Frequency Counter

Hash Mapseasy

Course · Section 9: Data Structures, Modern Operators and Strings · Lecture 124: Maps: Fundamentals

Implement `frequency(arr)` that returns an object mapping each unique element of `arr` to the number of times it appears. Example: `frequency(['a','b','a','c','b','a'])` → `{ a: 3, b: 2, c: 1 }`.

Sample tests

Input: frequency(["a","b","a","c","b","a"])

Output: {"a":3,"b":2,"c":1}

Input: frequency([])

Output: {}

+ 1 hidden test run on Submit.

Hints

Common pitfalls

Counting with nested loops is O(n^2); a map makes it linear.

Learning resources

MDN: Map

Approach & explanation (try first)

A hash map tallies each value in one pass. O(n) time and space.

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