How do I unit-test a class that depends on another class to change the state of something?

Question

I have a class A which contains an instance of class B, and function foo of A calls function set of B, which updates the state of B. Here is a code example (in Javascript):

A = function () {
    this.init = function (b) {
        this.b = b
    }

    this.foo = function (val) {
        this.b.set(val)
    }

    this.bar = function () {
        return this.b.get()
    }

}

B = function () {
    this.set = function (val) {
        this.v = val
    }

    this.get = function () {
        return this.v
    }
}

How do I unit-test the foo function, while keeping the test for A non-dependent on the implementation of B (using mocks and stubs and what not)?

Answer 1

I figured out the best way to do this, while making sure that A's test doesn't depend on its implementation, would be to create a mock B that has a working get and set, but writes to a temporary variable.

Code example to test A:

// Mock B
b = new function () {
    this.set = function (val) {
        this.v = val
    }

    this.get = function () {
        return this.v
    }
}

// Create an instance of A with Mock B
a = new A().init(b)

// Test A

// ...