higher-order-functions.md

What is a higher order function?

• Functions that operate on other functions
  • either by taking them as arguments or by returning them
• Functions with callbacks are considered higher order functions

const functionToPass = someNumber => someNumber * 2;


// this function does NOT return a function, it returns the RESULT of a function call + arithmetic
const higherOrderFunctionWithCallback = (number, callBackFunction) => callBackFunction(number) + number;

• Functions that return functions are also higher order functions

const higherOrderFunctionThatReturnsAFunction = number1 => number2 => number1 + number2;

// in traditional, pre-arrow function javascript, it would be written like this:
function sameAsAbove(number1) {
  return function(number2) {
    return number1 + number2;
  }
}

What problem does it solve?

• This is a concept that underpins a lot of the complexity hidden away in libraries/frameworks, there is value in understanding it

• Partial functions in javascript are functions you want to provide arguments to 'over time', or not all at once

• These are sometimes written using bind (this is less common now)

const add = (num1, num2) => num1 + num2;

// the first argument in the bind updates the `this` context inside of the funciton scope, which is pointless
// inside of an arrow function, every other argument is assigned to parameters in the function definition, in order
const partialAdd = add.bind(null, 5);


// results in 11
partialAdd(6);

• Often we want a 'base value' for our functions to be used multiple times later, but want to keep our functions generic

const builder = buildingBase => (rooms, floors, address) => ({...buildingBase, rentalUnits, floors, address});

const houseBuilder = builder({type: 'house', hasLawn: true});

const house1 = houseBuilder(4, 2, '123 Mocking Bird lane');
const house2 = houseBuilder(3, 1, '234 Mocking Bird lane');

const apartmentBuildingBuilder = builder({type: 'apartment'});

const apartmentBuilding1 = apartmentBuildingBuilder(22, 170, '345 Mocking Bird Lane');
// etc