Callback Argument.

Eighth class. A callback is a function passed as an argument to another function. The receiving function decides when to call it.

function applyTwice(fn, value) { return fn(fn(value)); }

const double = (n) => n * 2; console.log(applyTwice(double, 3));

Here double is not called while being passed — the function itself is passed. applyTwice calls it. map, filter, and find use the same idea next.

Common mistake: applyTwice(double(3), 3) passes a number instead of a function; pass double without parentheses.