89 lines
2.9 KiB
Markdown
89 lines
2.9 KiB
Markdown
---
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id: 587d7b87367417b2b2512b42
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title: Mutate an Array Declared with const
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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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The <code>const</code> declaration has many use cases in modern JavaScript.
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Some developers prefer to assign all their variables using <code>const</code> by default, unless they know they will need to reassign the value. Only in that case, they use <code>let</code>.
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However, it is important to understand that objects (including arrays and functions) assigned to a variable using <code>const</code> are still mutable. Using the <code>const</code> declaration only prevents reassignment of the variable identifier.
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```js
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"use strict";
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const s = [5, 6, 7];
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s = [1, 2, 3]; // throws error, trying to assign a const
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s[2] = 45; // works just as it would with an array declared with var or let
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console.log(s); // returns [5, 6, 45]
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```
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As you can see, you can mutate the object <code>[5, 6, 7]</code> itself and the variable <code>s</code> will still point to the altered array <code>[5, 6, 45]</code>. Like all arrays, the array elements in <code>s</code> are mutable, but because <code>const</code> was used, you cannot use the variable identifier <code>s</code> to point to a different array using the assignment operator.
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</section>
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## Instructions
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<section id='instructions'>
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An array is declared as <code>const s = [5, 7, 2]</code>. Change the array to <code>[2, 5, 7]</code> using various element assignment.
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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>s</code> should be a constant variable (by using <code>const</code>).
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testString: getUserInput => assert(getUserInput('index').match(/const\s+s/g), '<code>s</code> should be a constant variable (by using <code>const</code>).');
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- text: Do not change the original array declaration.
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testString: getUserInput => assert(getUserInput('index').match(/const\s+s\s*=\s*\[\s*5\s*,\s*7\s*,\s*2\s*\]\s*;?/g), 'Do not change the original array declaration.');
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- text: <code>s</code> should be equal to <code>[2, 5, 7]</code>.
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testString: assert.deepEqual(s, [2, 5, 7], '<code>s</code> should be equal to <code>[2, 5, 7]</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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const s = [5, 7, 2];
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function editInPlace() {
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'use strict';
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// change code below this line
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// s = [2, 5, 7]; <- this is invalid
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// change code above this line
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}
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editInPlace();
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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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const s = [5, 7, 2];
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function editInPlace() {
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'use strict';
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// change code below this line
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// s = [2, 5, 7]; <- this is invalid
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s[0] = 2;
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s[1] = 5;
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s[2] = 7;
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// change code above this line
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}
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editInPlace();
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```
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</section>
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