109 lines
3.7 KiB
Markdown
109 lines
3.7 KiB
Markdown
|
---
|
||
|
title: Higher Order Functions
|
||
|
---
|
||
|
## Higher Order Functions
|
||
|
|
||
|
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.
|
||
|
|
||
|
When you use `Array.map`, you provide a function as its only argument, which it applies to every element contained in the array.
|
||
|
|
||
|
```javascript
|
||
|
const arr = [ 1, 2, 3 ];
|
||
|
|
||
|
const arrDoubled = arr.map(function(num) {
|
||
|
return num * 2;
|
||
|
});
|
||
|
|
||
|
console.log(arrDoubled); // [ 2, 4, 6 ]
|
||
|
```
|
||
|
|
||
|
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.
|
||
|
|
||
|
```javascript
|
||
|
function multiplyBy(num1) {
|
||
|
return function(num2) {
|
||
|
return num1 * num2;
|
||
|
}
|
||
|
}
|
||
|
|
||
|
const multiplyByTwo = multiplyBy(2);
|
||
|
|
||
|
const arr = [ 1, 2, 3 ];
|
||
|
|
||
|
const arrDoubled = arr.map(multiplyByTwo);
|
||
|
|
||
|
console.log(arrDoubled); // [ 2, 4, 6 ]
|
||
|
```
|
||
|
|
||
|
### Filter Method Example
|
||
|
|
||
|
One amazing example of higher order function is the filter function.
|
||
|
|
||
|
* Filter: It is the function on the array which loops through the array and filter out the value we are looking for.
|
||
|
|
||
|
Below example:
|
||
|
|
||
|
```javascript
|
||
|
const animals = [
|
||
|
{name: 'Fluffykins', species: 'rabbit'},
|
||
|
{name:'Caro', species: 'dog'},
|
||
|
{name: 'Hamilton', species: 'dog'},
|
||
|
{name: 'Harold', species: 'fish'},
|
||
|
{name: 'Ursula', species: 'cat'}
|
||
|
]
|
||
|
```
|
||
|
|
||
|
This example illustrates filtering using imperative programming to simply return a list of species belonging to **dog**.
|
||
|
|
||
|
```javascript
|
||
|
let dogs = [];
|
||
|
for(let i = 0; i < animals.length; i++) {
|
||
|
if(animals[i].species === 'dog')
|
||
|
dogs.push(animals[i])
|
||
|
}
|
||
|
```
|
||
|
|
||
|
Now if we wanted to do the same but this time using filter.
|
||
|
|
||
|
```javascript
|
||
|
const dogs = animals.filter(function(animal) {
|
||
|
return animal.species === 'dog';
|
||
|
})
|
||
|
```
|
||
|
|
||
|
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.
|
||
|
|
||
|
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.
|
||
|
|
||
|
Another reason there is a lot less code, is because this code below
|
||
|
|
||
|
```javascript
|
||
|
for(let i = 0; i < animals.length; i++) {
|
||
|
if(animal[i].species === 'dog') {
|
||
|
dogs.push(animals[i])
|
||
|
}
|
||
|
}
|
||
|
```
|
||
|
|
||
|
is already handled inside the filter function for us so we dont have to worry about it.
|
||
|
|
||
|
The callback function and the filter function just **compose** in one another.
|
||
|
|
||
|
We can decouple the callback function from the filter function like this...
|
||
|
|
||
|
```javascript
|
||
|
const isDog = function(animal) {
|
||
|
return animal.species === 'dog'
|
||
|
}
|
||
|
|
||
|
const dogs = animals.filter(isDog)
|
||
|
```
|
||
|
|
||
|
#### More Information
|
||
|
|
||
|
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.
|
||
|
|
||
|
- [More info about Closures](https://eloquentjavascript.net/05_higher_order.html)
|
||
|
- [Eloquent Javascript](https://eloquentjavascript.net/05_higher_order.html)
|
||
|
- [Medium Article](https://medium.freecodecamp.org/higher-order-functions-in-javascript-d9101f9cf528)
|