7. In JS [] = =! [] Why is it true?

One ： introduction

js It's a weakly typed language , Type conversion is the most basic .

Before I look at this question , Let's review js Type conversion rules ：

Two ： Judge true/false

stay js in ,`if` Can be judged true/false, by false The value is ：

• false
• undefined
• null
• ''
• 0
• NaN

3、 ... and ： Non operators

! You can convert the current value into boolean type

Four ： operation

• +( The meaning of string splicing )
• -
• *
• /

 1/'a'= NaN

 // +  As the meaning of splicing 
 1+true = 2 // true To 1  Go back and 1 Add up 
 1+null = 0  // null  turn 0  Go back and 1 Add up 
 1+undefined = NaN // undefined Not numbers   So the sum is NaN
 1+{} = 1[object Object]
 1 + +'123' = 124 //  Execute first +'123'  Convert it to a number   And again 1 Add up 

//  Non digital addition  
true+{} = true[object Object] 
/**
 reason :
 There are two methods in the object   valueOf toString  
 Will be adjusted first valueOf   If valueOf The result is the raw data type , Then the original data type shall prevail , 
 If it's a reference data type , Call toString()

console.log({}.valueOf().toString());[object Object]

 step ：
{}.valueOf() => {}  .toString()=>[object Object]

*/

5、 ... and ： Compare  

• <
• >
• ==
• ===

// console.log('a' < 'bbbb'); // true  First to ascii Compare the code again 
// console.log(1 < 'aaa'); // false  If it can be converted into a string   It can't be turned into numbers   Just go back to false

// console.log(null == undefined); // true  null and undefined  The two equal signs are true     however null and undefined Compare with other types   All back to false
// console.log(null == 0); // false
// console.log(undefined == 0); // false

// console.log({} == {}); // false    Compare memory addresses 
// console.log(NaN == NaN); // false


// console.log('1' == 1); // true  Will first string 1 Convert to numbers 1 Compare again 

// console.log(1 == true); // true  If it is boolean type    Will also put boolean Turn it into numbers and compare 

// console.log({} == '[object Object]'); // true  object   and   character string   Numbers  symbol When comparing , Will convert the object to the original data type ( call toString Method ), Compare again 

6、 ... and ： Problem solving  

console.log([] == ![]); // true

1.   First js Comparison The ternary operator has the highest priority Will execute first ![], [] The boolean type is true, This is OK , Add one in front ! That is to say false
2. To the right of the equal sign is false, Boolean types do == operation It will be converted into numbers first Then compare That is, the right side of the equal sign is 0
3. []==0 Operation time   [] Would call valueof Get yourself [], Call again toString Method obtain ''
4. ''==0 operation '' It's going to be 0 Compare again
5. 0==0 So for true