Author: John Au-Yeung

Sometimes, we want to mock console when it is used by a third-party-library with Jest and JavaScript.

In this article, we’ll look at how to mock console when it is used by a third-party-library with Jest and JavaScript.

How to mock console when it is used by a third-party-library with Jest and JavaScript?

To mock console when it is used by a third-party-library with Jest and JavaScript, we call the jest.spyOn method.

For instance, we write

const consoleWarnMock = jest.spyOn(console, 'warn').mockImplementation();

to mock the console.warn method with the spyOn method in our test.

And then we call mockImplementation to return a mocked version of console.warn.

When we’re done with the test, then we call

consoleWarnMock.mockRestore();

to restore the original console.warn method.

Conclusion

To mock console when it is used by a third-party-library with Jest and JavaScript, we call the jest.spyOn method.

Sometimes, we want to check if an element is a child of a parent with JavaScript.

In this article, we’ll look at how to check if an element is a child of a parent with JavaScript.

How to check if an element is a child of a parent with JavaScript?

To check if an element is a child of a parent with JavaScript, we can use the parent element’s contains method.

For instance, we write

const contains = (parent, child) => {
  return parent !== child && parent.contains(child);
};

to create the contains function that checks if parent isn’t child and that the parent element contains the child element with parent.contains.

Conclusion

To check if an element is a child of a parent with JavaScript, we can use the parent element’s contains method.

Sometimes, we want to fix HTML5 canvas ctx.fillText not adding line breaks with JavaScript.

In this article, we’ll look at how to fix HTML5 canvas ctx.fillText not adding line breaks with JavaScript.

How to fix HTML5 canvas ctx.fillText not adding line breaks with JavaScript?

To fix HTML5 canvas ctx.fillText not adding line breaks with JavaScript, we set the location of the text when we call fillText.

For instance, we write

const c = document.getElementById("c").getContext("2d");
c.font = "11px Courier";
console.log(c);
const txt = "line 1\nline 2\nthird line..";
const x = 30;
const y = 30;
const lineHeight = 15;
const lines = txt.split("\n");

for (const line of lines) {
  c.fillText(line, x, y + i * lineHeight);
}

to loop through the lines with a for-of loop.

In it, we call the context’s fillText method with the line to render.

Then y coordinates of the text is set to y + i * lineHeight which positions the text below the previous line.

Conclusion

To fix HTML5 canvas ctx.fillText not adding line breaks with JavaScript, we set the location of the text when we call fillText.

Sometimes, we want to get HTML5 video element dimensions with JavaScript.

In this article, we’ll look at how to get HTML5 video element dimensions with JavaScript.

How to get HTML5 video element dimensions with JavaScript?

To get HTML5 video element dimensions with JavaScript, we can use the videoHeight and videoWidth properties.

For instance, we write

<video id="foo" src="foo.mp4"></video>

to add a video element.

Then we write

const vid = document.getElementById("foo");
const height = vid.videoHeight;
const width = vid.videoWidth;

to select the video element with getElementById.

And then we get the intrinsic video height and width with videoHeight and videoWidth.

Conclusion

To get HTML5 video element dimensions with JavaScript, we can use the videoHeight and videoWidth properties.

Sometimes, we want to fix Bootstrap 4 drop down menu is not working with JavaScript.

In this article, we’ll look at how to fix Bootstrap 4 drop down menu is not working with JavaScript.

How to fix Bootstrap 4 drop down menu is not working with JavaScript?

To fix Bootstrap 4 drop down menu is not working with JavaScript, we add popper.js before loading Bootstrap.

For instance, we write

<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.11.0/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/js/bootstrap.min.js"></script>
<link
  rel="stylesheet"
  href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css"
/>

to add the popper.js script tag before the Bootstrap 4 script tags.

Conclusion

To fix Bootstrap 4 drop down menu is not working with JavaScript, we add popper.js before loading Bootstrap.