Detailed interpretation of implicit type conversion caused by double equal signs in JavaScript (graphic tutorial)

The conversion caused by the == judgment in JavaScript will trigger a series of execution steps of the interpreter. Next, we will explain in detail the implicit type conversion caused by the double equal sign in JavaScript. Friends in need can refer to it

Introduction

The if statement should be a statement used more often by programmers. Many times, if judgment is required. The if statement generally uses a double equal sign to judge the two elements before and after. Is it consistent? If it is consistent, the return value is true, and then the following statement is executed. Otherwise, other statements are executed. The implicit type conversion mentioned in this article refers to the conversion caused by ==. To give a simple example, the double equal sign is not the congruent sign. The congruent sign is "===" and three equal signs. The statement "1" == 1, then generally the previous string "1" is converted to Number 1 and then compare. Through this example, you should understand what implicit type conversion is!

Implicit type conversion steps

1. First check whether there is NaN before or after the double equal sign. If there is NaN, it will always return false.

2. Check whether there is a Boolean before or after the double equal sign. If there is a Boolean, convert the Boolean into a number. (false is 0, true is 1)

3. Then check whether there is a string before and after the double equal sign. There are three situations:

1. The other party is an object, and the object uses toString() or valueOf() for conversion;
2. The other party is a number, and the string is converted to a number; (already given the example)
3. The other party is a string, direct comparison;
4. Others return false
four . If it is a number and the other party is an object, the object is compared with valueOf() or toString(), otherwise it will return false

5. Null and undefined will not perform type conversion, but they are equal

The above conversion sequence must be kept in mind. Type questions often arise during interviews.

.toString() method and .valueOf() method Numerical conversion

Normally we think that to convert an object into a string, we need to call the toString() method, and to convert it into a number, we need to call valueOf() method, but it is not that simple when actually applied. See the following code example:

var obj = {
 webName: "haorooms前端博客",
 url:"www.jb51.net"
}
console.log(obj.toString()); //[object Object]

Similarly, let’s look at the valueOf() method:

var arr = [1, 2, 3];
console.log(arr.valueOf());//[1, 2, 3]

From the above code, you can It can be seen that the valueOf() method does not convert the object into a number that can reflect this object. Instead, we use toString()

var arr = [1, 2, 3];
console.log(arr.toString());//1,2,3

Note: Many friends believe that to convert to a string, you must first call the toString() method. In fact, this is a wrong understanding. We should understand it this way. Calling the toString() method can Converting to a string, but not necessarily converting a string, is to first call the toString() method.

Let’s take a look at the following code:

var arr = {};
arr.valueOf = function () { return 1; }
arr.toString = function () { return 2; }
console.log(arr == 1);//true

var arr = {};
arr.valueOf = function () { return []; }
arr.toString = function () { return 1; }
console.log(arr == 1);//true

We can see from the above code that the conversion first calls valueOf(). If valueOf() is not a numerical value, then toString will be called for conversion!

var arr = {};
arr.valueOf = function () { return "1"; }
arr.toString = function () { return "2"; }
console.log(arr == "1");//true

If "1" is a string, the first thing it calls is valueOf().

var arr = [2];
console.log(arr + "1");//21

In the above example, toString() is called; because arr.toString() is followed by 2.

The conversion process is like this. First, arr will first call the valueOf() method, but this method of numbers is simply inherited and has not been rewritten (of course this rewriting is not implemented by us). The return value is The array object itself is not a value type, so the toString() method is called instead, thus achieving the purpose of converting to a string.

Summary

Most objects that are implicitly converted to a value type first try to call the valueOf() method. But the Date object is an exception. The valueOf() and toString() methods of this object have been carefully rewritten. The default is to call the toString() method, such as using operators. If it is in other arithmetic operation environments, valueOf will be called instead. ()method.

var date = new Date();
console.log(date + "1"); //Sun Apr 17 2014 17:54:48 GMT+0800 (CST)1
console.log(date + 1);//Sun Apr 17 2014 17:54:48 GMT+0800 (CST)1
console.log(date - 1);//1460886888556
console.log(date * 1);//1460886888557

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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