[RxJS] Introduction to RxJS Marble Testing

Marble testing is an expressive way to test observables by utilizing marble diagrams. This lesson will walk you through the syntax and features, preparing you to start writing marble tests today!

Grep two files from the rxjs

• https://github.com/ReactiveX/rxjs/blob/master/spec/helpers/marble-testing.ts
• https://github.com/ReactiveX/rxjs/blob/master/spec/helpers/test-helper.ts

/*
    RxJS marble testing allows for a more natural style of testing observables.
    To get started, you need to include a few helpers libraries, marble-testing.ts and test-helper.ts,
    in your karma.conf or wallaby.js configuration file. 
    These files provide helpers for parsing marble diagrams and asserting against the subscription points and result
    of your observables under test. For these examples I will be using Jasmine, but Mocha and Chai works just as well.

    Let's get started with the basics of marble testing!

    First, let's understand the pieces that make up a valid marble diagram.

    Dash: Indicates a passing of time, you can think of each dash as 10ms when it comes to your tests.
    -----                 <----- 50ms
    Characters: Each character inside the dash indicates an emission.
    -----a-----b-----c    <----- Emit 'a' at 60ms, 'b' at 120ms, 'c' at 180ms
    Pipes |: Pipes indicate the completion point of an observable.
    -----a|               <----- Emit 'a' at 60ms then complete (70ms)
    Parenthesis (): Parenthesis indicate multiple emissions in same time frame, think Observable.of(1,2,3)
    -----(abc|)           <----- Emit 'a''b''c' at 60ms then complete (60ms)
    Caret ^: Indicates the starting point of a subscription, used with expectSubscription assertion.
    ^-------              <----- Subscription point at caret.
    Exclamation Point - !: Indicates the end point of a subscription, also used with expectSubscription assertion.
    ^------!              <----- Subscription starts at caret, ends at exclamation point.
    Pound Sign - #: Indicates an error
    ---a---#              <----- Emit 'a' at 40ms, error at 80ms
    There are also a few methods included to parse marble sequences and transpose values.

    cold(marbles: string, values?: object, error?: any) : Subscription starts when test begins
    cold(--a--b--|, {a: 'Hello', b: 'World'})           <----- Emit 'Hello' at 30ms and 'World' at 60ms, complete at 90ms
    hot(marbles: string, values?: object, error?: any) : Behaves like subscription starts at point of caret
    hot(--^--a---b--|, {a: 'Goodbye', b: 'World'})      <----- Subscription begins at point of caret
*/

For example we want to test:

const source =       "---a---b---c--|";
const expected =   "---a---b---c--|";

they should be equal. 

Here each '-' means 1. frames.

'|' means completed.

The method we need to use is 'expectObservable' & 'cold':

    it('should parse marble diagrams', () => {
        const source = cold('---a---b---c---|');
        const expected =    '---a---b---c---|';

        expectObservable(source).toBe(expected)
    });

Cold will treat the beginning of the diagram as a subscription point. Now the test passing.

But if we change a little bit:

    it('should parse marble diagrams', () => {
        const source = cold('---a---b---c---|');
        const expected =    '---a--b---c---|';

        expectObservable(source).toBe(expected)
    });

It reports error:

    Expected 
    {"frame":30,"notification":{"kind":"N","value":"a","hasValue":true}}
    {"frame":70,"notification":{"kind":"N","value":"b","hasValue":true}}
    {"frame":110,"notification":{"kind":"N","value":"c","hasValue":true}}
    {"frame":150,"notification":{"kind":"C","hasValue":false}}
    
    to deep equal 
    {"frame":30,"notification":{"kind":"N","value":"a","hasValue":true}}
    {"frame":60,"notification":{"kind":"N","value":"b","hasValue":true}}
    {"frame":100,"notification":{"kind":"N","value":"c","hasValue":true}}
    {"frame":140,"notification":{"kind":"C","hasValue":false}}

Test 'concat' opreator:

    it('should work with cold observables', () => {
        const obs1 = cold('-a---b-|');
        const obs2 = cold('-c---d-|');
        const expectedConcatRes = '-a---b--c---d-|';

        expectObservable(obs1.concat(obs2)).toBe(expectedConcatRes)
    });

'Hot' observable: Hot will actually let you identify the subscription point yourself:

When testing hot observables you can specify the subscription point using a caret '^', similar to how you specify subscriptions when utilizing the expectSubscriptions assertion.

    it('should work with hot observables', () => {
        const obs1 =     hot('---a--^--b---|');
        const obs2 =  hot('-----c---^-----------------d-|');
        const expected =           '---b--------------d-|';

        expectObservable(obs1.concat(obs2)).toBe(expected);
    });

Algin the ^, easy for read

Spread subscription and marble diagram:

    /*
        For certain operators you may want to confirm the point at which
        an observable is subscribed or unsubscribed. Marble testing makes this 
        possible by using the expectSubscriptions helper method. The cold and hot
        methods return a subscriptions object, including the frame at which the observable 
        would be subscribed and unsubscribed. You can then assert against these
        subscription points by supplying a diagram which indicates the expected behavior.

        ^ - Indicated the subscription point.
        ! - Indicates the point at which the observable was unsubscribed.

        Example subscriptions object: {"subscribedFrame":70,"unsubscribedFrame":140}
    */
    it('should identify subscription points', () => {
        const obs1 = cold('-a---b-|');
        const obs2 = cold('-c---d-|')
        const expected =  '-a---b--c---d-|';
        const sub1 =      '^------!'
        const sub2 =      '-------^------!'

        expectObservable(obs1.concat(obs2)).toBe(expected);
        expectSubscriptions(obs1.subscriptions).toBe(sub1);
        expectSubscriptions(obs2.subscriptions).toBe(sub2);
    })

Object to map the key and value:

    /*
        Both the hot and cold methods, as well the the toBe method accept an object map as a
        second parameter, indicating the values to output for the appropriate placeholder.
        When the test is executed these values rather than the matching string in the marble diagram.
    */
    it('should correctly sub in values', () => {
        const values = {a: 3, b: 2};
        const source = cold(  '---a---b---|', values);
        const expected =      '---a---b---|';

        expectObservable(source).toBe(expected, values);
    });

    /*
        Multiple emissions occuring in same time frame can be represented by grouping in parenthesis.
        Complete and error symbols can also be included in the same grouping as simulated outputs.
    */
    it('should handle emissions in same time frame', () => {
        const obs1 = Observable.of(1,2,3,4);
        const expected = '(abcd|)';

        expectObservable(obs1).toBe(expected, {a: 1, b: 2, c: 3, d: 4});
    });

    /*
        For asynchronous tests RxJS supplies a TestScheduler.
        How it works...
    */
    it('should work with asynchronous operators', () => {
        const obs1 = Observable
            .interval(10, rxTestScheduler)
            .take(5)
            .filter(v => v % 2 === 0);
        const expected = '-a-b-(c|)';

        expectObservable(obs1).toBe(expected, {a: 0, b: 2, c: 4});
    });

Error handling:

    /*
        Observables that encounter errors are represented by the pound (#) sign.
        In this case, our observable is retried twice before ultimately emitting an error.
        A third value can be supplied to the toBe method specifying the error to be matched.
    */
    it('should handle errors', () => {
        const source = Observable.of(1,2,3,4)
            .map(val => {
                if(val > 3){
                    throw 'Number too high!';
                };
                return val;
            })
        .retry(2);

        const expected = '(abcabcabc#)';

        expectObservable(source).toBe(expected, {a: 1, b: 2, c: 3, d: 4}, 'Number too high!');
    });