javascript函数中变量的范围

iteye_14264

于 2011-12-08 11:17:52 发布

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分类专栏： javascript 文章标签： javascript function var scope ViewUI

本文链接：https://blog.youkuaiyun.com/iteye_14264/article/details/82230982

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本文通过两个测试案例深入探讨了JavaScript中变量声明提升的概念及其影响。第一例展示了局部变量声明如何被提升到函数顶部，并解释了为何在声明之前访问这些变量会返回undefined。第二例则说明了当未在函数内部重新声明已存在的全局变量时，该变量在函数内的访问不会受到变量提升的影响。

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var first = 'hi there';
var first = (function() {
    console.log("first", first); // undefined
    var first = "hello world";
})();
//相当于
var first = 'hi there';
var first = (function() {
    var first;//the variable declaration moved to the top 
    console.log("first", first); // undefined
    var first = "hello world";
})();

// second test case
var second = 'hi there';
var second = (function() {
    console.log("second", second ); // "hi there"
    // here, we DO NOT declare a local "second" variable
})();

// in the second test case, we don't declare the local variable, so the console.log call refers to the global variable and the value is not "undefined"
//link:http://jsfiddle.net/pomeh/ynqBs/