deepFreeze.js

import test from 'ava';

/**
 * deep object freeze that avoids the Readonly<{set foo()}> trap, and avoids
 * needing to pull in something heavier like harden()
 *
 * @template {object} O
 * @param {O} o
 * @returns {O}
 */
function freeze(o) {
    // freeze the object first to break any cycles if we re-encounter this
    // object in the loop below (or any recursive sub-loop thereof)
    Object.freeze(o);

    for (const [_key, {value}] of Object.entries(Object.getOwnPropertyDescriptors(o))) {
        // skip accessor properties
        if (value === undefined) continue;

        // skip primitives
        if (typeof value !== 'object') continue;

        // skip already frozen objects
        if (Object.isFrozen(value)) continue;

        freeze(value);
    }
    // NOTE: this intentionally discards the Readonly<T> wrapper
    return o;
}

test('freeze', t => {
    let cv = 7;
    const o = {
        a: 3,
        bs: [4, 5, 6],
        get c() { return cv },
        set c(v) { cv = v },

        /** @type {any} this field is just a hack to prove cycle safety */
        d: null,
    };
    o.d = o;
    freeze(o);

    // nope to regular property change
    t.is(o.a, 3);
    t.throws(() => { o.a = 9 }, {message: /Cannot assign to read only property/});
    t.is(o.a, 3);

    // nope to array element change, frozen
    t.is(o.bs[1], 5);
    t.throws(() => { o.bs[1] = 9 }, {message: /Cannot assign to read only property/});
    t.is(o.bs[1], 5);

    // nope to array push, frozen
    t.deepEqual(o.bs, [4, 5, 6]);
    t.throws(() => o.bs.push(9), {message: /Cannot add property 3/})
    t.deepEqual(o.bs, [4, 5, 6]);

    // nope to array pop, frozen
    t.deepEqual(o.bs, [4, 5, 6]);
    t.throws(() => o.bs.pop(), {message: /Cannot delete property '2'/})
    t.deepEqual(o.bs, [4, 5, 6]);

    // explicitly defined property change is okay tho
    t.is(o.c, 7);
    o.c = 9;
    t.is(o.c, 9);
});