Category: JavaScript Answers

Sometimes, we want to change image opacity using JavaScript.

In this article, we’ll look at how to change image opacity using JavaScript.

Change Image Opacity Using JavaScript

To change image opacity using JavaScript, we can set the style.opacity and style.filter properties of the img element to the value we want.

For instance, if we have:

<img src='https://picsum.photos/200/300'>

Then we write:

const element = document.querySelector('img');
element.style.opacity = "0.5";
element.style.filter = 'alpha(opacity=90)';

to select the img element with document.querySelector.

Then we set the style.opacity property of element to 0.5 to set the CSS opacity property value.

Likewise, we set the style.filter property of element to 'alpha(opacity=90)' to set the CSS filter property value.

Now we should see the opacity and filter values applied to the image.

Conclusion

To change image opacity using JavaScript, we can set the style.opacity and style.filter properties of the img element to the value we want.

Sometimes, we want to get a list of registered custom elements with JavaScript

In this article, we’ll look at how to get a list of registered custom elements with JavaScript

Get a List of Registered Custom Elements with JavaScript

To get a list of registered custom elements with JavaScript, we can use the global customElements.get method.

For instance, we can write:

class PopUpInfo extends HTMLElement {
  constructor() {
    super();
    const shadow = this.attachShadow({
      mode: 'open'
    });

    const wrapper = document.createElement('span');
    const info = document.createElement('span');
    info.innerText = 'foo'
    shadow.appendChild(wrapper);
    wrapper.appendChild(info);
  }
}

customElements.define('popup-info', PopUpInfo);
console.log(customElements.get('popup-info'))

to register the popup-info custom element.

Then we use the customElements.get method with the element tag name to check if it has been registered.

We created the PopUpInfo class that creates some span elements and attach them to the shadow DOM with appendChild.

shadow is the shadow DOM root, which is created by the this.attachShadow method.

We then call customElements.define with the tag name and custom element class to register the element.

Therefore, the console log should log the custom element class that we just created.

Conclusion

To get a list of registered custom elements with JavaScript, we can use the global customElements.get method.

Sometimes, we want to get an element by a custom attribute using JavaScript.

In this article, we’ll look at how to get an element by a custom attribute using JavaScript.

Get an Element by a Custom Attribute Using JavaScript

To get an element by a custom attribute using JavaScript, we can use the document.querySelector method with a select string with the tag and attribute of the element we’re looking for.

For instance, we can write:

<div data-automation="something">
</div>

to add a div.

Then we write:

const div = document.querySelector("div[data-automation='something']")
console.log(div)

to select the div with document.querySelector.

We just put data-automation='something' in the square brackets to select the element with attribute data-automation with value something.

Conclusion

To get an element by a custom attribute using JavaScript, we can use the document.querySelector method with a select string with the tag and attribute of the element we’re looking for.

Sometimes, we may run into the ‘Warning: unreachable code after return statement’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘Warning: unreachable code after return statement’ when we’re developing JavaScript apps.

Fix the ‘Warning: unreachable code after return statement’ When Developing JavaScript Apps

To fix the ‘Warning: unreachable code after return statement’ when we’re developing JavaScript apps, we should make sure we don’t have any code below the return statement of a function.

For instance, if we have:

function f() {
  let x = 3;
  x += 4;
  return x;  
  x -= 3;    
}

function f() {
  return     
    3 + 4;   
}

then we’ll get the warning.

In the first function, we have x -= 3; below return x which is unreachable since it’s after the return statement.

Likewise, 3 + 4 is below return so that’s also unreachable.

To fix this, we write:

function f() {
  let x = 3;
  x += 4;
  x -= 3;
  return x;  
}

function f() {
  return 3 + 4
}

Now the return statement are both on the last line.

Conclusion

To fix the ‘Warning: unreachable code after return statement’ when we’re developing JavaScript apps, we should make sure we don’t have any code below the return statement of a function.

Sometimes, we may run into the ‘TypeError: invalid assignment to const "x"’ when we’re developing JavaScript apps.

In this article, we’ll look at how to fix the ‘TypeError: invalid assignment to const "x"’ when we’re developing JavaScript apps.

Fix the ‘TypeError: invalid assignment to const "x"’ When Developing JavaScript Apps

To fix the ‘TypeError: invalid assignment to const "x"’ when we’re developing JavaScript apps, we should make sure we aren’t assigning a variable declared with const to a new value.

On Firefox, the error message for this error is TypeError: invalid assignment to const "x".

On Chrome, the error message for this error is TypeError: Assignment to constant variable.

And on Edge, the error message for this error is TypeError: Assignment to const.

For example, the following code will throw this error:

const COLUMNS = 80;

// ...

COLUMNS = 100;

We tried to assign 100 to COLUMNS which is declared with const, so we’ll get this error.

To fix this, we write:

const COLUMNS = 80;
const WIDER_COLUMNS = 100;

to declare 2 variables, or we can use let to declare a variable that we can reassign a value to:

let COLUMNS = 80;

// ...

COLUMNS = 100;

Conclusion

To fix the ‘TypeError: invalid assignment to const "x"’ when we’re developing JavaScript apps, we should make sure we aren’t assigning a variable declared with const to a new value.

On Firefox, the error message for this error is TypeError: invalid assignment to const "x".

On Chrome, the error message for this error is TypeError: Assignment to constant variable.

And on Edge, the error message for this error is TypeError: Assignment to const.