Dynamic Tests

Given Mocha’s use of function expressions to define suites and test cases, it’s straightforward to generate your tests dynamically. No special syntax is required — plain old JavaScript can be used to achieve functionality similar to “parameterized” tests, which you may have seen in other frameworks.

Take the following example:

const assert = require("assert");

function add(args) {
  return args.reduce((prev, curr) => prev + curr, 0);
}

describe("add()", function () {
  const tests = [
    { args: [1, 2], expected: 3 },
    { args: [1, 2, 3], expected: 6 },
    { args: [1, 2, 3, 4], expected: 10 },
  ];

  tests.forEach(({ args, expected }) => {
    it(`correctly adds ${args.length} args`, function () {
      const res = add(args);
      assert.strictEqual(res, expected);
    });
  });
});

The above code will produce a suite with three specs:

$ mocha

  add()
    ✓ correctly adds 2 args
    ✓ correctly adds 3 args
    ✓ correctly adds 4 args

Tests added inside a .forEach handler often don’t play well with editor plugins, especially with “right-click run” features. Another way to parameterize tests is to generate them with a closure. This following example is equivalent to the one above:

describe("add()", function () {
  const testAdd = ({ args, expected }) =>
    function () {
      const res = add(args);
      assert.strictEqual(res, expected);
    };

  it("correctly adds 2 args", testAdd({ args: [1, 2], expected: 3 }));
  it("correctly adds 3 args", testAdd({ args: [1, 2, 3], expected: 6 }));
  it("correctly adds 4 args", testAdd({ args: [1, 2, 3, 4], expected: 10 }));
});

With top-level await you can collect your test data in a dynamic and asynchronous way while the test file is being loaded.

See also --delay for CommonJS modules without top-level await.

testfile.mjs

import assert from "assert";

// top-level await: Node >= v14.8.0 with ESM test file
const tests = await new Promise((resolve) => {
  setTimeout(resolve, 5000, [
    { args: [1, 2], expected: 3 },
    { args: [1, 2, 3], expected: 6 },
    { args: [1, 2, 3, 4], expected: 10 },
  ]);
});

// in suites ASYNCHRONOUS callbacks are NOT supported
describe("add()", function () {
  tests.forEach(({ args, expected }) => {
    it(`correctly adds ${args.length} args`, function () {
      const res = args.reduce((sum, curr) => sum + curr, 0);
      assert.strictEqual(res, expected);
    });
  });
});