How can I partially apply a function in JavaScript?

Partial application is a technique used to fix a number of arguments to a function, producing another function of smaller arity. This is particularly useful in situations where you want to create a new function by pre-filling some of the arguments of an existing function.

Depending on the function you want to partially apply, you can either prepend or append the arguments to the function. Both techniques are fairly similar.

Prepend arguments to a function

In order to partially apply a function by prepending arguments, you can use the spread operator (...) to prepend partials to the list of arguments of fn. Simply pass partials before any other arguments that are supplied.

const partial = (fn, ...partials) => (...args) => fn(...partials, ...args);

const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetHello = partial(greet, 'Hello');
greetHello('John'); // 'Hello John!'

Append arguments to a function

Similarly, you can partially apply a function by appending arguments using the spread operator (...). You need only pass them to fn after any other arguments that are supplied.

const partialRight = (fn, ...partials) => (...args) => fn(...args, ...partials);

const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetJohn = partialRight(greet, 'John');
greetJohn('Hello'); // 'Hello John!'