What are higher-order functions? Give examples.
Quick answer
Functions that take or return other functions — the basis of map/filter/reduce, event handlers and middleware.
In detail
Because functions are first-class values, they can be passed and returned. A higher-order function does one or both. `map`, `filter`, `reduce`, `setTimeout`, and decorators (a function that wraps another to add logging/caching) are all examples.
1const withLog = fn => (...a) => { console.log(a); return fn(...a); };
2const add = (x, y) => x + y;
3withLog(add)(2, 3); // logs [2,3], returns 5
4
5const times = n => x => x * n;
6times(3)(10); // 30Why interviewers ask this: Tests functional thinking, central to modern JS/React.
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