Compilation of JavaScript error-prone knowledge points

Preface

This article is about some error-prone knowledge points that I collected and organized in the process of learning JavaScript. It will focus on variable scope, type comparison, this pointer, function parameters, closure issues and object copying and The six aspects of assignment are introduced and explained from the shallower to the deeper, which also involves some ES6 knowledge points.

JavaScript knowledge points

1. Variable scope

var a = 1;

function test() {
    var a = 2;

    console.log(a); // 2
}

test();

A is declared and assigned in the function scope above, and it is above the console, so the output follows the proximity principle a is equal to 2.

var a = 1;

function test2() {
    console.log(a); // undefined

    var a = 2;
}

test2();

Although a is declared and assigned in the function scope above, it is located under the console, and the a variable is promoted. It has been declared but has not been assigned a value during output, so "undefined" is output.

var a = 1;

function test3() {
    console.log(a); // 1

    a = 2;
}

test3();

A in the function scope above is reassigned, not re-declared, and is located under the console, so a in the global scope is output.

let b = 1;

function test4() {
    console.log(b); // b is not defined

    let b = 2;
}

test4();

The ES6 let is used in the function scope above to redeclare the variable b. Unlike var, let does not have the function of variable promotion, so the output error "b is not defined" is reported.

function test5() {
    let a = 1;

    {
        let a = 2;
    }
    
    console.log(a); // 1
}

test5();

In the function scope above, let is used to declare a as 1, and a is declared as 2 in the block-level scope. Because the console is not in the block-level scope within the function, 1 is output. .

2. Type comparison

var arr = [],
    arr2 = [1];

console.log(arr === arr2); // false

Comparison of two different arrays above, console is false.

var arr = [],
    arr2 = [];

console.log(arr === arr2); // false

Comparison of the two identical arrays above, because the comparison between arrays and arrays is always false, so the console is false.

var arr = [],
    arr2 = {};

console.log(typeof(arr) === typeof(arr2)); // true

The above uses typeof to compare arrays and objects. Because typeof obtains NULL, the types of arrays and objects are all object, so the console is true.

var arr = [];

console.log(arr instanceof Object); // true
console.log(arr instanceof Array); // true

The above uses instanceof to determine whether a variable belongs to an instance of an object. Because arrays are also a type of object in JavaScript, both consoles are true.

3.this points to

var obj = {
    name: 'xiaoming',
    getName: function () {
        return this.name;
    }
};

console.log(obj.getName());  // 'xiaoming'

This in the object method above points to the object itself, so "xiaoming" is output.

var obj = {
    myName: 'xiaoming',
    getName: function () {
        return this.myName;
    }
};

var nameFn = obj.getName;console.log(nameFn()); // undefined

The method in the object is assigned to a variable above. At this time, this in the method will no longer point to the obj object, but to the window object, so the console is "undefined".

var obj = {
    myName: 'xiaoming',
    getName: function () {
        return this.myName;
    }
};

var obj2 = {
    myName: 'xiaohua'
};
    
var nameFn = obj.getName;console.log(nameFn.apply(obj2)); // 'xiaohua'

The above also assigns the method in the obj object to the variable nameFn, but points this to the obj2 object through the apply method, so the final console is 'xiaohua'.

4. Function parameters

function test6() {
    console.log(arguments); // [1, 2]
}

test6(1, 2);

The above uses the arguments object in the function to obtain the parameter array passed into the function, so the output array is [1, 2].

function test7 () {
    return function () {
        console.log(arguments); // 未执行到此，无输出
    }
}

test7(1, 2);

The above also uses arguments to obtain parameters, but because test7(1, 2) does not execute the function in return, there is no output. If test7(1, 2)(3, 4) is executed, it will output [ 3, 4].

var args = [1, 2];

function test9() {
    console.log(arguments); // [1, 2, 3, 4]
}

Array.prototype.push.call(args, 3, 4);

test9(...args);

上方利用Array.prototype.push.call()方法向args数组中插入了3和4，并利用ES6延展操作符(…)将数组展开并传入test9，所以console为[1, 2, 3, 4]。

5.闭包问题

var elem = document.getElementsByTagName('div'); // 如果页面上有5个div

for(var i = 0; i < elem.length; i++) {
    elem[i].onclick = function () {
        alert(i); // 总是5
    };
}

上方是一个很常见闭包问题，点击任何div弹出的值总是5，因为当你触发点击事件的时候i的值早已是5，可以用下面方式解决：

var elem = document.getElementsByTagName('div'); // 如果页面上有5个div

for(var i = 0; i < elem.length; i++) {
    (function (w) {
        elem[w].onclick = function () {
            alert(w); // 依次为0,1,2,3,4
        };
    })(i);
}

在绑定点击事件外部封装一个立即执行函数，并将i传入该函数即可。

6.对象拷贝与赋值

var obj = {
    name: 'xiaoming',
    age: 23
};
    
var newObj = obj;

newObj.name = 'xiaohua';

console.log(obj.name); // 'xiaohua'
console.log(newObj.name); // 'xiaohua'

上方我们将obj对象赋值给了newObj对象，从而改变newObj的name属性，但是obj对象的name属性也被篡改，这是因为实际上newObj对象获得的只是一个内存地址，而不是真正 的拷贝，所以obj对象被篡改。

var obj2 = {
    name: 'xiaoming',
    age: 23
};
    
var newObj2 = Object.assign({}, obj2, {color: 'blue'});

newObj2.name = 'xiaohua';console.log(obj2.name); // 'xiaoming'

console.log(newObj2.name); // 'xiaohua'
console.log(newObj2.color); // 'blue'

上方利用Object.assign()方法进行对象的深拷贝可以避免源对象被篡改的可能。因为Object.assign() 方法可以把任意多个的源对象自身的可枚举属性拷贝给目标对象，然后返回目标对象。

var obj3 = {
    name: 'xiaoming',
    age: 23
};
    
var newObj3 = Object.create(obj3);

newObj3.name = 'xiaohua';console.log(obj3.name); // 'xiaoming'

console.log(newObj3.name); // 'xiaohua'

我们也可以使用Object.create()方法进行对象的拷贝，Object.create()方法可以创建一个具有指定原型对象和属性的新对象。

结语

学习JavaScript是一个漫长的过程，不能一蹴而就。希望本文介绍的几点内容能够帮助学习JavaScript的同学更加深入的了解和掌握JavaScript的语法，少走弯路。

以上就是JavaScript易错知识点整理的内容，更多相关内容请关注PHP中文网（www.php.cn）！