How do you mock an import?

[onosendi]

The module isn't getting mocked:

// a.js

const foo = 'bar';

export { foo };

// a.spec.js

import t from 'tap';

const foo = await t.mockImport('./a.js', {
  foo: 'baz',
});

t.test('', async () => {
  console.log(foo);
  // [Module: null prototype] { foo: 'bar' }
})

[isaacs]

The challenge here is that "mock" and "import" are both somewhat overloaded terms.

t.mockImport(module, mocks) is not "import module and then apply the mocks to it." It's "import module, but mock its imports with the following values."

So for example, if you had this:

// a.js
import { foo } from './b.js'
export const fooInA = foo

t.test('', async t => {
  const { fooInA } = await t.mockImport('./a.js', {
    // when a.js imports from './b.js', it gets this object instead
    // of loading the actual contents of ./b.js
    './b.js': { foo: 'mocked foo' },
  })
  t.equal(fooInA, 'mocked foo')
})

If you want to import something, and then apply some mocks to it, you can use t.createMock(), t.capture(), or t.intercept(), depending on the use case.

import a from './a.js'
const fooBaz = t.createMock(a, { foo: 'baz' })
// imports are a null-prototype object with configurable:false properties
// so make a plain old object out of it first.
const aPlainObj = { ...a }
const aIntercepted = t.intercept(aPlainObj, 'foo', { value: 'baz' })
t.equal(aPlainObj.foo, 'baz')
aPlainObj.foo = 'bar'
t.equal(aPlainObj.foo, 'bar')
// now aIntercepted has info about the get and set operations
// and when the test ends, the intercept is removed and the property
// goes back to how it was before the test mucked with it.