Chicken first or egg？

最新推荐文章于 2024-11-10 19:10:52 发布

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最新推荐文章于 2024-11-10 19:10:52 发布

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分类专栏： js

本文链接：https://blog.youkuaiyun.com/qq_31070475/article/details/53204002

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本文探讨了JavaScript中函数声明与变量声明的区别，解释了为什么某些代码会导致不同的执行结果，并对比了函数声明与函数表达式的执行时机差异。

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Let’s look at the following code:

<script>
    foo();//execute function foo
    function foo()
    {
        console.log("execute function foo");
    }
    console.log(a);//undefined 
    var a=1;
</script>

Can you see the difference?Why they lead to different results?
In fact,function and variable declaration executed during the compilation before runtime.
The code above will execute like this:

<script>
    function foo()
    {
        console.log("execute function foo");
    }
    var a;
    foo();//execute function foo
    console.log(a);//undefined 
    a=1;
</script>

Each declaration will execute first in its own scope before run.

<script>
    foo();
    function foo()
    {
        console.log(a);//undefined
        var a=1;
    }
</script>

However，function expression does not work like function declaration.

<script>
    foo();//TypeError: foo is not a function
    var foo=function()
    {
        console.log("foo()");
    }
</script>

It likes

<script>
    foo();//TypeError: foo is not a function
    var foo;
</script>

Tips:If you write more than one same function,the behind covers the front.

<script>
    foo();//2
    function foo()
    {
        console.log("1");
    }

    function foo()
    {
        console.log("2");
    }
</script>

Q:fill in the blanks with the correct answers:

<script>
    var foo=function()
    {
        console.log("1");
    }
    foo();//①
    var foo=function()
    {
        console.log("2");
    }
    foo();//②
</script>

<script>
    function foo()
    {
        console.log("1");
    }
    foo();//③
    var foo=function()
    {
        console.log("2");
    }
    foo();//④
    function foo()
    {
        console.log("3");
    }
    foo();//⑤
</script>

A: [1,2,3,2,2].