105 lines
2.5 KiB
Markdown
105 lines
2.5 KiB
Markdown
---
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id: 598f48a36c8c40764b4e52b3
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challengeType: 1
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forumTopicId: 301207
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title: 防止对象改变
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---
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## Description
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<section id='description'>
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通过之前的挑战可以看出，<code>const</code>声明并不会真的保护你的数据不被改变。为了确保数据不被改变，JavaScript 提供了一个函数<code>Object.freeze</code>来防止数据改变。
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当一个对象被冻结的时候，你不能再对它的属性再进行增、删、改的操作。任何试图改变对象的操作都会被阻止，却不会报错。
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```js
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let obj = {
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name:"FreeCodeCamp",
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review:"Awesome"
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};
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Object.freeze(obj);
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obj.review = "bad"; // will be ignored. Mutation not allowed
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obj.newProp = "Test"; // will be ignored. Mutation not allowed
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console.log(obj);
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// { name: "FreeCodeCamp", review:"Awesome"}
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```
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</section>
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## Instructions
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<section id='instructions'>
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在这个挑战中，你将使用<code>Object.freeze</code>来防止数学常量被改变。你需要冻结<code>MATH_CONSTANTS</code>对象，使得没有人可以改变<code>PI</code>的值，抑或增加或删除属性。
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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: 不要替换<code>const</code>关键字。
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testString: getUserInput => assert(getUserInput('index').match(/const/g));
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- text: <code>MATH_CONSTANTS</code>应该为一个常量 (使用<code>const</code>)。
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testString: getUserInput => assert(getUserInput('index').match(/const\s+MATH_CONSTANTS/g));
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- text: 不要改变原始的<code>MATH_CONSTANTS</code>。
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testString: getUserInput => assert(getUserInput('index').match(/const\s+MATH_CONSTANTS\s+=\s+{\s+PI:\s+3.14\s+};/g));
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- text: <code>PI</code>等于<code>3.14</code>。
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testString: assert(PI === 3.14);
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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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function freezeObj() {
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'use strict';
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const MATH_CONSTANTS = {
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PI: 3.14
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};
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// change code below this line
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// change code above this line
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try {
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MATH_CONSTANTS.PI = 99;
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} catch(ex) {
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console.log(ex);
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}
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return MATH_CONSTANTS.PI;
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}
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const PI = freezeObj();
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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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function freezeObj() {
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'use strict';
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const MATH_CONSTANTS = {
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PI: 3.14
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};
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// change code below this line
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Object.freeze(MATH_CONSTANTS);
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// change code above this line
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try {
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MATH_CONSTANTS.PI = 99;
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} catch(ex) {
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console.log(ex);
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}
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return MATH_CONSTANTS.PI;
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}
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const PI = freezeObj();
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```
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</section>
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