Simple mockable dependency injection

const deps = getDependencies(this, defaultDependencies)

...doSomethingWith(deps)

Used in the live function to be tested.

This gets the external dependencies to use. You provide the default set, but you may get back overridden ones, if overrides have been set for the given key (we are using this in this example).

The choice of key is up to you, but it must be typeof === 'object'.

Similarly, the shape of the dependencies is up to you.

setDependencies(key, mockDependencies)

functionToBeTested.call(key, args)

Used in the test harness.

Registers the alternative (usually mock) dependencies for that object/key.

And that's it.

This uses a WeakMap if it can, so no cleanup.