The difference between equality judgment ===, == and Object.is() in js

I believe that people who are new to JS will be confused by its equality judgment. Take a look at the following code. How many can you answer correctly?

NaN === NaN // false
NaN == NaN // false
Object.is(NaN, NaN) // true
0 == false  // true
1 == true   // true
Number(null) === 0  // true
null == 0   // false

Javascript provides three different value comparison operations, namely strict equality, loose equality, and Object.is. After checking the information today, I made a summary. I hope the following content can be helpful to everyone. Please help. If there are any mistakes, you are welcome to correct me.

[Related course recommendations: JavaScript video tutorial]

1. Strict equality x === y judgment logic

1. If the data type of x is different from the data type of y, return false;

2. If x is Number type

● x is NaN, return false

● y is NaN, return false

● The value of x is equal to the value of y, return true

● x is 0, y is -0, return true

● x is -0, y is 0, return true

● Otherwise return false

3. Other types refer to SameValueNonNumber(x, y)

● Assertion: x, y is not Number type;

● Assertion: x, y have the same data type;

● x is undefined, y is undefined return true;

● x is null, y is null, return true;

● x is a string type, if and only if the character sequences of x and y are exactly the same ( The length is the same and the characters at each position are the same) return true, otherwise return false;

● If x is a Boolean type, return true when x and y are both true or false, otherwise return false ;

● If x is a symbol type, when x and y are the same symbol value, return true, otherwise return false;

● If x, y are the same object value, return true , otherwise return false;

NaN === NaN // false
undefined === undefined // true
null === null   // true
undefined === null  // false

2. Relaxed equality x == y

1. If x and y have the same type, return the result of x===y ;

2. If x is null, y is undefined, return true;

3. If x is undefined, y is null, return true;

4. If If x is a numerical value and y is a string, return x == ToNumber(y);

5. If x is a string and y is a numerical value, return ToNumber(x) == y;

6. If x is a Boolean type, return the result of ToNumber(x)==y;

7. If y is a Boolean type, return the result of x==ToNumber(y);

8. If x is one of String, Number or Symbol and Type(y) is Object, return the result of x==ToPrimitive(y)

9. If Type(x) is Object and Type (y) is one of String, Number or Symbol, and returns the result of ToPrimitive(x)==y

10. Others return false

12 == '0xc' // true, 0xc是16进制
12 == '12'  // true
12 == '12c' // false, 说明ToNumber转换是用的Number()方法

Note:

Number(null) === 0

But

null == 0 // false,

2.1 ToNumber converts a value to a numeric type

1. If it is a boolean type, true returns 1, false returns 0;

2. If it is a numerical value, it is simply passed in and returned;

3. If it is null, it returns 0

4. If it is undefined, it returns NaN;

5. If it is a string, if the string only contains numbers, convert it into a decimal number; if it is a valid floating point format, convert it into the corresponding floating point value; if it is binary or hexadecimal Convert it into the corresponding decimal value;

6. If it is an object, call the object's valueOf() method, and then convert it according to the previous rules. If the valueOf return value is NaN, call the toString() method, and then Convert the returned string according to the previous rules

2.2 ToPrimitive

toPrimitive(A) by trying to call A.toString() and A.valueOf() methods , convert parameter A into a primitive value (Primitive);

The primitive types in JS are: Number, String, Boolean, Null, Undefined;

The return of the valueOf() method of different types of objects Value:

Object	Return value
Array	Return the array object itself.
Boolean	Boolean value
Date	The stored time is from January 1, 1970 The number of milliseconds starting at midnight UTC
Function	The function itself
Number	Number value
Object	The object itself. This is the default, you can override the valueOf method of the custom object
String	String value

// Array：返回数组对象本身
var array = ["ABC", true, 12, -5];
console.log(array.valueOf() === array);   // true
// Date：当前时间距1970年1月1日午夜的毫秒数
var date = new Date(2013, 7, 18, 23, 11, 59, 230);
console.log(date.valueOf());   // 1376838719230
// Number：返回数字值
var num =  15.26540;
console.log(num.valueOf());   // 15.2654
// 布尔：返回布尔值true或false
var bool = true;
console.log(bool.valueOf() === bool);   // true
// new一个Boolean对象
var newBool = new Boolean(true);
// valueOf()返回的是true，两者的值相等
console.log(newBool.valueOf() == newBool);   // true
// 但是不全等，两者类型不相等，前者是boolean类型，后者是object类型
console.log(newBool.valueOf() === newBool);   // false
// Function：返回函数本身
function foo(){}
console.log( foo.valueOf() === foo );   // true
var foo2 =  new Function("x", "y", "return x + y;");
console.log( foo2.valueOf() );
/*
ƒ anonymous(x,y
) {
return x + y;
}
*/
// Object：返回对象本身
var obj = {name: "张三", age: 18};
console.log( obj.valueOf() === obj );   // true
// String：返回字符串值
var str = "http://www.xyz.com";
console.log( str.valueOf() === str );   // true
// new一个字符串对象
var str2 = new String("http://www.xyz.com");
// 两者的值相等，但不全等，因为类型不同，前者为string类型，后者为object类型
console.log( str2.valueOf() === str2 );   // false

3.同值相等

同值相等由 Object.is 方法判断：

● 两个值都是 undefined

● 两个值都是 null

● 两个值都是 true 或者都是 false

● 两个值是由相同个数的字符按照相同的顺序组成的字符串

● 两个值指向同一个对象

● 两个值都是数字并且

○ 都是正零 +0，

○ 或者都是负零 -0，

○ 或者都是 NaN

○ 都是除零和 NaN 外的其它同一个数字

Object.is('foo', 'foo');     // true
Object.is(window, window);   // true
Object.is('foo', 'bar');     // false
Object.is([], []);           // false
var foo = { a: 1 };
var bar = { a: 1 };
Object.is(foo, foo);         // true
Object.is(foo, bar);         // false
Object.is(null, null);       // true
Object.is(true, 'true')     // false
// 特例
Object.is(0, -0);            // false
Object.is(0, +0);            // true
Object.is(-0, -0);           // true
Object.is(NaN, 0/0);         // true

4.零值相等

与同值相等类似，不过会认为 +0 与 -0 相等。

小结

=== 不做类型转换，当两边的数类型不相同时，直接返回false；当前类型相同且都是数值类型的时候，有一个是NaN，那么结果就是false, 另外 +0 === -0

==运算符，当两边操作数类不相同时会做隐式转换，然后才进行比较，这样的话就会出现 false == 0, '' == false 等现象， 但是Object.is不会做这种转换

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