Question 1

Why should you use forEach instead of map for side effects in JavaScript?

Accepted Answer

Because map returns a new array, which is wasted if you only want side effects. forEach returns undefined and signals that you are not transforming data. Using map for side effects is misleading and wastes memory.

Question 2

Why does reduce throw TypeError on an empty array without an initial value?

Accepted Answer

Because without an initial value, reduce uses the first element as the accumulator. An empty array has no first element, so it throws TypeError: Reduce of empty array with no initial value. Always pass an initial value.

Question 3

Why does [10, 2, 1].sort() give [1, 10, 2] in JavaScript?

Accepted Answer

Because sort defaults to string comparison. It converts elements to strings and sorts lexicographically. '10' comes before '2' because '1' < '2'. Use a numeric comparator: sort((a, b) => a - b).

Question 4

Why does map((n) => { n * 2; }) return undefined values in JavaScript?

Accepted Answer

Because the arrow function has a block body (braces) with no return statement. Block bodies need an explicit return. Use an expression body (n => n * 2) or add return (n => { return n * 2; }).

Why should you use forEach instead of map for side effects in JavaScript?

More FAQs in Common Higher-Order Function Mistakes in JavaScript

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