Detailed explanation of the implicit conversion mechanism of double equal sign (==) in Javascript

There are two types of equality relationships in Javascript: 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). This article mainly introduces the double equal sign (==) implicit conversion mechanism in JavaScript to you in detail. It is very good and has reference value. Friends who need it can refer to it. I hope it can help everyone.

So there are some common sense knowledge:

1. For basic types such as string and number, there is a difference between == and ===

　1) Between different types Compare, == compare "values ​​converted into the same type" to see if the "values" are equal, === if the types are different, the result will be unequal

　2) Compare the same type, directly perform "value" "Compare, the results are the same

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

Perform "pointer address" comparison

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 is false

In other words, the double equal sign (==) will perform type conversion during operation, but the triple equal sign (===) will not.

For example:

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

The five basic data types (primitive values, also called simple data types) in the Javascript language: 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. Such storage facilitates quick search of variable values;

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

1, if both sides are simple types:

　1,1, if both sides are simple types and the types are the same, they will be compared 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 numbers 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 implicitly converted to a simple type and then compared. .

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), perform "pointer address" comparison.

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 it does not return a numerical type representation 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);  //true
var 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 for equality, 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');  //true
var 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', we find 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, and toString will no longer be called at this time. method. 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.

PS: js double equal sign implicit conversion rules

When using the double equal sign for comparison, when the two operand types are different, the double equal sign will perform an implicit conversion Convert, convert to the same type and then compare. The following are the conversion rules, which can be found on page P51 of the Little Red Book. (I always forget, and I still feel that my good memory is not as good as my bad writing. It always has a profound impact when I write it)

1. One of the operands is a Boolean value. Convert the Boolean value into a numerical value and then compare it. False is 0, true is 1.

2. One is a string and the other is a number. Convert strings to numeric values ​​and compare.

3. One operator is an object and the other is not an object. First use valueOf() to get the object value type, and then compare according to other rules.

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