First-class and higher-order functions

Model answer. First-class means functions are ordinary values: you can store them in variables, put them in arrays or objects, pass them as arguments and return them from other functions.

A higher-order function is a function that takes a function as an argument or returns one.

Standard examples: map, filter, reduce, sort (with a comparator), setTimeout, addEventListener, Promise.prototype.then.

// takes a function
const shout = words => words.map(w => w.toUpperCase());

// returns a function
const multiplier = factor => n => n * factor;
const triple = multiplier(3);
triple(7);   // 21

Higher-order functions let you separate what to do (the callback) from how to iterate or schedule it, which keeps code short and reusable.