2018-10-10 22:03:03 +00:00
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---
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id: 56533eb9ac21ba0edf2244be
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2021-03-04 17:49:46 +00:00
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title: 全局作用域和函数
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2018-10-10 22:03:03 +00:00
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challengeType: 1
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2020-04-29 10:29:13 +00:00
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videoUrl: 'https://scrimba.com/c/cQM7mCN'
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forumTopicId: 18193
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2021-01-13 02:31:00 +00:00
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dashedName: global-scope-and-functions
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2018-10-10 22:03:03 +00:00
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---
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2020-12-16 07:37:30 +00:00
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# --description--
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2018-10-10 22:03:03 +00:00
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2021-03-04 17:49:46 +00:00
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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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2018-10-10 22:03:03 +00:00
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2021-11-06 15:56:52 +00:00
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未使用 `let` 或 `const` 关键字声明的变量会在 `global` 范围内自动创建。 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。 你应该总是用 `let` 或 `const` 声明你的变量。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --instructions--
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2018-10-10 22:03:03 +00:00
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2021-11-06 15:56:52 +00:00
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使用 `let` 或 `const`,在任何函数之外声明一个名为 `myGlobal` 的全局变量。 并给它一个初始值 `10`。
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2018-10-10 22:03:03 +00:00
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2021-11-06 15:56:52 +00:00
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在函数 `fun1` 中,***不要*** 使用 `let` 或 `const` 关键字,将 `5` 分配给 `oopsGlobal` 。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --hints--
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2018-10-10 22:03:03 +00:00
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2021-03-04 17:49:46 +00:00
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应定义 `myGlobal`。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert(typeof myGlobal != 'undefined');
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2018-10-10 22:03:03 +00:00
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```
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2021-03-04 17:49:46 +00:00
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`myGlobal` 的值应为 `10`。
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2018-10-10 22:03:03 +00:00
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```js
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2020-12-16 07:37:30 +00:00
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assert(myGlobal === 10);
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2018-10-10 22:03:03 +00:00
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```
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2021-11-06 15:56:52 +00:00
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`myGlobal` 应该使用 `let` 或 `const` 关键字声明
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2018-10-10 22:03:03 +00:00
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```js
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2021-11-06 15:56:52 +00:00
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assert(/(let|const)\s+myGlobal/.test(code));
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2018-10-10 22:03:03 +00:00
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```
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2021-03-04 17:49:46 +00:00
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`oopsGlobal` 应为全局变量,值为 `5`。
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2020-04-29 10:29:13 +00:00
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2018-10-10 22:03:03 +00:00
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```js
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2020-12-16 07:37:30 +00:00
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assert(typeof oopsGlobal != 'undefined' && oopsGlobal === 5);
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2018-10-10 22:03:03 +00:00
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```
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2020-04-29 10:29:13 +00:00
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2021-01-13 02:31:00 +00:00
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# --seed--
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## --before-user-code--
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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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## --after-user-code--
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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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## --seed-contents--
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```js
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// Declare the myGlobal variable below this line
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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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2020-12-16 07:37:30 +00:00
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# --solutions--
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2021-01-13 02:31:00 +00:00
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```js
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2021-11-06 15:56:52 +00:00
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const myGlobal = 10;
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2021-01-13 02:31:00 +00:00
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function fun1() {
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oopsGlobal = 5;
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}
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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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