Testing assignments to function scope variables within a void function: Best practices

Question

Testing assignments to function scope variables within a void function: Best practices

I have a function that needs to be tested for code coverage. The function, called downloadPdfDataContent, creates a link element and appends it to the document body in order to trigger a file download. After downloading, the link is removed from the DOM using URL.revokeObjectURL.

const downloadPdfDataContent = (title: string, url: string): void => {
  const link = document.createElement('a');
  link.target = title;
  link.href = url;
  link.download = title;
  document.body.appendChild(link);
  link.click();
  document.body.removeChild(link);
  URL.revokeObjectURL(url);
};

How can I test the assignment of variables and interactions with the document object within this function scope? It seems like using spyOn could be a solution, but I'm not sure how to go about it given the locally scoped variable. One thought was to return the variable, but if possible, I'd like to avoid that approach.

angular typescript unit-testing jestjs jasmine

Answer 1

Answer №1

To easily mock DOM manipulation, you can utilize jest.spyOn().

For example:

app.js:

export const handleButtonClick = (elementId) => {
  const button = document.getElementById(elementId);
  button.click();
};

app.test.js:

import { handleButtonClick } from '.';

describe('93848293', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  it('should trigger click event', () => {
    const mButton = ({
      click: jest.fn(),
    } as unknown) as HTMLButtonElement;
    const getElementByIdSpy = jest.spyOn(document, 'getElementById').mockReturnValueOnce(mButton);
    handleButtonClick('myButton');
    expect(getElementByIdSpy).toBeCalledWith('myButton');
    expect(mButton.click).toBeCalledTimes(1);
  });
});

test result:

 PASS  examples/93848293/app.test.js (5.673 s)
  93848293
    ✓ should trigger click event (3 ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |                   
 app.js   |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        6.987 s

Answer 2

To easily mock DOM manipulation, you can utilize jest.spyOn().

For example:

app.js:

export const handleButtonClick = (elementId) => {
  const button = document.getElementById(elementId);
  button.click();
};

app.test.js:

import { handleButtonClick } from '.';

describe('93848293', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  it('should trigger click event', () => {
    const mButton = ({
      click: jest.fn(),
    } as unknown) as HTMLButtonElement;
    const getElementByIdSpy = jest.spyOn(document, 'getElementById').mockReturnValueOnce(mButton);
    handleButtonClick('myButton');
    expect(getElementByIdSpy).toBeCalledWith('myButton');
    expect(mButton.click).toBeCalledTimes(1);
  });
});

test result:

 PASS  examples/93848293/app.test.js (5.673 s)
  93848293
    ✓ should trigger click event (3 ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |                   
 app.js   |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        6.987 s

Answer 3

Answer №2

To create a reliable test, consider setting up an onclick handler for the main document itself during your testing process. By adding unique identifiers to your link elements, such as a specific ID, you can verify that a click event occurred on the intended target. It's also important to validate that the URL associated with the link matches the expected values.

In a unit test scenario, capturing any click event would be sufficient for evaluation purposes. When running tests in a browser environment and aiming to prevent actual page navigation, utilize the preventDefault method in your event handler to block any unintentional redirects from occurring.

Answer 4

To create a reliable test, consider setting up an onclick handler for the main document itself during your testing process. By adding unique identifiers to your link elements, such as a specific ID, you can verify that a click event occurred on the intended target. It's also important to validate that the URL associated with the link matches the expected values.

In a unit test scenario, capturing any click event would be sufficient for evaluation purposes. When running tests in a browser environment and aiming to prevent actual page navigation, utilize the preventDefault method in your event handler to block any unintentional redirects from occurring.

Testing assignments to function scope variables within a void function: Best practices

Answer №1

Answer №2

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