Dictionary in JavaScript: Choose Object Keys or Map Keys

The task

Store desk labels under textual keys in an object, then compare numeric 7 and string “7” as keys in both structures. Use a null-prototype object for the explicit property-key dictionary.

Runnable Node 22 example

const assert = require('node:assert/strict');

const desks = Object.create(null);
desks.north = 'A2';
desks[7] = 'numeric entry';
desks['7'] = 'text entry';
assert.equal(desks[7], 'text entry');
const lookup = new Map([[7, 'numeric entry'], ['7', 'text entry']]);
assert.equal(lookup.size, 2);
assert.equal(lookup.get(7), 'numeric entry');
assert.equal(lookup.get('7'), 'text entry');
assert.equal(desks.north, 'A2');
console.log(`object=${desks[7]}; map=${lookup.get(7)}; entries=${lookup.size}`);

Expected output and explanation

The object assignments address the same property key, so the later text entry replaces the earlier value. The Map retains separate numeric and string keys and reports two entries. That difference can determine whether a lookup silently overwrites a value.

The null-prototype object avoids an inherited Object.prototype for this small dictionary, but it still has property-key semantics. Map offers entry operations such as get and has rather than ordinary property access for stored entries. Choose consistently rather than writing lookup[key] and expecting a Map entry.

Pitfall to check

If keys come from external input, define their type and permitted range before storing them. A null-prototype object is not permission to assign arbitrary configuration fields into some other object.