JS convert pseudo array to array

Brief introduction

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stay JS in , Pseudo array It's very common , It's also called An array of class . Pseudo arrays may give JS A little trouble for beginners .

This article will explain in detail What is a pseudo array , And in ES5 and ES6 Convert pseudo arrays into real arrays .

What is a pseudo array ？

The main features of pseudo arrays ： It's an object , And the object has length attribute

such as

let arrayLike = {  "0": "a",  "1": "b",  "2": "c",  "length": 3}

Like above arrayLike object , Yes length attribute ,key It is also an ordered sequence . Traversable , You can also query the length . But you can't call array methods . such as push、pop Other methods .

stay ES6 Before , There is also a common pseudo array ：arguments.

arguments It also looks like an array , But it has no method of array .

such as arguments.push(1) , Doing so will definitely report an error .

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except arguments outside ,NodeList The collection of objects representing nodes is also a pseudo array , Such as through document.querySelectorAll Get the node set, etc .

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transformation

There is more than one way to convert a pseudo array into a real array , Let's start with ES5 Speak up .

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ES5 How to do it

stay ES6 Before it came out , Developers usually need to use the following methods to convert pseudo arrays into arrays .

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Method 1

//  adopt  makeArray  Method , Turn an array into a pseudo array function makeArray(arrayLike) {  let result = [];  for (let i = 0, len = arrayLike.length; i < len; i++) {    result.push(arrayLike[i]);  }  return result;}function doSomething () {  let args = makeArray(arguments);  console.log(args);}doSomething(1, 2, 3);//  Output : [1, 2, 3]

This method works , But write a lot more code .

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Method 2

function doSomething () {  let args = Array.prototype.slice.call(arguments);  console.log(args);}doSomething(1, 2, 3);//  Output : [1, 2, 3]

The function of this method and Method 1 It's the same , Although the amount of code has been reduced , But it can't intuitively make other developers feel that this is a transformation .

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ES6 How to do it

until ES6 Provides Array.from The method perfectly solves the above problems .

function doSomething () {  let args = Array.from(arguments);  console.log(args);}doSomething(' One ', ' Two ', ' 3、 ... and ');//  Output : [' One ', ' Two ', ' 3、 ... and ']

Array.from Its main function is to convert pseudo arrays and traversable objects into arrays .

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say “ The main role ” The reason is that Array.from It also provides 2 Parameters can be transferred . This can extend many kinds of small play .

Array.from The second argument to is a function , similar map Traverse Method . Used to traverse .

Array.from The third parameter of accepts a this object , To change this Point to .

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Usage of the third parameter （ Not commonly used ）

let helper = {  diff: 1,  add (value) {    return value + this.diff; //  Notice there's one here  this  }};function translate () {  return Array.from(arguments, helper.add, helper);}let numbers = translate(1, 2, 3);console.log(numbers); // 2, 3, 4

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Array.from Other ways of playing

Create a length of 5 Array of , And every element of the initialization array is 1

let array = Array.from({length: 5}, () => 1)console.log(array)//  Output : [1, 1, 1, 1, 1]

The function of the second parameter and map Traverse About the same , therefore map Traverse What's the way to play , The same function can be done here . I won't go into details .

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Convert a string to an array

let msg = 'hello';let msgArr = Array.from(msg);console.log(msgArr);//  Output : ["h", "e", "l", "l", "o"]

If you pass a real array to Array.from Will return an identical array .

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