!=, ==, !==, === usage summary

This time I will bring you a summary of the use of !=, ==, !==, ===, what are the precautions when using !=, ==, !==, === , the following is a practical case, let’s take a look.

var num = 1;  
var str = '1'; 
var test = 1; 
test == num  //true　相同类型　相同值 
test === num //true　相同类型　相同值 
test !== num //false test与num类型相同，其值也相同,　非运算肯定是false 
num == str  //true 　把str转换为数字，检查其是否相等。 
num != str  //false == 的 非运算 
num === str //false 类型不同，直接返回false 
num !== str //true  num 与 str类型不同 意味着其两者不等　非运算自然是true啦

== and != If the types are different, try converting the type first, then perform value comparison, and finally return the value comparison result.

And

=== and !== will only compare their values ​​if they are of the same type.

First of all, == equality is equal, === identity is equal.

==, when the value types on both sides are different, type conversion must be performed first, and then compared.

===, no type conversion is performed, and different types must not be equal.

The following are explained separately:

Let’s talk about === first, this is relatively simple. The following rules are used to determine whether two values ​​​​are === equal:

1. If the types are different, [not equal]

2. If both are numerical values ​​and are the same A value, then [equal]; (except!) if at least one of them is NaN, then [not equal]. (To determine whether a value is NaN, you can only use isNaN() to determine)

3. If both are strings, and the characters in each position are the same, then [equal ]; otherwise [not equal].

4. If both values ​​are true, or both are false, then [equal].

5. If both values ​​refer to the same object or function, then [equal]; otherwise [not equal].

6. If both values ​​are null, or both are undefined, then [equal].

Let’s talk about ==, according to the following rules:

1. If the two value types are the same, perform === comparison.

2. If two value types are different, they may be equal. Perform type conversion and then compare according to the following rules:

a. If one is null and the other is undefined, then [equal].

b. If one is a string and the other is a numerical value, convert the string into a numerical value and then compare.

c. If any value is true, convert it to 1 and then compare; if any value is false, convert it to 0 and compare.

d. If one is an object and the other is a numerical value or string, convert the object into a value of the basic type and then compare. The object is converted to the base type using its toString or valueOf method. JS core built-in classes will try valueOf before toString; the exception is Date, which uses toString conversion. Non-js core objects, such as

e or any other combination (it’s more troublesome, I don’t quite understand it), are [not equal].

Example:

"1" == true

The types are not equal, true will be converted to the value 1 first, and now it becomes "1" == 1, then convert "1" into 1, compare 1 == 1, equal.

= Assignment operator
== equals
=== strictly equals

Example:
var a = 3;
var b = "3";

a==b returns true
a===b returns false

Because the types of a and b are different

=== Used for strict comparison judgment

var data = ({"val":"7","flag":"true"});
<FONT face=Verdana></FONT>

How to judge the value of flag below?

Because true with double quotes == is presumed to be a string true

If without double quotes==== it is a Boolean value true

This is very important, I have never had it before Figure this out

Writing method 1

if(data.flag=true){…}else{..}

No matter how you write it, it is correct. You cannot get the value of else at all. The reason is this This way of writing is equivalent to

if(true){…}

Writing 2

if(data.flag==true){…}else{..}

There is no such way of writing

##Writing 3

if(data.flag='true'){…}else{..}

if(true){…}

4

if(data.flag=='true'){…}else{..}

这个才是正确的写法

“=”：这个表示赋值，不是表示运算符

“==”：表示等于(值)

“===”：表示全等于（类型和值）

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