Using Jest without a package.json#

I wanted to try out Jest for writing JavaScript unit tests, in a project that wasn’t set up with package.json and other NPM related things.

Jest looks for *.spec.js tests in a __tests__ directory. It expects to find configuration in a package.json file but it can be passed configuration using the -c option - which can be a path to a JSON configuration file or can be a JSON literal.

I created a file I wanted to test in plugins.js which looked like this. The module.exports at the bottom was required so Jest could later import the code:

1
var datasette = datasette || {};
2
datasette.plugins = (() => {
3
    var registry = {};
4
    return {
5
        register: (hook, fn, parameters) => {
6
            if (!registry[hook]) {
7
                registry[hook] = [];
8
            }
9
            registry[hook].push([fn, parameters]);
10
        },
11
        call: (hook, args) => {
12
            args = args || {};
13
            var results = [];
14
            (registry[hook] || []).forEach(([fn, parameters]) => {
15
                /* Call with the correct arguments */
16
                var result = fn.apply(fn, parameters.map(parameter => args[parameter]));
17
                if (result !== undefined) {
18
                    results.push(result);
19
                }
20
            });
21
            return results;
22
        }
23
    };
24
})();
25

26
module.exports = datasette;

Then I created __tests__/plugins.spec.js with this:

1
const datasette = require("../plugins.js");
2

3
describe("Datasette Plugins", () => {
4
  test("it should have datasette.plugins", () => {
5
    expect(!!datasette.plugins).toEqual(true);
6
  });
7
  test("registering a plugin should work", () => {
8
    datasette.plugins.register("numbers", (a, b) => a + b, ["a", "b"]);
9
    var result = datasette.plugins.call("numbers", { a: 1, b: 2 });
10
    expect(result).toEqual([3]);
11
    datasette.plugins.register("numbers", (a, b) => a * b, ["a", "b"]);
12
    var result2 = datasette.plugins.call("numbers", { a: 1, b: 2 });
13
    expect(result2).toEqual([3, 2]);
14
  });
15
});

Now I can run Jest in the same directory as plugins.js like this:

1
% npx jest -c '{}'
2
 PASS  __tests__/plugins.spec.js
3
  Datasette Plugins
4
    ✓ it should have datasette.plugins (3 ms)
5
    ✓ registering a plugin should work (1 ms)
6

7
Test Suites: 1 passed, 1 total
8
Tests:       2 passed, 2 total
9
Snapshots:   0 total
10
Time:        1.163 s
11
Ran all test suites.

Newsletter

Using Jest without a package.json#