67 lines
2.3 KiB
Markdown
67 lines
2.3 KiB
Markdown
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---
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title: Global Variables
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localeTitle: 全局变量
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---
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全局变量在整个程序的可访问性函数之外声明，而局部变量使用`var`存储在函数中，仅在该函数的[范围内使用](https://developer.mozilla.org/en-US/docs/Glossary/Scope) 。如果在不使用`var`情况下声明变量，即使它在函数内部，它仍将被视为全局变量：
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```javascript
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var x = 5; //global
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function someThing(y) {
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var z = x + y;
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console.log(z);
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}
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function someThing(y) {
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x = 5; //still global!
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var z = x + y;
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console.log(z);
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}
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function someThing(y) {
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var x = 5; //local
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var z = x + y;
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console.log(z);
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}
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```
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全局变量也是当前范围的对象，例如浏览器窗口：
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```javascript
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var dog = “Fluffy”;
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console.log(dog); //Fluffy;
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var dog = “Fluffy”;
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console.log(window.dog); //Fluffy
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```
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最小化全局变量是最佳实践。由于变量可以在程序中的任何位置访问，因此它们可能导致奇怪的行为。
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参考文献：
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* [var -Javascript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var)
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* [您不了解JavaScript：范围和闭包](https://github.com/getify/You-Dont-Know-JS/tree/master/scope%20%26%20closures)
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附加信息：
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* [JavaScript最佳实践：避免使用全局](http://www.w3.org/wiki/JavaScript_best_practices#Avoid_globals)
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## \* [javascript中的全局var和window.variable有什么区别？](https://stackoverflow.com/questions/6349232/whats-the-difference-between-a-global-var-and-a-window-variable-in-javascript)
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JavaScript变量的范围是全局变量或本地变量。 全局变量在函数外部声明，其值可在整个程序中访问/更改。
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你应该总是使用**var**来声明你的变量（在本地生成），否则它会安装GLOBALLY
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注意全局变量，因为它们存在风险。大多数情况下，您应该使用闭包来声明变量。 例：
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```javascript
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(function(){
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var myVar = true;
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})();
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```
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#### 更多信息：
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* [JavaScript最佳实践：避免使用全局](http://www.w3.org/wiki/JavaScript_best_practices#Avoid_globals)
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* [全局变量很糟糕](http://c2.com/cgi/wiki?GlobalVariablesAreBad)
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* [MDN - 全局变量](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var)
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