Under what circumstances does a print 1 in the following code? (Look at implicit conversions)

The title as follows

var a = ? ; if(a == 1 && a == 2 && a == 3){ conso.log(1); }Copy the code

solution

1. Use the toString

let a = {
  i: 1,
  toString () {
    return a.i++
  }
}

if(a == 1 && a == 2 && a == 3) {
  console.log('1');
}
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2. Use the valueOf

let a = {
  i: 1,
  valueOf () {
    return a.i++
  }
}

if(a == 1 && a == 2 && a == 3) {
  console.log('1');
}
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An array of 3.

Var a = [1, 2, 3]; a.join = a.shift; if(a == 1 && a == 2 && a == 3) { console.log('1'); }Copy the code

4. ES6 symbol

let a = {[Symbol.toPrimitive]: ((i) => () => ++i) (0)};
if(a == 1 && a == 2 && a == 3) {
  console.log('1');
}
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