Detailed explanation of JavaScript class array objects and arguments instances

Array-like object

The so-called array-like object:

has a length attribute and several index attributes Object

For example:

var array = ['name', 'age', 'sex'];
  
var arrayLike = {
    0: 'name',
    1: 'age',
    2: 'sex',
    length: 3
}

Even so, why is it called an array-like object?
Let us look at these two objects from three aspects: reading and writing, obtaining length, and traversing.

Read and write

console.log(array[0]); // name
console.log(arrayLike[0]); // name
  
array[0] = 'new name';
arrayLike[0] = 'new name';

Length

console.log(array.length); // 3
console.log(arrayLike.length); // 3

Traversal

for(var i = 0, len = array.length; i < len; i++) {
   ……
}
for(var i = 0, len = arrayLike.length; i < len; i++) {
    ……
}

Isn’t it similar?
Can that type of array object use array methods? For example:

arrayLike.push('4');

However, the above code will report an error: arrayLike.push is not a function
So it is still an array class after all... …

Call the array method

What if the array-like method wants to use the array method willfully?
Since it cannot be called directly, we can use Function.call to call indirectly:

var arrayLike = {0: 'name', 1: 'age', 2: 'sex', length: 3 }
  
Array.prototype.join.call(arrayLike, '&'); // name&age&sex
  
Array.prototype.slice.call(arrayLike, 0); // ["name", "age", "sex"] 
// slice可以做到类数组转数组
  
Array.prototype.map.call(arrayLike, function(item){
    return item.toUpperCase();
}); 
// ["NAME", "AGE", "SEX"]

Array-like to object

In the above example, one method of converting an array to an array has been mentioned, and three more methods will be added:

var arrayLike = {0: 'name', 1: 'age', 2: 'sex', length: 3 }
// 1. slice
Array.prototype.slice.call(arrayLike); // ["name", "age", "sex"] 
// 2. splice
Array.prototype.splice.call(arrayLike, 0); // ["name", "age", "sex"] 
// 3. ES6 Array.from
Array.from(arrayLike); // ["name", "age", "sex"] 
// 4. apply
Array.prototype.concat.apply([], arrayLike)

So why do we talk about array-like objects? And what are the applications of class arrays?
When it comes to array-like objects, the Arguments object is an array-like object. In client-side JavaScript, some DOM methods (document.getElementsByTagName(), etc.) also return array-like objects.

Arguments object

Next, we will focus on the Arguments object.
The Arguments object is only defined in the function body, including the parameters and other properties of the function. In a function body, arguments refers to the Arguments object of the function.
For example:

function foo(name, age, sex) {
    console.log(arguments);
}
  
foo('name', 'age', 'sex')

The print result is as follows:

We can see that in addition to the index attribute and length attribute of the array, there is also a callee attribute. Next, we will introduce them one by one.

length attribute

The length attribute of the Arguments object indicates the length of the actual parameter, for example:

function foo(b, c, d){
    console.log("实参的长度为：" + arguments.length)
}
  
console.log("形参的长度为：" + foo.length)
  
foo(1)
  
// 形参的长度为：3
// 实参的长度为：1

callee attribute

Arguments The callee attribute of the object through which the function itself can be called.
Let’s talk about a solution to a classic closure interview question using callee:

var data = [];
  
for (var i = 0; i < 3; i++) {
    (data<i> = function () {
       console.log(arguments.callee.i) 
    }).i = i;
}
  
data[0]();
data[1]();
data[2]();
  
// 0
// 1
// 2</i>

Next, let’s talk about several points to note about the arguments object:

Binding of arguments and corresponding parameters

function foo(name, age, sex, hobbit) {
  
    console.log(name, arguments[0]); // name name
  
    // 改变形参
    name = 'new name';
  
    console.log(name, arguments[0]); // new name new name
  
    // 改变arguments
    arguments[1] = 'new age';
  
    console.log(age, arguments[1]); // new age new age
  
    // 测试未传入的是否会绑定
    console.log(sex); // undefined
  
    sex = 'new sex';
  
    console.log(sex, arguments[2]); // new sex undefined
  
    arguments[3] = 'new hobbit';
  
    console.log(hobbit, arguments[3]); // undefined new hobbit
  
}
  
foo('name', 'age')

The values ​​of the parameters passed in, the actual parameters and arguments will be shared. When no parameters are passed in, the values ​​of the actual parameters and arguments will not be shared.
Except for this In addition, the above is in non-strict mode. If it is in strict mode, the actual parameters and arguments will not be shared.

Passing parameters

Passing parameters from one function to another

// 使用 apply 将 foo 的参数传递给 bar
function foo() {
    bar.apply(this, arguments);
}
function bar(a, b, c) {
   console.log(a, b, c);
}
  
foo(1, 2, 3)

Powerful ES6

Using ES6’s... operator, we can easily convert to an array .

function func(...arguments) {
    console.log(arguments); // [1, 2, 3]
}
  
func(1, 2, 3);

Application

There are actually many applications for arguments, which will be discussed in the next series , that is, in the JavaScript topic series, we will see arguments in jQuery's extend implementation, function currying, recursion and other scenarios. This article will not go into details.
If you want to summarize these scenarios, the ones that can be thought of for the time being include:

1. Parameters of variable length

2. Function currying

3. Recursive call

4. Function overloading ...

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