Reading your linked page and some other related sources (like [this github issue](https://github.com/facebook/jest/issues/1524#issuecomment-262366820)) *some* users have found that:

&gt;...using the `--runInBand` helps in an environment with **limited resources**.

and

&gt;... `--runInBand` took our tests from &gt;1.5 hours (actually I don&#39;t know how long because Jenkins timed out at 1.5 hours) to around 4 minutes. (Note: we have really **poor resources** for our build server)

As we can see, those users had improvements in their performances on their machines even though they had limited resources on them. If we read what does the `--runInBand` flag does from the [docs](https://facebook.github.io/jest/docs/cli.html#runinband) it says:

&gt;Alias: -i. Run all tests serially in the current process, **rather than creating a worker pool of child processes** that run tests. This can be useful for debugging.

Therefore, taking into consideration these comments and the docs, I believe the improvement in performance is due to the fact that now the process runs in a single thread. This greatly helps a *limited-resource-computer* because it does not have to spend memory and time dealing and handling multiple threads in a thread pool, a task that could prove to be too expensive for its limited resources.

However, I believe this is the case only if the machine you are using also has limited resources. If you used a more &quot;powerful&quot; machine (i.e.: several cores, decent RAM, SSD, etc.) using multiple threads probably will be better than running a single one.  

When you run tests in multi-threads, jest creates a cache for every thread. When you run with `--runInBand` jest uses one cache storage for all tests.

I found it after runs 20 identical tests files, first with key `--runInBand`, a first test takes 25 seconds and next identical tests take 2-3s each. 

When I run tests without `--runInBand` key, each identical test file executes in 25 seconds.  

Quite simply, how do I say if a given `TextView` contains a specific string in `Espresso`.

The equivalent of: `myStrings.contains(&quot;Subby&quot;);`

Espresso - TextView contains String

I try to make a `uuid` (v 3.0.1) package work in Node/Typescript app, but I&#39;m not sure what should I import and how to use it.

This is `index.d.ts` (from `@types/uuid` v 2.0.29):

    declare namespace uuid {
    	interface V1Options {
    		node?: number[];
    		clockseq?: number;
    		msecs?: number | Date;
    		nsecs?: number;
    	}
    
    	type V4Options = { random: number[] } | { rng: () =&gt; number[]; }
    
    	interface UuidStatic {
    		(options?: V4Options): string;
    		(options: V4Options | null, buffer: number[], offset?: number): number[];
    		(options: V4Options | null, buffer: Buffer, offset?: number): Buffer;
    
    		v1(options?: V1Options): string;
    		v1(options: V1Options | null, buffer: number[], offset?: number): number[];
    		v1(options: V1Options | null, buffer: Buffer, offset?: number): Buffer;
    		v4: UuidStatic;
    		parse(id: string): number[];
    		parse(id: string, buffer: number[], offset?: number): number[];
    		parse(id: string, buffer: Buffer, offset?: number): Buffer;
    		unparse(buffer: number[] | Buffer, offset?: number): string;
    	}
    }
    
    declare const uuid: uuid.UuidStatic
    export = uuid

I cant find exported class here. 

For example `index.d.ts` from `angular2-uuid` looks like that:

    export declare class UUID {
        constructor();
        static UUID(): string;
        private static pad4(num);
        private static random4();
    }

And it&#39;s quite obvious to use:

    let id = UUID.UUID();

So. How to use (import and call) `uuid`?

guid/uuid in Typescript Node.js app

I read that the [--runInBand](https://facebook.github.io/jest/docs/troubleshooting.html#tests-are-extremely-slow-on-docker-and-or-continuous-integration-ci-server) flag speeds up Jest test duration by 50% on CI servers. I can&#39;t really find an explanation online on what that flag does except that it lets tests run in the same thread and sequentially.

Why does running the test in the same thread and sequentially make it faster? Intuitively, shouldn&#39;t that make it slower?

Why does Jest --runInBand speed up tests?

<p>I read that the <a href="https://facebook.github.io/jest/docs/troubleshooting.html#tests-are-extremely-slow-on-docker-and-or-continuous-integration-ci-server" target="_blank" rel="noopener noreferrer">--runInBand</a> flag speeds up Jest test duration by 50% on CI servers. I can't really find an explanation online on what that flag does except that it lets tests run in the same thread and sequentially.</p>
<p>Why does running the test in the same thread and sequentially make it faster? Intuitively, shouldn't that make it slower?</p>


I&#39;m trying to write a simple test for a simple React component, and I want to use Jest to confirm that a function has been called when I simulate a click with enzyme. According to the Jest docs, I should be able to use `spyOn` to do this: [spyOn](http://facebook.github.io/jest/docs/jest-object.html#jestspyonobject-methodname).

However, when I try this, I keep getting `TypeError: Cannot read property &#39;_isMockFunction&#39; of undefined` which I take to mean that my spy is undefined. My code looks like this:

    import React, { Component } from &#39;react&#39;;
    import logo from &#39;./logo.svg&#39;;
    import &#39;./App.css&#39;;
    class App extends Component {
    
      myClickFunc = () =&gt; {
          console.log(&#39;clickity clickcty&#39;)
      }
      render() {
        return (
          &lt;div className=&quot;App&quot;&gt;
            &lt;div className=&quot;App-header&quot;&gt;
              &lt;img src={logo} className=&quot;App-logo&quot; alt=&quot;logo&quot; /&gt;
              &lt;h2&gt;Welcome to React&lt;/h2&gt;
            &lt;/div&gt;
            &lt;p className=&quot;App-intro&quot; onClick={this.myClickFunc}&gt;
              To get started, edit &lt;code&gt;src/App.js&lt;/code&gt; and save to reload.
            &lt;/p&gt;
          &lt;/div&gt;
        );
      }
    }
    
    export default App;


and in my test file:

    import React from &#39;react&#39;;
    import ReactDOM from &#39;react-dom&#39;;
    import App from &#39;./App&#39;;
    import { shallow, mount, render } from &#39;enzyme&#39;

    describe(&#39;my sweet test&#39;, () =&gt; {
     it(&#39;clicks it&#39;, () =&gt; {
        const spy = jest.spyOn(App, &#39;myClickFunc&#39;)
        const app = shallow(&lt;App /&gt;)
        const p = app.find(&#39;.App-intro&#39;)
        p.simulate(&#39;click&#39;)
        expect(spy).toHaveBeenCalled()
     })
    })

Anyone have an insight into what I&#39;m doing wrong?

Jest spyOn function called

What&#39;s the best way to correctly mock the following example?

The problem is that after import time, `foo` keeps the reference to the original unmocked `bar`.

`module.js`:

    export function bar () {
        return &#39;bar&#39;;
    }

    export function foo () {
        return `I am foo. bar is ${bar()}`;
    }



`module.test.js`:

    import * as module from &#39;../src/module&#39;;

    describe(&#39;module&#39;, () =&gt; {
        let barSpy;

        beforeEach(() =&gt; {
            barSpy = jest.spyOn(
                module,
                &#39;bar&#39;
            ).mockImplementation(jest.fn());
        });


        afterEach(() =&gt; {
            barSpy.mockRestore();
        });

        it(&#39;foo&#39;, () =&gt; {
            console.log(jest.isMockFunction(module.bar)); // outputs true

            module.bar.mockReturnValue(&#39;fake bar&#39;);

            console.log(module.bar()); // outputs &#39;fake bar&#39;;

            expect(module.foo()).toEqual(&#39;I am foo. bar is fake bar&#39;);
            /**
             * does not work! we get the following:
             *
             *  Expected value to equal:
             *    &quot;I am foo. bar is fake bar&quot;
             *  Received:
             *    &quot;I am foo. bar is bar&quot;
             */
        });
    });

I could change:

    export function foo () {
        return `I am foo. bar is ${bar()}`;
    }

to:

    export function foo () {
        return `I am foo. bar is ${exports.bar()}`;
    }

but this is pretty ugly in my opinion to do everywhere.

How to mock functions in the same module using Jest?

Jest documentation reads:

&gt; toBe just checks that a value is what you expect. It uses === to check strict equality.  
  
And for `toEqual`:
&gt; Use .toEqual when you want to check that two objects have the same value. This matcher recursively checks the equality of all fields, rather than checking for object identity—this is also known as &quot;deep equal&quot;. For example, toEqual and toBe behave differently in this test suite, so all the tests pass.

```javascript
const x = { a: { b: 3 } };
const y = { a: { b: 3 } };

expect(x).toEqual(y);
expect(x).toBe(y);
```

In this case, `toEqual` passes but `toBe` fails. I understand that `toEqual` passes because it does a _deep equal_ check. Why is `toBe` failing in this case?

Also, are there best practices for using `toBe` and `toEqual` (not just in Jest but in other testing frameworks, too)?

What is the difference between &#39;toBe&#39; and &#39;toEqual&#39; in Jest?

When running Karma to test my Angular4 application, I get this error  `Found the synthetic property @enterAnimation. Please include either &quot;BrowserAnimationsModule&quot; or &quot;NoopAnimationsModule&quot; in your application.` though I already imported the module in **app.module.ts**

            // animation module
            import { BrowserAnimationsModule } from &#39;@angular/platform-browser/animations&#39;; 
        ...
    @NgModule({
        imports: [...
            BrowserAnimationsModule,
            ...
          ],

and in my **component**:

   

     import { Component, OnInit } from &#39;@angular/core&#39;;
        import {
          trigger,
          state,
          style,
          animate,
          transition
        } from &#39;@angular/animations&#39;;
        
        @Component({
          selector: &#39;app-about&#39;,
          animations: [
            trigger(
              &#39;enterAnimation&#39;, [
                transition(&#39;:enter&#39;, [
                  style({ transform: &#39;translateX(100%)&#39;, opacity: 0 }),
                  animate(&#39;500ms&#39;, style({ transform: &#39;translateX(0)&#39;, opacity: 1 }))
                ]),
                transition(&#39;:leave&#39;, [
                  style({ transform: &#39;translateX(0)&#39;, opacity: 1 }),
                  animate(&#39;500ms&#39;, style({ transform: &#39;translateX(100%)&#39;, opacity: 0 }))
                ])
              ]
            ),
            trigger(
              &#39;enterAnimationVetically&#39;, [
                transition(&#39;:enter&#39;, [
                  style({ transform: &#39;translateY(100%)&#39;, opacity: 0 }),
                  animate(&#39;500ms&#39;, style({ transform: &#39;translateY(0)&#39;, opacity: 1 }))
                ]),
                transition(&#39;:leave&#39;, [
                  style({ transform: &#39;translateY(0)&#39;, opacity: 1 }),
                  animate(&#39;500ms&#39;, style({ transform: &#39;translateY(100%)&#39;, opacity: 0 }))
                ])]
            )
          ],
    ...

The application runs perfectly with `ng serve` yet, I got this error with karma.


Found the synthetic property @enterAnimation. Please include either &quot;BrowserAnimationsModule&quot; or &quot;NoopAnimationsModule&quot; in your application. Angular4

In React components importing assets
    (ex, import logo from &quot;../../../assets/img/logo.png)
gives such error 

&gt; ({&quot;Object.&lt;anonymous&gt;&quot;:function(module,exports,require,__dirname,__filename,global,jest){�PNG                                                                                   
    SyntaxError: Invalid or unexpected token
      at ScriptTransformer._transformAndBuildScript (node_modules/jest-runtime/build/script_transformer.js:305:17)

 my jest config is 

    &quot;jest&quot;: {
    &quot;testRegex&quot;: &quot;.*\\.spec\\.js$&quot;,
    &quot;moduleFileExtensions&quot;: [
      &quot;js&quot;,
      &quot;jsx&quot;,
      &quot;json&quot;
    ],
    &quot;moduleDirectories&quot;: [
      &quot;node_modules&quot;,
      &quot;src&quot;,
      &quot;assets&quot;
    ],
    &quot;moduleNameMapper&quot;: {
      &quot;\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$/&quot;: &quot;&lt;rootDir&gt;/__mocks__/fileMock.js&quot;,
      &quot;\\.(css|less|scss)$&quot;: &quot;&lt;rootDir&gt;/__mocks__/styleMock.js&quot;
    },
    &quot;verbose&quot;: true,
    &quot;bail&quot;: true
    }

what am i missing?

Importing images breaks jest test

I want to run just one test with Jest.

I use `it.only` or `describe.only`, but it still runs a whole lot of tests. I think it runs all the tests since my last commit, but it shouldn&#39;t have this behavior with the `only` flag explicitly set, right?

What causes this behavior and how can I run a single test?




Run only ONE test with Jest

I wonder if there is a better way to **disable console errors** *inside* a **specific Jest test** (i.e., **restore the original console** before/after each test).

Here is my current approach:

    describe(&quot;Some description&quot;, () =&gt; {
      let consoleSpy;
    
      beforeEach(() =&gt; {
        if (typeof consoleSpy === &quot;function&quot;) {
          consoleSpy.mockRestore();
        }
      });
    
      test(&quot;Some test that should not output errors to jest console&quot;, () =&gt; {
        expect.assertions(2);
    
        consoleSpy = jest.spyOn(console, &quot;error&quot;).mockImplementation();
     
        // some function that uses console error
        expect(someFunction).toBe(&quot;X&quot;);
        expect(consoleSpy).toHaveBeenCalled();
      });
    
      test(&quot;Test that has console available&quot;, () =&gt; {
        // shows up during jest watch test, just as intended
        console.error(&quot;test&quot;);
      });
    });

Is there a cleaner way of accomplishing the same thing? **I would like to avoid `spyOn`, but `mockRestore` only seems to work with it**.

Thanks!

Jest: Better way to disable console inside unit tests

I am new to jest and trying to figure out some basic stuff in my following code

    import * as actions from &#39;./IncrementalSearchActions&#39;;
    
    describe(&#39;Incremental Search Actions&#39;, () =&gt; {
        it(&#39;Should create an incremental search action&#39;)
    });

The questions/confusions I have around this are

 1. I get an error saying describe is not defined, how do I import the reuqired module?
 2. Is this supposed to be used with Karma/Jasmine?



Using jest in my react app, describe is not defined

I&#39;m trying to mock a call to a service but I&#39;m struggeling with the following message: **The module factory of `jest.mock()` is not allowed to reference any out-of-scope variables**.

I&#39;m using babel with ES6 syntax, jest and enzyme.


I have a simple component called `Vocabulary` which gets a list of `VocabularyEntry`-Objects from a `vocabularyService` and renders it. 

    import React from &#39;react&#39;;
    import vocabularyService from &#39;../services/vocabularyService&#39;;

    export default class Vocabulary extends React.Component {

    render() {
        
        let rows = vocabularyService.vocabulary.map((v, i) =&gt; &lt;tr key={i}&gt;
                &lt;td&gt;{v.src}&lt;/td&gt;
                &lt;td&gt;{v.target}&lt;/td&gt;
            &lt;/tr&gt;
        );
        // render rows
     }
     }

The `vocabularyServise` ist very simple:
     
      import {VocabularyEntry} from &#39;../model/VocabularyEntry&#39;;

      class VocabularyService {
    
      constructor() {
           this.vocabulary = [new VocabularyEntry(&quot;a&quot;, &quot;b&quot;)];
      }
    } 
    export default new VocabularyService();`


Now I want to mock the `vocabularyService` in a test:

    import {shallow} from &#39;enzyme&#39;;
    import React from &#39;react&#39;;
    import Vocabulary from &quot;../../../src/components/Vocabulary &quot;;
    import {VocabularyEntry} from &#39;../../../src/model/VocabularyEntry&#39;

    jest.mock(&#39;../../../src/services/vocabularyService&#39;, () =&gt; ({

    vocabulary: [new VocabularyEntry(&quot;a&quot;, &quot;a1&quot;)]

    }));

    describe(&quot;Vocabulary tests&quot;, () =&gt; {

    test(&quot;renders the vocabulary&quot;, () =&gt; {
        
        let $component = shallow(&lt;Vocabulary/&gt;);

        // expect something

    });
});

Running the test causes an error: Vocabulary.spec.js: babel-plugin-jest-hoist: The module factory of `jest.mock()` is not allowed to reference any out-of-scope variables.
Invalid variable access: VocabularyEntry.

As far as I unterstood, I cannot use the VocabularyEntry because it is not declares (as jest moves the mock definition to the top of the file).

Can anyone please explain how I can fix this? I saw solutions which required the references insinde the mock-call but I do not understand how I can do this with a class file.



Service mocked with Jest causes &quot;The module factory of jest.mock() is not allowed to reference any out-of-scope variables&quot; error

It will be the best explain in on example

`expected(someNumber).toBe(1).or.toBe(-2).or.toBe(22) // expect result is 1 or -2 or 22`

This is bad syntax, but can do sth like that in jest?

Content Type	Original Author	Original Content on Stackoverflow
Question	Yangshun Tay	View Question on Stackoverflow
Solution 1 - Testing	DarkCygnus	View Answer on Stackoverflow
Solution 2 - Testing	Ihor Bogdosarov	View Answer on Stackoverflow

Why does Jest --runInBand speed up tests?

Testing Problem Overview

Testing Solutions

Solution 1 - Testing

Solution 2 - Testing

guid/uuid in Typescript Node.js app

Espresso - TextView contains String

Attributions