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* Javascript -> JavaScript (English) * Update technical documentation page for required change * Update use-class-syntax-to-define-a-constructor-function.english.md * Update left-factorials.md
138 lines
4.6 KiB
Markdown
138 lines
4.6 KiB
Markdown
---
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title: Higher Order Functions
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---
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## Higher Order Functions
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A Higher Order Function is any function that returns a function when executed, takes a function as one or more of its arguments, or both. If you have used any of the `Array` methods like `map` or `filter`, or passed a callback function to jQuery's `$.get`, you have already worked with Higher Order Functions.
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## Map
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When you use `Array.map`, you provide a function as its only argument, which it applies to every element contained in the array.
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```javascript
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const arr = [ 1, 2, 3 ];
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const arrDoubled = arr.map(function(num) {
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return num * 2;
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});
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console.log(arrDoubled); // [ 2, 4, 6 ]
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```
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Higher order functions can also return a function. For example, you can make a function called `multiplyBy` that takes a number and returns a function that multiplies another number you provide by the first number provided. You can use this approach to create a `multiplyByTwo` function to pass to `Array.map`. This will give you the same result you saw above.
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```javascript
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function multiplyBy(num1) {
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return function(num2) {
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return num1 * num2;
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}
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}
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const multiplyByTwo = multiplyBy(2);
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const arr = [ 1, 2, 3 ];
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const arrDoubled = arr.map(multiplyByTwo);
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console.log(arrDoubled); // [ 2, 4, 6 ]
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```
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## Filter
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One amazing example of higher order function is the filter function.
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* Filter: It is the function on the array which loops through the array and filter out the value we are looking for.
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Below example:
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```javascript
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const animals = [
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{name: 'Fluffykins', species: 'rabbit'},
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{name:'Caro', species: 'dog'},
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{name: 'Hamilton', species: 'dog'},
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{name: 'Harold', species: 'fish'},
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{name: 'Ursula', species: 'cat'}
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]
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```
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This example illustrates filtering using imperative programming to simply return a list of species belonging to **dog**.
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```javascript
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let dogs = [];
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for(let i = 0; i < animals.length; i++) {
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if(animals[i].species === 'dog')
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dogs.push(animals[i])
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}
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```
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Now if we wanted to do the same but this time using filter.
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```javascript
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const dogs = animals.filter(function(animal) {
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return animal.species === 'dog';
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})
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```
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So what is happening here is that filter function takes in an argument, which is another function. A function passed as an argument inside another function is called a callback function. Here `function animal()` is an callback function.
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As you can see, the higher-order function has lot less code as compared to the traditional for-loop code. It's not because the syntax is shorter, but because there is lot less logic involved. The only logic used in the filter function in the above example is `return animal.species === 'dog'` which determines which animal goes into the array.
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Another reason there is a lot less code, is because this code below
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```javascript
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for(let i = 0; i < animals.length; i++) {
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if(animal[i].species === 'dog') {
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dogs.push(animals[i])
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}
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}
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```
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is already handled inside the filter function for us so we dont have to worry about it.
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The callback function and the filter function just **compose** in one another.
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We can decouple the callback function from the filter function like this...
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```javascript
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const isDog = function(animal) {
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return animal.species === 'dog'
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}
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const dogs = animals.filter(isDog)
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```
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## Reduce
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The Reduce function takes an array and "reduces" to a single variable.
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An example most people use is summing the numbers in an array:
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```javascript
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const nums = [1,2,3];
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const sum = nums.reduce((acc,curr)=> acc+curr);
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console.log(sum) // 6
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```
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The Reduce function usually takes two parameters, an accumulator and the current value, you can also specify a starting
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value:
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```javascript
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nums.reduce((acc,curr)=> acc+curr, 1);
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//returns 7
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```
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Reduce is a very powerful function, a more advanced way to use it would be to take an array and return an object counting the occurances of each element:
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```javascript
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[1,2,3,4,true,2].reduce((a,b)=>{
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a[b] = a[b] || 0;
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a[b]++;
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return a
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},{})
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//returns
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{ '1': 1, '2': 2, '3': 1, '4': 1, true: 1 }
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```
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There are limitations to this for example, the int 3 and the string '3' will be counted as the same item due to JavaScripts typing system, but this is still a very useful function.
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#### More Information
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See the guide on [Closures](https://guide.freecodecamp.org/javascript/closures) for more information on how `multiplyByTwo` keeps a reference to `num1` in the example above.
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- [More info about Closures](https://eloquentjavascript.net/05_higher_order.html)
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- [Eloquent JavaScript](https://eloquentjavascript.net/05_higher_order.html)
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- [Medium Article](https://medium.freecodecamp.org/higher-order-functions-in-javascript-d9101f9cf528)
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