89 lines
2.1 KiB
Markdown
89 lines
2.1 KiB
Markdown
---
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id: 587d7da9367417b2b2512b67
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title: Add Elements to the End of an Array Using concat Instead of push
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challengeType: 1
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forumTopicId: 301226
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dashedName: add-elements-to-the-end-of-an-array-using-concat-instead-of-push
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---
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# --description--
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Functional programming is all about creating and using non-mutating functions.
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The last challenge introduced the `concat` method as a way to combine arrays into a new one without mutating the original arrays. Compare `concat` to the `push` method. `push` adds an item to the end of the same array it is called on, which mutates that array. Here's an example:
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```js
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var arr = [1, 2, 3];
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arr.push([4, 5, 6]);
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```
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`arr` would have a modified value of `[1, 2, 3, [4, 5, 6]]`, which is not the functional programming way.
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`concat` offers a way to add new items to the end of an array without any mutating side effects.
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# --instructions--
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Change the `nonMutatingPush` function so it uses `concat` to add `newItem` to the end of `original` instead of `push`. The function should return an array.
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# --hints--
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Your code should use the `concat` method.
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```js
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assert(code.match(/\.concat/g));
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```
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Your code should not use the `push` method.
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```js
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assert(!code.match(/\.?[\s\S]*?push/g));
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```
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The `first` array should not change.
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```js
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assert(JSON.stringify(first) === JSON.stringify([1, 2, 3]));
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```
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The `second` array should not change.
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```js
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assert(JSON.stringify(second) === JSON.stringify([4, 5]));
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```
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`nonMutatingPush([1, 2, 3], [4, 5])` should return `[1, 2, 3, 4, 5]`.
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```js
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assert(
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JSON.stringify(nonMutatingPush([1, 2, 3], [4, 5])) ===
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JSON.stringify([1, 2, 3, 4, 5])
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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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function nonMutatingPush(original, newItem) {
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// Only change code below this line
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return original.push(newItem);
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// Only change code above this line
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}
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var first = [1, 2, 3];
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var second = [4, 5];
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nonMutatingPush(first, second);
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```
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# --solutions--
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```js
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function nonMutatingPush(original, newItem) {
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return original.concat(newItem);
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}
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var first = [1, 2, 3];
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var second = [4, 5];
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nonMutatingPush(first, second);
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```
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