js 中怎么使 if(aﾠ==1 && a== 2 && ﾠa==3) 返回 true？

toString / valueOf : 最直接的方案

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

　　

getter : 新式

var i = 0;
 
with({
  get a() {
    return ++i;
  }
}) {
  if (a == 1 && a == 2 && a == 3)
    console.log("wohoo");
}
 
// or
 
Object.defineProperty(window, 'a', {
    get: function() {
        return ++val;
    }
});
 
// or
 
window.__defineGetter__( 'a', function(){
    if( typeof i !== 'number' ){
        // define i in the global namespace so that it's not lost after this function runs
        i = 0;
    }
    return ++i;
});

es6 Proxy

var a = new Proxy({ i: 0 }, {
    get: (target, name) => name === Symbol.toPrimitive ? () => ++target.i : target[name],
});
console.log(a == 1 && a == 2 && a == 3);

别具匠心

// This works because == invokes toString which calls .join for Arrays.
a = [1,2,3];
a.join = a.shift;
console.log(a == 1 && a == 2 && a == 3);

新潮

// Another solution, using Symbol.toPrimitive which is an ES6 equivalent of toString/valueOf
let a = {[Symbol.toPrimitive]: ((i) => () => ++i) (0)};
 
console.log(a == 1 && a == 2 && a == 3);

覆盖式: 原理同上, if 后面有一个字符. 在 chrome 调试中可以看得到, 你没法覆盖掉真正的 if. 所以大括号必须新起一行

if‌=()=>!0;
 
var i = 1;
if‌(i == 1 && i == 2 && i == 3)
{
    console.log(i)
}

拓展: 数字变量名

var  a = 1;
var ﾠ1 = a;
var ﾠ2 = a;
var ﾠ3 = a;
console.log( a ==ﾠ1 && a ==ﾠ2 && a ==ﾠ3 );