SOURCE

// 实现setTimeout
function mySetTimeout(fn, time, ...args) {
    const start = +new Date()
    let timer, now
    let loop = () => {
        timer = window.requestAnimationFrame(loop)
        now = +new Date()
        if(now - start >= time){
            fn.apply(this, args)
            window.cancelAnimationFrame(timer)
        }
    }
    window.requestAnimationFrame(loop)
}

mySetTimeout(() => {
    console.log(3)
}, 2000)