103 lines
2.7 KiB
Markdown
103 lines
2.7 KiB
Markdown
---
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id: 598f48a36c8c40764b4e52b3
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title: Prevent Object Mutation
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challengeType: 1
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---
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## Description
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<section id='description'>
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As seen in the previous challenge, <code>const</code> declaration alone doesn't really protect your data from mutation. To ensure your data doesn't change, JavaScript provides a function <code>Object.freeze</code> to prevent data mutation.
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Once the object is frozen, you can no longer add, update, or delete properties from it. Any attempt at changing the object will be rejected without an error.
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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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In this challenge you are going to use <code>Object.freeze</code> to prevent mathematical constants from changing. You need to freeze the <code>MATH_CONSTANTS</code> object so that no one is able to alter the value of <code>PI</code>, add, or delete properties.
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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: Do not replace <code>const</code> keyword.
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testString: getUserInput => assert(getUserInput('index').match(/const/g), 'Do not replace <code>const</code> keyword.');
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- text: <code>MATH_CONSTANTS</code> should be a constant variable (by using <code>const</code>).
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testString: getUserInput => assert(getUserInput('index').match(/const\s+MATH_CONSTANTS/g), '<code>MATH_CONSTANTS</code> should be a constant variable (by using <code>const</code>).');
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- text: Do not change original <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), 'Do not change original <code>MATH_CONSTANTS</code>.');
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- text: <code>PI</code> equals <code>3.14</code>.
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testString: assert(PI === 3.14, '<code>PI</code> equals <code>3.14</code>.');
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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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