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How does filter() work? What does reduce() do and what are its arguments? What's the difference between splice() and slice()? How do find() and findIndex() differ from filter()? What does Array.isArray() solve that typeof can't? How does sort() compare values by default, and how do you fix numeric sorting? What is the difference between flat() and flatMap()? Why can array holes (sparse arrays) behave unexpectedly with iteration methods? How do you efficiently remove duplicate values from an array?
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What's the difference between map() and forEach()?

Beginner

Answer

map() returns a new array built from the return value of the callback for each element; forEach() only runs the callback for its side effects and always returns undefined.
const doubled = [1,2,3].map(n => n*2); // [2,4,6]
Real-world example Transforming an array of user objects into an array of display names.

Common follow-ups: Can you chain map() and filter() together?

Functional Programming
All 10 Arrays & Array Methods questions