When writing a multi-argument function, like the following
let adder = (x, y) => x + y; adder(2, 3); /* => 5 */
We are utilizing a syntactic sugar of the function syntax. The same function can be written as such:
let adder = (x) => (y) => x + y; adder(2, 3); /* => 5 */
As you can see, we can apply the function in the same way.
This is useful because it means we can partially apply (or curry) our functions to create other functions.
let adder = (x, y) => x + y; let twoAdder = adder(2); twoAdder(5); /* => 7 */