Merge with defaults in JavaScript
JavaScript interview exercise on Objects, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write withDefaults(defaults, options) that returns a new object where every key from defaults is present, overridden by options only when the option is not undefined. Neither input may be mutated.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.