ee1e8abd87
* feat(tools): add seed/solution restore script * chore(curriculum): remove empty sections' markers * chore(curriculum): add seed + solution to Chinese * chore: remove old formatter * fix: update getChallenges parse translated challenges separately, without reference to the source * chore(curriculum): add dashedName to English * chore(curriculum): add dashedName to Chinese * refactor: remove unused challenge property 'name' * fix: relax dashedName requirement * fix: stray tag Remove stray `pre` tag from challenge file. Signed-off-by: nhcarrigan <nhcarrigan@gmail.com> Co-authored-by: nhcarrigan <nhcarrigan@gmail.com>
80 lines
1.6 KiB
Markdown
80 lines
1.6 KiB
Markdown
---
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id: 56533eb9ac21ba0edf2244aa
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title: 理解未初始化的变量
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challengeType: 1
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videoUrl: 'https://scrimba.com/c/cBa2JAL'
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forumTopicId: 18335
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dashedName: understanding-uninitialized-variables
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---
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# --description--
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当 JavaScript 中的变量被声明的时候，程序内部会给它一个初始值`undefined`。当你对一个值为`undefined`的变量进行运算操作的时候，算出来的结果将会是`NaN`，`NaN`的意思是<dfn>"Not a Number"</dfn>。当你用一个值是`undefined`的变量来做字符串拼接操作的时候，它会输出字符串`"undefined"`。
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# --instructions--
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定义 3 个变量`a`、`b`、`c`，并且分别给他们赋值：`5`、`10`、`"I am a"`，这样它们值就不会是`undefined`了。
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# --hints--
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`a`应该被定义，并且值为`6`。
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```js
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assert(typeof a === 'number' && a === 6);
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```
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`b`应该被定义，并且值为`15`。
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```js
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assert(typeof b === 'number' && b === 15);
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```
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`c`的值不能包含`undefined`，应该为 "I am a String!"。
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```js
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assert(!/undefined/.test(c) && c === 'I am a String!');
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```
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不要修改第二条注释下的代码。
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```js
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assert(
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/a = a \+ 1;/.test(code) &&
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/b = b \+ 5;/.test(code) &&
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/c = c \+ " String!";/.test(code)
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);
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```
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# --seed--
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## --after-user-code--
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```js
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(function(a,b,c){ return "a = " + a + ", b = " + b + ", c = '" + c + "'"; })(a,b,c);
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```
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## --seed-contents--
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```js
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// Only change code below this line
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var a;
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var b;
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var c;
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// Only change code above this line
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a = a + 1;
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b = b + 5;
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c = c + " String!";
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```
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# --solutions--
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```js
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var a = 5;
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var b = 10;
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var c = "I am a";
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a = a + 1;
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b = b + 5;
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c = c + " String!";
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```
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