Practical introduction to Functional Programming with JS


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Currying is the technique of translating the evaluation of a function that takes multiple arguments into evaluating a sequence of functions, each with a single argument.

This becomes very easy using lambda functions and closures. Let's dive straight into code:

Get the even numbers in an array
// standard way
function add(x,y,z){
return x + y + z;
console.log("standard: ", add(1,2,3));
// curried style
const add2 = x => // if you have only one argument in the lambda function you can omit the parenthesis
y =>
z =>
x + y + z;
console.log("curried: ", add2(1)(2)(3) );

So why is this useful? Because we are now able to pass the arguments at different points in time; this means that we can use currying to "construct" a function.


Generic filter callback to check any property of an object
const checkProperty = propertyName =>
expectedValue =>
object =>
object[propertyName] === expectedValue;
const candies = [
{"soft":true, "flavour":"strawberry"},
{"soft":false, "flavour":"strawberry"},
{"soft":false, "flavour":"cherry"},
{"soft":true, "flavour":"orange"},
{"soft":false, "flavour":"lemon"},
const softCandies = candies.filter( checkProperty("soft")(true) ); // checkProperty("soft")(true) returns a function that accepts an object, exactly what we need
const strawberryCandies = candies.filter( checkProperty("flavour")("strawberry") );

And now, time to get your hands dirty: remember the applyCoupon function we wrote in the previous chapter? It was very specific; now we want to create a curryable function that takes as arguments (in this order) category, discount between 0 and 1 (a 2$ item with a 0.1 discount will cost 1.8$) and an item, and that returns the item with the correct price.

const item = {
           "name": "Biscuits",
           "type": "regular",
           "category": "food",
           "price": 2.0
applyCoupon("food")(0.1)(item).price === 1.8

Remember to apply the discount only to the right items!

Implement `applyCoupon` as a curriable function
const applyCoupon = // this should be a curried function
// {...}
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