What are stubs?

Test stubs are functions (spies) with pre-programmed behavior.

They support the full test spy API in addition to methods which can be used to alter the stub’s behavior.

As spies, stubs can be either anonymous, or wrap existing functions. When wrapping an existing function with a stub, the original function is not called.

When to use stubs?

Use a stub when you want to:

  1. Control a method’s behavior from a test to force the code down a specific path. Examples include forcing a method to throw an error in order to test error handling.

  2. When you want to prevent a specific method from being called directly (possibly because it triggers undesired behavior, such as a XMLHttpRequest or similar).

The following example is yet another test from PubSubJS which shows how to create an anonymous stub that throws an exception when called.

"test should call all subscribers, even if there are exceptions" : function(){
    var message = 'an example message';
    var stub = sinon.stub().throws();
    var spy1 = sinon.spy();
    var spy2 = sinon.spy();

    PubSub.subscribe(message, stub);
    PubSub.subscribe(message, spy1);
    PubSub.subscribe(message, spy2);

    PubSub.publishSync(message, undefined);

    assert(spy1.called);
    assert(spy2.called);
    assert(stub.calledBefore(spy1));
}

Note how the stub also implements the spy interface. The test verifies that all callbacks were called, and also that the exception throwing stub was called before one of the other callbacks.

Defining stub behavior on consecutive calls

Calling behavior defining methods like returns or throws multiple times overrides the behavior of the stub. As of Sinon version 1.8, you can use the onCall method to make a stub respond differently on consecutive calls.

Note that in Sinon version 1.5 to version 1.7, multiple calls to the yields* and callsArg* family of methods define a sequence of behaviors for consecutive calls. As of 1.8, this functionality has been removed in favor of the onCall API.

Stub API

Properties

var stub = sinon.stub();

Creates an anonymous stub function

var stub = sinon.stub(object, "method");

Replaces object.method with a stub function. An exception is thrown if the property is not already a function.

The original function can be restored by calling object.method.restore(); (or stub.restore();).

var stub = sinon.stub(object, "method", func);

Replaces object.method with a func, wrapped in a spy.

As usual, object.method.restore(); can be used to restore the original method.

var stub = sinon.stub(obj);

Stubs all the object’s methods.

Note that it’s usually better practice to stub individual methods, particularly on objects that you don’t understand or control all the methods for (e.g. library dependencies).

Stubbing individual methods tests intent more precisely and is less susceptible to unexpected behavior as the object’s code evolves.

If you want to create a stub object of MyConstructor, but don’t want the constructor to be invoked, use this utility function.

var stub = sinon.createStubInstance(MyConstructor)

stub.withArgs(arg1[, arg2, ...]);

Stubs the method only for the provided arguments.

This is useful to be more expressive in your assertions, where you can access the spy with the same call. It is also useful to create a stub that can act differently in response to different arguments.

"test should stub method differently based on arguments": function () {
    var callback = sinon.stub();
    callback.withArgs(42).returns(1);
    callback.withArgs(1).throws("TypeError");

    callback(); // No return value, no exception
    callback(42); // Returns 1
    callback(1); // Throws TypeError
}

"stub.onCall(n); Added in v1.8

Defines the behavior of the stub on the nth call. Useful for testing sequential interactions.

"test should stub method differently on consecutive calls": function () {
    var callback = sinon.stub();
    callback.onCall(0).returns(1);
    callback.onCall(1).returns(2);
    callback.returns(3);

    callback(); // Returns 1
    callback(); // Returns 2
    callback(); // All following calls return 3
}

There are methods onFirstCall, onSecondCall,onThirdCall to make stub definitions read more naturally.

onCall can be combined with all of the behavior defining methods in this section. In particular, it can be used together with withArgs.

"test should stub method differently on consecutive calls with certain argument": function () {
    var callback = sinon.stub();
    callback.withArgs(42)
        .onFirstCall().returns(1)
        .onSecondCall().returns(2);
    callback.returns(0);

    callback(1); // Returns 0
    callback(42); // Returns 1
    callback(1); // Returns 0
    callback(42); // Returns 2
    callback(1); // Returns 0
    callback(42); // Returns 0
}

Note how the behavior of the stub for argument 42 falls back to the default behavior once no more calls have been defined.

stub.onFirstCall();

Alias for stub.onCall(0);

stub.onSecondCall();

Alias for stub.onCall(1);

stub.onThirdCall();

Alias for stub.onCall(2);

stub.reset();

Resets the history of the stub, like calling spy.reset();

Note: Updated in sinon@2.0.0

stub.resetBehavior();

Resets the stub’s behaviour to the default behaviour

var stub = sinon.stub();

stub.returns(54)

stub(); // 54

stub.resetBehavior();

stub(); // undefined

stub.returns(obj);

Makes the stub return the provided value.

stub.returnsArg(index);

Causes the stub to return the argument at the provided index.

stub.returnsArg(0); causes the stub to return the first argument.

stub.returnsThis();

Causes the stub to return its this value.

Useful for stubbing jQuery-style fluent APIs.

stub.throws();

Causes the stub to throw an exception (Error).

stub.throws("TypeError");

Causes the stub to throw an exception of the provided type.

stub.throws(obj);

Causes the stub to throw the provided exception object.

stub.throwsArg(index);

Causes the stub to throw the argument exception at the provided index. Throws a TypeError if called with fewer than index arguments.

stub.throwsArg(0); causes the stub to throw the first argument exception.

stub.callsArg(index);

Causes the stub to call the argument at the provided index as a callback function. stub.callsArg(0); causes the stub to call the first argument as a callback.

stub.callsArgOn(index, context);

Like stub.callsArg(index); but with an additional parameter to pass the this context.

stub.callsArgWith(index, arg1, arg2, ...);

Like callsArg, but with arguments to pass to the callback.

stub.callsArgOnWith(index, context, arg1, arg2, ...);

Like above but with an additional parameter to pass the this context.

stub.yields([arg1, arg2, ...])

Similar to callsArg.

Causes the stub to call the first callback it receives with the provided arguments (if any).

If a method accepts more than one callback, you need to use callsArg to have the stub invoke other callbacks than the first one.

stub.yieldsOn(context, [arg1, arg2, ...])

Like above but with an additional parameter to pass the this context.

stub.yieldsTo(property, [arg1, arg2, ...])

Causes the spy to invoke a callback passed as a property of an object to the spy.

Like yields, yieldsTo grabs the first matching argument, finds the callback and calls it with the (optional) arguments.

stub.yieldsToOn(property, context, [arg1, arg2, ...])

Like above but with an additional parameter to pass the this context.

"test should fake successful ajax request": function () {
    sinon.stub(jQuery, "ajax").yieldsTo("success", [1, 2, 3]);

    jQuery.ajax({
        success: function (data) {
            assertEquals([1, 2, 3], data);
        }
    });
}

stub.yield([arg1, arg2, ...])

Invoke callbacks passed to the stub with the given arguments.

If the stub was never called with a function argument, yield throws an error.

Also aliased as invokeCallback.

stub.yieldTo(callback, [arg1, arg2, ...])

Invokes callbacks passed as a property of an object to the stub.

Like yield, yieldTo grabs the first matching argument, finds the callback and calls it with the (optional) arguments.

"calling callbacks": function () {
    var callback = sinon.stub();
    callback({
        "success": function () {
            console.log("Success!");
        },
        "failure": function () {
            console.log("Oh noes!");
        }
    });

    callback.yieldTo("failure"); // Logs "Oh noes!"
}

stub.callArg(argNum)

Like yield, but with an explicit argument number specifying which callback to call.

Useful if a function is called with more than one callback, and simply calling the first callback is not desired.

"calling the last callback": function () {
    var callback = sinon.stub();
    callback(function () {
        console.log("Success!");
    }, function () {
        console.log("Oh noes!");
    });

    callback.callArg(1); // Logs "Oh noes!"
}

stub.callArgWith(argNum, [arg1, arg2, ...])

Like callArg, but with arguments.

stub.callsArgAsync(index);

Same as their corresponding non-Async counterparts, but with callback being deferred (executed not immediately but after short timeout and in another “thread”)

stub.callsArgAsync(index);

stub.callsArgOnAsync(index, context);

stub.callsArgWithAsync(index, arg1, arg2, ...);

stub.callsArgOnWithAsync(index, context, arg1, arg2, ...);

stub.yieldsAsync([arg1, arg2, ...]);

stub.yieldsOnAsync(context, [arg1, arg2, ...]);

stub.yieldsToAsync(property, [arg1, arg2, ...]);

stub.yieldsToOnAsync(property, context, [arg1, arg2, ...])

Same as their corresponding non-Async counterparts, but with callback being deferred (executed not immediately but after short timeout and in another “thread”)