Explain in vernacular: how to achieve a==1 && a==2 && a==3 as true

       I encountered this interesting interview question two days ago. I really wanted to break my head and didn't understand "how can a be equal to three numbers at the same time"?
       I finally realized the answer when I saw it. Although the answer is relatively abstract, today I will briefly explain a solution to this problem in my own words:

Before explaining this question, we first have to understand the JS grammar (day), fan (pit), that is:

• Direct comparison (double equals comparison) if two value types are the same
• If the two values ​​are not of the same type, the corresponding type conversion is performed according to object→string→number (boolean value→number) until the two values ​​are of the same type and then compared.

说白了就是JS的隐性转换！

Without further ado, let’s go to the picture directly:

Let’s do two experiments to verify:

1. Object and Boolean Comparison

[] == true;  //false  

Arrays belong to objects and are implicitly converted to empty strings after encountering a double equals sign

2. Object and String Comparison

[1,2,3] == '1,2,3' // true

Here the three arrays in the array are implicitly converted to '123' of the string, so they are equal!

---

OK! Understand the knowledge (heaven) knowledge (pit) of the above JS, then this problem is easily solved!

Let's first see what the answer is:

//实现a==1 && a==2 && a==3 为true
var a = {
    
    
    i : 1,
    toString(){
    
    
        return a.i++
    }
}
console.log(a==1 && a==2 && a==3 ) //true

【Thinking】:

1. a becomes an object

2. The object encounters a double equal sign ------> implicit conversion (the toString method on the object will be called)

   -------> Since "++" is after i, first return the value of i and then perform the ++ operation -------> can be true

In this way, a==1 && a==2 && a==3it can be true! !

In fact, the test point of this question is to examine the understanding of JS implicit conversion. If you think about it carefully, it is actually not difficult~~~