How to determine whether variables are equal in javascript

Judgment method: 1. Use the equality operator "==" or "==="; 2. Use the toString() method in the array; 3. Use the "Object.is()" method; 4 , first use "JSON.stringify()" to convert the object or array into a string type, and then use "===" for equality comparison.

The operating environment of this tutorial: Windows 7 system, ECMAScript version 6, Dell G3 computer.

Ways to determine whether variables are equal

To determine whether variables are equal, there are roughly the following ways

• == and ===;
• Use the toString method in the array;
• in ES6 The Object.is method;
• uses JSON.stringify to convert to String type for comparison;

== and ===

##== are conversion type comparison operations operator, === is a strict comparison operator. When the data types are the same, using == will generally not cause problems.

let num = 5;
let num2 = 5;

num == num2;//true
num === num2;//true

== will do some implicit type conversion.

let num = 5;
let str = '5';

num == str;//true
num === str;//false

'' == false;//true
'' === false;//false

null == undefined;//true
null === undefined;//false

bugs that are not easy to find, so it is more recommended to use === for comparison , which is one of the reasons why TS is popular.

=== to compare reference types.

let a = {xx: 1};
let b = a;

a === b;//true

let c = {xx: 1};
let d = {xx: 1};

c === d;//false

=== compares whether the reference of the variable is the same, rather than the value . When the reference is different, # will be returned ##false. It can be seen that

is not a silver bullet that is invincible. When comparing whether objects are equal, other more reliable methods are needed. [Recommended learning:

]

Array toString methodThe front end is given to the back end When passing parameters from the end, the backend sometimes requires multiple parameters separated by

. The Array toString method is more useful. This method can also be used for array comparison. <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:js;toolbar:false;">let arr = [1,3,5,7,9]; let arr2 = [1,3,5,7,9]; arr.toString() === arr2.toString();//true &quot;1,3,5,7,9&quot;</pre><div class="contentsignin">Copy after login</div></div> However, there are certain limitations. It cannot be used to compare arrays of two dimensions and above, and it cannot contain

, undefined, and object##. #, function, etc., otherwise it is easy to make errors, as follows

[1,3,5,[2,4,6]].toString();//"1,3,5,2,4,6"
[1,null,undefined,'',2].toString();//"1,,,,2"
[{xx:2},window,1,2].toString();//"[object Object],[object Window],1,2"

Object. is

=== and is also used to compare whether two values ​​are equal.

Object.is(1,1);//true
Object.is('str','str');//true
Object.is({},{});//false

-0, NaN and NaN.

+0 === -0 //true
NaN === NaN //false

Object.is(+0, -0) //false
Object.is(NaN, NaN) //true

if (!Object.is) {
  Object.is = function(x, y) {
    if (x === y) {
      // 针对+0 不等于 -0的情况
      return x !== 0 || 1 / x === 1 / y;
    } else {
     // 针对NaN的情况
      return x !== x && y !== y;
    }
  };
}

JSON.stringify

let obj = {name: 'lin', age: 24};
let obj2 = {name: 'lin', age: 24};

obj === obj2;//false
JSON.stringify(obj) === JSON.stringify(obj2);//true

=== cannot accurately compare objects, but it also has certain limitations. When encountering undefined, function and symbol values ​​are ignored. In addition, it is worth mentioning that using JSON.parse

JSON.stringify can realize object deep copy, with the same limitations as above.

Different scenarios may have different requirements. If only basic data types are compared, then ===

Once and for all", then it is undoubtedly necessary to pay a certain cost to use a custom method. In the daily repeated development, you still need to explore more.

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