JavaScript定时器及回调用法

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循环定时任务

// 假设现在有这样一个需求：我需要请求一个接口，根据返回结果判断需不需要重复请求，直到达到某一条件为止,停止请求执行某操作
<script type="text/javascript">
    var timer;
    var index = 0;
    function tim(){
        timer = setInterval(function(){
            index++;
            console.log(Math.random())
            if (index === 5) {
                window.clearInterval(timer)
            }
        }, 500);
    }
</script>

循环请求并获取返回值 - 拿到返回值后执行操作

回调函数方法

<script>
    function getSomething(cb) {
         var r = 0;
         setTimeout(function() {
             r = 2;
             cb(r);
         }, 100);
    }

    function compute(x) {
         alert(x * 2);
    }
    getSomething(compute);
</script>

promise方法

function getSomething() {
     var r = 0;
     return new Promise(function(resolve) {
         setTimeout(function() {
          r = 2;
          resolve(r);
         }, 10);
     });
}

function compute(x) {
     alert(x * 2);
}

getSomething().then(compute);

generator

function getSomething() {
     var r = 0;
     setTimeout(function() {
         r = 2;
         it.next(r);
     }, 10);
}

function *compute(it) {
     var x = yield getSomething();
     alert(x * 2);
}

var it = compute();
it.next();

promise + generator

function getSomething() {
    var r = 0;
    return new Promise(function(resolve) {
        setTimeout(function() {
            r = 2;
            resolve(r);
        }, 10);
    });
}

function* compute() {
    var x = yield getSomething();
    alert(x * 2);
}
var it = compute();
it.next().value.then(function(value) {
    it.next(value);
});

ES7 - 终极方案

<script type="text/javascript">
    function getSomething() {
        var r = 0;
        return new Promise(function(resolve) {
            setTimeout(function() {
                r = 2;
                resolve(r);
            }, 10);
        });
    }

    async function compute() {
        var x = await getSomething();
        alert(x * 2);
    }
    
    // 直接调用compute()方法 -> x为同步请求返回结果，输出4
    compute();
    
     // 表示getSomething获取返回值后执行resolve状态 -> 
    // 当.then()前的方法执行完后再执行then()内部的程序，这样就避免了，数据没获取到等的问题
    getSomething().then(compute);
</script>