behave like arrays on the surface but do not share all of their methods. The arguments object provides a length attribute but does not implement the forEach() method, for example.

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

function printArguments() {
  arguments.forEach(function(item) {  // TypeError: arguments.forEach is not a function
    console.log(item);
  });
}

But you can call them indirectly using Function.prototype.call().

function printArguments() {
  Array.prototype.forEach.call(arguments, function(item) {
    console.log(item);
  });
}

Array prototype methods can be used on strings as well, since they provide sequential access to their characters in a similar way to arrays:

Array.prototype.forEach.call('a string', function(chr) {
  console.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 so forth, it has become clear that there are times when it would be helpful for JavaScript code to be able to quickly 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 order to access the memory contained in a buffer, you need to use a view. A view provides a context — that is, a data type, starting offset, and number of elements — that turns the data into an actual typed array.

ArrayBuffer

The ArrayBuffer is a data type that is used to represent a generic, fixed-length binary data buffer. You can't directly manipulate 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

Typed array views have self descriptive names and provide views for all the usual numeric types like Int8, Uint32, Float64 and so forth. There is one special typed array view, {jsxref("Uint8ClampedArray")}}, which clamps the values between 0 and 255. This is useful for Canvas data processing, for example.

For more information, see JavaScript typed arrays and the reference documentation for the different TypedArray objects.

• « Previous
• Next »