JavaScript Practice Lesson 9 of 21

Curry and partial application

A closure (Phase 6) doesn't just hide private state - it can also hold onto an argument for later. Partial application means fixing one argument now and getting back a smaller function that only needs the rest. Currying takes that idea further: a function that normally takes several arguments is rewritten to take them one at a time, each call returning a new function until enough arguments have arrived.

The trick is a single if: check whether the second argument showed up. If it did, you have everything you need - finish the job now. If it didn't, return a new function that remembers the first argument (through a closure) and waits for the rest.

Your task: write add(a, b) so it works both ways - add(2, 3) returns 5 immediately, and add(2)(3) also returns 5, by returning a function that remembers 2 and waits for the second number.

You'll practice:

Checking whether an argument was actually passed
Returning a function that closes over an already-received argument

// Write add(a, b): if called with two args, return a + b right away.
// If called with just one arg, return a function that takes the second
// arg and returns the sum - so both add(2, 3) and add(2)(3) equal 5.
function add(a, b) {

}