if(a==1&&a==2&&a==3)为true

var a = {
  i: 1,
  valueOf: function() {
    console.log('valueOf function');
    return this.i++;
  },
  toString: function() {
    console.log('toString function');
    return this.i++;
  }
}

if (a==1&&a==2&&a==3) {
  console.log('true')
} else {
  console.log('false')
}