81 lines
1.9 KiB
Markdown
81 lines
1.9 KiB
Markdown
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---
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id: 587d7b89367417b2b2512b48
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title: Use the Spread Operator to Evaluate Arrays In-Place
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challengeType: 1
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forumTopicId: 301222
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dashedName: use-the-spread-operator-to-evaluate-arrays-in-place
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---
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# --description--
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ES6 introduces the <dfn>spread operator</dfn>, which allows us to expand arrays and other expressions in places where multiple parameters or elements are expected.
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The ES5 code below uses `apply()` to compute the maximum value in an array:
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```js
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var arr = [6, 89, 3, 45];
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var maximus = Math.max.apply(null, arr); // returns 89
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```
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We had to use `Math.max.apply(null, arr)` because `Math.max(arr)` returns `NaN`. `Math.max()` expects comma-separated arguments, but not an array. The spread operator makes this syntax much better to read and maintain.
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```js
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const arr = [6, 89, 3, 45];
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const maximus = Math.max(...arr); // returns 89
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```
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`...arr` returns an unpacked array. In other words, it *spreads* the array. However, the spread operator only works in-place, like in an argument to a function or in an array literal. The following code will not work:
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```js
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const spreaded = ...arr; // will throw a syntax error
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```
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# --instructions--
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Copy all contents of `arr1` into another array `arr2` using the spread operator.
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# --hints--
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`arr2` should be correct copy of `arr1`.
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```js
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assert(arr2.every((v, i) => v === arr1[i]) && arr2.length);
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```
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`...` spread operator should be used to duplicate `arr1`.
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```js
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assert(code.match(/Array\(\s*\.\.\.arr1\s*\)|\[\s*\.\.\.arr1\s*\]/));
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```
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`arr2` should remain unchanged when `arr1` is changed.
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```js
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assert((arr1, arr2) => {
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arr1.push('JUN');
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return arr2.length < arr1.length;
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});
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```
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# --seed--
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## --seed-contents--
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```js
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const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
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let arr2;
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arr2 = []; // Change this line
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console.log(arr2);
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```
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# --solutions--
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```js
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const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
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let arr2;
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arr2 = [...arr1];
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```
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