InputgroupByLength(["one","two","three","four"])
Output{"3":["one","two"],"4":["four"],"5":["three"]}
InputgroupByLength([])
Output{}
InputgroupByLength(["a"])
Output{"1":["a"]}

Answer

function groupByLength(words) {
  return words.reduce((acc, w) => {
    (acc[w.length] ||= []).push(w);
    return acc;
  }, {});
}
groupByLength(["one", "two", "three"]);
// { 3: ["one", "two"], 5: ["three"] }

Object keys are always strings, so acc[3] and acc["3"] are the same bucket. An empty input returns {}, not [].

sandbox.js

function groupByLength(words) {
  // TODO
}

Output

Hit “Run tests” to check your solution.

All problems

Next question

JavaScriptMedium11/15

Write groupByLength(words) that groups an array of strings into an object keyed by length

By FrontendPro Editorial Team Updated 8/7/2026

Explore the JavaScript interview guide

Examples

Write groupByLength so that the following cases hold.

InputgroupByLength(["one","two","three","four"])
Output{"3":["one","two"],"4":["four"],"5":["three"]}
InputgroupByLength([])
Output{}
InputgroupByLength(["a"])
Output{"1":["a"]}