Examples of functions returning functions

functions-in-functions.js

// One classic purpose of a function returning a function is to do partial application.
// Let's say we have a function that adds two numbers:
function add(x, y) {
  return x + y;
}

// Then, we want to make a function that adds 9000 to the argument:
function itsOver9000(x) {
  return add(9000, x);
}

// But what if we weren't happy just adding 9000,
// but wanted a way to more generally create functions that add a constant amount to their input?
function createAdder(constantAmountToAdd) {
  return function(x) {
    return add(constantAmountToAdd, x);
  };
}

// You would use it like so:
const addTwo = createAdder(2);
addTwo(5); // ==> 7
addTwo(10); // ==> 12

const addNineThousand = createAdder(9000);
addNineThousand(1); // ==> 9001
addNineThousand(-1); // ==> -8999

// With a simple function that takes two arguments and adds them, it can seem a little silly
// do to all this. What are we really getting? But when the arguments get more complicated,
// the benefits from this become more compelling. 

// For instance, let's say that we have a function that saves to a database:
function save(databaseConnectionOptions, dataToSave) {
  // https://www.youtube.com/watch?v=Qtb11P1FWnc
}

// We don't want to have to pass the database connection options all around the app.
// These connection options could include sensitive information like credentials,
// so we want to limit them to as small a part of the program as possible. It's
// also just cumbersome to have to make that information available anywhere you want
// to save some data. Instead, we'll use partial application to save the connection
// options up front:
function createSaveFunctionWithConnectionOptionsApplied(databaseConnectionOptions) {
  return function(dataToSave) {
    return save(databaseConnectionOptions, dataToSave);
  };
}

// First, we create the function:
const saveData = createSaveFunctionWithConnectionOptionsApplied({user: 'admin', password: 'gohan'});

// Then, we can save without worrying about credentials:
saveData({username: 'Rust', booksRead: ['Atlas Shrugged']});