a == 1 && a == 2 && a == 3

const a = {
    n: 1,
    // valueOf: function () {
    //     return this.n++;
    // },
    toString: function () {
        return this.n++;
    }
}

let a = [1, 2, 3, ];
a.toString = a.shift; // 从开头删除一个

let n = 1;
Object.defineProperty(window, 'a', {
    get() {
        return n++;
    }
})

const a = new Proxy({}, {
    n: 1,
    get: function () {
        return () => this.n++;
    }
})