test.js

const data = {
  a: 1,
  b: null,
  c: undefined,
  d: {
    _: 'object',
    e: {},
    f: null,
    g: function () {},
    get h () {
      return 3
    },
    j: 1,
    k: "string"
  },
  l: {
    m: [
      {n: 1},
      {n: 2},
      {n: 3}
    ],
    o: {
      p: {
        q: true,
        r: false,
        s: {
          t: 1,
          "spaced keys": "hey"
        },
        u: [
          [0, 0, 1],
          [0, 2, 0],
          [1, 0, 0],
        ]
      }
    }
  }
}
const write = w => document.body.innerHTML += `${w}<br/>`
const results = { total: 0, passed: 0 };
const test = (condition, data) => {
  results.total++;

  if (condition) {
    results.passed++;
    write(`  ✅ pass: ${data}`)
    return;
  }
  write(`  🔴 fail: ${data}`)
  console.assert(condition, data);
}

console.group('mb tests');
test(mb(['a'])(data) === 1, 'a');
test(mb(['a', 'bad key', 'bad key', 'bad key'])(data) === undefined, 'bad key');
test(mb(['b'])(data) === null, 'b');
test(mb(['c'])(data) === undefined, 'c');
test(mb(['d'])(data)._ === 'object', 'd');
test(JSON.stringify(mb(['d', 'e'])(data)) === '{}', 'e');
test(mb(['d', 'f'])(data) === null, 'f');
test(typeof mb(['d', 'g'])(data) === 'function', 'g');
test(mb(['d', 'h'])(data) === 3, 'h');
test(mb(['d', 'j'])(data) === 1, 'j');
test(mb(['d', 'k'])(data) === 'string', 'k');
test(mb(['d', 'k', 'length'])(data) === 6, 'length');
test(mb(['l', 'm'])(data).length === 3, 'm');
test(mb(['l', 'm', 0, 'n'])(data) === 1, 'n');
test(mb(['l', 'm', 1, 'n'])(data) === 2, 'n');
test(mb(['l', 'm', 2, 'n'])(data) === 3, 'n');
test(mb(['l', 'o', 'p', 'q'])(data) === true, 'q');
test(mb(['l', 'o', 'p', 'r'])(data) === false, 'r');
test(mb(['l', 'o', 'p', 's', 't'])(data) === 1, 't');
test(mb(['l', 'o', 'p', 'u', 1, 1])(data) === 2, 'u');
test(mb(['l', 'o', 'p', 's', 'spaced keys'])(data) === 'hey', 'spaced keys');

test(mb([0, 1])([[1,2]]) === 2, 'array');
test(mb([0, 1])([0]) === undefined, 'missing intermediate');

console.info(`Passed tests ${results.passed} out of ${results.total}.`);

if (results.passed == results.total) {
  console.info('Everything seems fine.');
}

console.groupEnd();