Example of implicit conversion mechanism of double equal sign in Javascript

In Javascript, there are two types of equality signs: double equal sign (==) and triple equal sign (===). The double equal sign (==) indicates value equality, while the triple equal sign (===) indicates strict equality (whether the values ​​and types are completely equal).

So there are some common sense knowledge:

1. For basic types such as string, number, == and === are different
1) Comparison between different types, == compares "values ​​converted into the same type" to see if the "values" are equal, === if the types are different, the result is unequal
2) Comparison of the same type, directly Compare "value" and the result is the same

2. For advanced types such as Array and Object, there is no difference between == and ===
Compare "pointer address"

3. There is a difference between basic types and advanced types, == and ===
　1) For ==, convert the advanced types into basic types and perform "value" comparison
　2) Because the types are different, = The result of == is false

In other words, the double equal sign (==) will perform type conversion during operation, while the triple equal sign (== =) No.

For example:

alert('55' == 55); //true
alert('55' === 55); //false

Five basic data types in Javascript language (original value, also called simple data type): namely Undefined, Null, Boolean, Number and String types. Since these primitive types occupy a fixed amount of space, they can be stored in a smaller area of ​​memory - the stack. This storage makes it easy to quickly look up the value of the variable. (For details, see: http://www.w3school.com.cn/js/pro_js_value.asp)

The implicit conversion mechanism using the double equal sign (==) in Javascript to determine equality:

1, if both sides are simple types:

　1,1, both sides are simple types and the types are the same, then compare directly.

console.log(1==1); //true
console.log("1"=="1"); //true
console.log(false==false); //true
console.log(null==null); //true
console.log(undefined==undefined); //true

　1.2. Both sides are simple types. If the types are different, they will be converted to numerical comparison first (Boolean has only two values: true==1, false==0; null and undefined are equal; String Number is equal to numeric value, empty string ""==0;)

console.log(1==true); //true
console.log(0==false); //true
console.log(1=="1"); //true
console.log(0==""); //true
console.log(0==null); //false
console.log(0==undefined); //false
console.log(null==undefined); //true

2, if one side is a simple type and the other side is a reference type (advanced type), the advanced type is implicit Convert to a simple type and compare.

console.log(Object==Object); //true
console.log(Object=={}); //false
console.log(0=={}); //false
console.log(0==[]); //true
console.log(Array==Array); //true
console.log(Object==Array); //false

3, if both sides are reference types (advanced types), "pointer address" comparison is performed.

Key points-toString() and valueOf()

The first impression many people have when seeing these two methods is that the toString() method converts an object into a string, and the valueOf method converts it into a string. Convert an object to a numeric value.

This idea is very one-sided. Let’s take a look at the following two examples:

var obj={
    name:"熊仔其人",
    getName:function(){ return $(this).name; }
};

console.log(obj.toString()); //[object Object]

Define an obj object, call its toString method, the return value is [object Object], and find that does not return a string representation of its contents as we thought.

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

Define an array arr, call its valueOf method, the return value is [1, 2, 3], and find that the representation of numeric type is not returned as we imagined.

In fact, the real understanding is this: calling the object's toString() method can convert the object into a string, but if you want to convert it into a string, you do not necessarily have to call the toString method.

Let’s take a look at the code below.

var obj= { };     
obj.valueOf=function(){ return 1; }
obj.toString=function(){ return 2; }
console.log(obj==1);    //truevar obj2= { };     
obj2.valueOf=function(){ return 2; }
obj2.toString=function(){ return 1; }
console.log(obj2==1);    //false                        
var obj3={ };
obj3.valueOf=function(){ return []; }
obj3.toString=function(){ return 1; }
console.log(obj3==1);    //true

In the above code, we defined an object obj, obj2, and defined the return value of the valueOf and toString methods. By comparing it with 1 for equality, we found that the valueOf method was called first.

Then an object obj3 is defined, and the return value of the valueOf and toString methods is defined. By comparing it with 1, it is found that it calls the toString method.

Then let's look at the following piece of code:

var obj= { };     
obj.valueOf=function(){ return 'a'; }
obj.toString=function(){ return 2; }
console.log(obj=='a');    //truevar obj2= { };     
obj2.valueOf=function(){ return 'b'; }
obj2.toString=function(){ return 'a'; }
console.log(obj2=='a');    //false

An object obj is defined in the above code 2. By comparing it with the string 'a', it is found that it calls the valueOf method.

Then the comparison between object obj2 and 'a' returns false, and it is found that the toString method is not called.

From this we can draw the conclusion:

When the object is converted to a simple type, the valueOf method will be called first. If it can be compared with a simple value, it will be compared directly. The toString method is no longer called at this time. If the valueOf method cannot be compared with a simple value after calling the valueOf method, the toString method will be called again to finally get the comparison result.

But one thing to note is that the Date object does not meet the above rules. The toString and valueOf methods of the Date object have been redefined, and the toString method will be called by default.

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