Create an empty map that does not have properties

An object with no prototype: no inherited keys such as toString or __proto__.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function createDictionary() { return Object.create(null); }

Usage

const counts = createDictionary();
'toString' in counts; // → false
'toString' in {}; // → true
counts.__proto__ = 'a key like any other';
Object.keys(counts); // → ['__proto__']

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• Keys from user input, such as "__proto__", stay ordinary data, as the last lines show.
• For keys added and removed at runtime, a Map is often clearer. JSON.stringify serializes a null-prototype object but turns a Map into {}.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Pick properties from an object — Build a new object with only the requested own properties.
• Omit properties from an object — Keep the own enumerable fields that are not in a supplied list.
• Create an object from pairs of key and value — Turn [key, value] pairs into a plain object with Object.fromEntries.