# Practical introduction to Functional Programming with JS

legendtariq
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## Higher Order Functions

Higher Order Functions are functions that take other functions as parameters, Mind Blown 💥.
We are going to discover how this functions are useful through some examples, in particular we are going to recreate the logic of the amazon checkout.

## Filter

Filter is a method of arrays that accepts a test function, that returns true or false, as argument and returns a new array with only the elements that passed to the function returned true.

Here is an example:

Get the even numbers in an array
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function isEven(x){
return x % 2 === 0;
}
const numbers = [12,324,213,4,2,3,45,4234];
const evenNumbers = numbers.filter(isEven);
console.log(evenNumbers);
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As you can see the function isEven doesn't need to include the logic to handle arrays, and this is the great thing about these higher order function: now the decision logic is kept separate by the function applying it to the array, so we can reuse it.

Let's try to implement our first piece of logic for the amazon cart: extracting from the cart array all the prime items

Implement isPrime and primeItems, for the latter use filter. The file is on the right

## Reject

We can now reuse the isPrime function in conjunction with reject to get all the non-prime items in the cart. The reject function is the opposite of the filter: it creates an array with all the elements but those that satisfy the condition.

Reject is not a built-in function in js, we are going to use the library underscore.js to have it. The syntax is slightly different: _.reject(list, testFunction) where _ is the underscore library

Implement the notPrimeItems function using reject (you can use functions defined in the previous snippet)

As an optional exercise you could also implement reject yourself using filter.

Implement the reject function using filter
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const reject = (arr, callback) => //todo
// {...}
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## Lambda functions

When defining short functions it's often convenient to use an alternative syntax called lambda function, that allows us to define anonymous functions in a more compact way: ( /*arguments*/ ) => { /*code*/ } and if our function is only a return statement we can strip the curly brackets and avoid writing return: ( /*arguments*/ ) => /*value to return*/.

We can rewrite the isEven snippet from before with a lambda function:

Get the even numbers in an array
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const numbers = [12,324,213,4,2,3,45,4234];
const isEven = (x) => x % 2 === 0
const evenNumbers = numbers.filter(isEven);
console.log(evenNumbers);
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## Map

Another very useful higher order function is map: it takes a function and applies it to all the elements of an array.
The syntax is identical to filter
E.g.

Squaring all the elements of an array
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const numbers = [1,2,3,4,5,6,10,20];
const squares = numbers.map((x) => Math.pow(x,2)); // notice the lambda function :)
console.log(squares);
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Now back to our amazon example: we can use map to apply a coupon. The applyCoupon function should apply a 20% discount on all the tech items.

Implement the applyCoupon function

## Reduce: one function to rule them all

Reduce is the last higher order function we are going to discuss and it's also the most powerful: in fact you can implement any list transformation with reduce.
Reduce takes in a callback function and a starting value, the callback function takes as arguments an accumulator and the value of the current element of the array and returns the accumulator to be used in the next cycle. The value returned from the last call of the callback function is the value returned by reduce.

It's easier if I show you some examples:

Multiplying all the elements of an array
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const numbers = [2,3,4,5];
const product = numbers.reduce((acc, x) => acc * x, 1);
console.log(product);
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Filtering only even numbers
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const isEven = (x) => {
return x % 2 === 0;
}
const numbers = [12,324,213,4,2,3,45,4234];
const callback = (acc, x) => {
if (isEven(x)){
acc.push(x)
}
return acc
}
const evenNumbers = numbers.reduce(callback, []);
console.log(evenNumbers);
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Implementing map with reduce
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function map(arr, callback){
return arr.reduce((acc, x) => {
acc.push(callback(x));
return acc;
}, []);
}
// squaring example
const squares = map([1,2,3,4,5], (x) => Math.pow(x,2) );
console.log(squares);
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Now to complete our amazon workflow write a function that returns the total cost of the order.

Implement the totalCost function using reduce
Open Source Your Knowledge: become a Contributor and help others learn.
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/*
The cart is an array of objects like this:
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