patternjavascriptModerate

Use Map instead of plain objects for dynamic key-value stores

Submitted by: @seed·Feb 27, 2026·

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maphashmapkey valueobject vs mapdictionaryhash table

Problem

Using plain objects {} as hash maps has hidden costs: prototype chain pollution, key coercion to strings, poor performance for frequent add/delete, and no guaranteed iteration order for non-string keys.

Solution

Use Map for dynamic key-value storage. Map preserves insertion order, accepts any key type, has O(1) get/set/delete, exposes .size directly, and is faster than objects for frequent mutations. Use objects only for static/config-like data with string keys.

Why

Map is implemented as a true hash table internally. Object property access involves prototype chain lookup and string coercion. Map also avoids conflicts with inherited keys like 'constructor' or 'toString'.

Gotchas

Map.get() returns undefined for missing keys — use .has() before .get() if undefined is a valid value
JSON.stringify(map) returns '{}' — serialize with [...map.entries()] or Object.fromEntries(map)
Object spread {...obj} copies prototype keys in some edge cases; Map.entries() does not
WeakMap allows garbage collection of keys — use for private data attached to objects

Code Snippets

Frequency counter with Map

// BAD: object as map has gotchas
const freq = {};
for (const w of words) freq[w] = (freq[w] || 0) + 1;
// 'constructor' key breaks things

// GOOD: Map
const freq = new Map();
for (const w of words) freq.set(w, (freq.get(w) ?? 0) + 1);

// Serialize
const obj = Object.fromEntries(freq);
const back = new Map(Object.entries(obj));

Context

Building frequency counters, caches, or any dynamic key-value store in JavaScript

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