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JS谜题-函数中的函数 #18

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sunmaobin opened this issue May 26, 2017 · 0 comments
Open

JS谜题-函数中的函数 #18

sunmaobin opened this issue May 26, 2017 · 0 comments
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@sunmaobin
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JS/NodeJS/ES6+
Milestone
2016年

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@sunmaobin
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请问以下结果能否在点击button后，延迟3秒打印结果？

<html>
<body>
<button id="btn">Click</button>
<script src="jquery.js"></script>
<script>
(function(){
    var delayFun = function () {
       alert("This is sample function");
    };

    $("#btn").click(function(){
        setTimeout("delayFun",3000);
    });
})();
</script>
</body>
</html>

答案

不能。

原因

由于delayFun在一个匿名函数中定义，外部无法访问。而setTimeout已经脱离了当前执行环境，它的环境是window，所以3S后从window中找不到delayFun。

延伸

如果这里去掉setTimeout，即：

$("#btn").click(function(){
    delayFun();
});

点击button能立即打印结果吗？

答案：可以。

原因：click回调函数是在最外层的匿名函数中，也就是函数中的函数（即：闭包），闭包有访问外层函数中变量的权利。
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