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:

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var datasette = datasette || {};
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datasette.plugins = (() => {
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    var registry = {};
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    return {
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        register: (hook, fn, parameters) => {
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            if (!registry[hook]) {
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                registry[hook] = [];
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            }
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            registry[hook].push([fn, parameters]);
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        },
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        call: (hook, args) => {
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            args = args || {};
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            var results = [];
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            (registry[hook] || []).forEach(([fn, parameters]) => {
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                /* Call with the correct arguments */
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                var result = fn.apply(fn, parameters.map(parameter => args[parameter]));
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                if (result !== undefined) {
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                    results.push(result);
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                }
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            });
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            return results;
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        }
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    };
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})();
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module.exports = datasette;

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

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const datasette = require("../plugins.js");
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describe("Datasette Plugins", () => {
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  test("it should have datasette.plugins", () => {
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    expect(!!datasette.plugins).toEqual(true);
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  });
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  test("registering a plugin should work", () => {
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    datasette.plugins.register("numbers", (a, b) => a + b, ["a", "b"]);
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    var result = datasette.plugins.call("numbers", { a: 1, b: 2 });
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    expect(result).toEqual([3]);
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    datasette.plugins.register("numbers", (a, b) => a * b, ["a", "b"]);
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    var result2 = datasette.plugins.call("numbers", { a: 1, b: 2 });
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    expect(result2).toEqual([3, 2]);
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  });
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});

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

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% npx jest -c '{}'
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 PASS  __tests__/plugins.spec.js
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  Datasette Plugins
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    ✓ it should have datasette.plugins (3 ms)
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    ✓ registering a plugin should work (1 ms)
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Test Suites: 1 passed, 1 total
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Tests:       2 passed, 2 total
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Snapshots:   0 total
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Time:        1.163 s
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Ran all test suites.