137 lines
3.2 KiB
Markdown
137 lines
3.2 KiB
Markdown
---
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id: 56533eb9ac21ba0edf2244be
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challengeType: 1
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videoUrl: 'https://scrimba.com/c/cQM7mCN'
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forumTopicId: 18193
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localeTitle: 全局作用域和函数
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---
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## Description
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<section id='description'>
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在 JavaScript 中，<dfn>作用域</dfn>涉及到变量的作用范围。在函数外定义的变量具有 <dfn>全局</dfn> 作用域。这意味着，具有全局作用域的变量可以在代码的任何地方被调用。
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这些没有使用<code>var</code>关键字定义的变量，会被自动创建在全局作用域中，形成全局变量。当在代码其他地方无意间定义了一个变量，刚好变量名与全局变量相同，这时会产生意想不到的后果。因此你应该总是使用var关键字来声明你的变量。
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</section>
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## Instructions
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<section id='instructions'>
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在函数外声明一个<code>全局</code>变量<code>myGlobal</code>，并给它一个初始值<code>10</code>
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在函数<code>fun1</code>的内部，<strong>不</strong>使用<code>var</code>关键字来声明<code>oopsGlobal</code>，并赋值为<code>5</code>。
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: 应定义<code>myGlobal</code>。
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testString: assert(typeof myGlobal != "undefined");
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- text: <code>myGlobal</code>的值应为<code>10</code>。
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testString: assert(myGlobal === 10);
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- text: 应使用<code>var</code>关键字定义<code>myGlobal</code>。
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testString: assert(/var\s+myGlobal/.test(code));
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- text: <code>oopsGlobal</code>应为全局变量且值为<code>5</code>。
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testString: assert(typeof oopsGlobal != "undefined" && oopsGlobal === 5);
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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// Declare your variable here
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function fun1() {
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// Assign 5 to oopsGlobal Here
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}
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// Only change code above this line
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function fun2() {
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var output = "";
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if (typeof myGlobal != "undefined") {
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output += "myGlobal: " + myGlobal;
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}
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if (typeof oopsGlobal != "undefined") {
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output += " oopsGlobal: " + oopsGlobal;
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}
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console.log(output);
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}
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```
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</div>
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### Before Test
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<div id='js-setup'>
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```js
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var logOutput = "";
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var originalConsole = console
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function capture() {
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var nativeLog = console.log;
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console.log = function (message) {
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logOutput = message;
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if(nativeLog.apply) {
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nativeLog.apply(originalConsole, arguments);
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} else {
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var nativeMsg = Array.prototype.slice.apply(arguments).join(' ');
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nativeLog(nativeMsg);
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}
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};
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}
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function uncapture() {
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console.log = originalConsole.log;
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}
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var oopsGlobal;
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capture();
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```
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</div>
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### After Test
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<div id='js-teardown'>
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```js
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fun1();
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fun2();
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uncapture();
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(function() { return logOutput || "console.log never called"; })();
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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// Declare your variable here
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var myGlobal = 10;
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function fun1() {
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// Assign 5 to oopsGlobal Here
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oopsGlobal = 5;
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}
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// Only change code above this line
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function fun2() {
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var output = "";
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if(typeof myGlobal != "undefined") {
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output += "myGlobal: " + myGlobal;
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}
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if(typeof oopsGlobal != "undefined") {
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output += " oopsGlobal: " + oopsGlobal;
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}
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console.log(output);
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}
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```
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</section>
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