Map.prototype.keys() — JS Technical Interview Questions & Answers

Returns an iterator over the Map's keys.

Map.prototype.keys()

  • Map.prototype.keys() is a JS topic. In plain words you use it for fail students only in a marks list. Don’t start with a slogan — start with that picture.
  • Smallest example: filter(m => m < 40). Type it, run it, and say what you see. If you can do that from memory, you know Map.prototype.keys().
  • From the example next to this theory: Returns an iterator over the Map's keys.
  • Trap: mutating while filtering. Fix that before you talk about advanced JS.
  • Viva: what is Map.prototype.keys()? Then show filter(m => m < 40). Then name the trap.

Technical Interview Tip: What is Map.prototype.keys()? Show this: filter(m => m < 40). Trap: mutating while filtering.

Code Examples for Map.prototype.keys()

Map.prototype.keys() — working example

const scores = new Map([
  ["Ada", 10],
  ["Lin", 8],
]);
scores.set("Ada", 11);
console.log(scores.get("Ada"), scores.has("Lin"), scores.size);
scores.delete("Lin");
console.log([...scores.keys()]);

Map.prototype.keys() — map keeps insertion-ordered key→value pairs; keys can be any type (unlike plain objects).

How to Explain Map.prototype.keys() to an Interviewer

What is Map.prototype.keys()?

Used for fail students only a marks list Not just the heading

Give one small example of Map.prototype.keys().

filter(m => m < 40) Returns an iterator over the Map's keys

What mistake do beginners make with Map.prototype.keys()?

mutating while filtering Show the fix

Where do you use Map.prototype.keys()?

a marks list fail students only