Category: JavaScript Answers

Sometimes, we want to find the index of an object in an array from a given object.

In this article, we’ll look at how to find the index of an object in an array from a given object.

Find the Index of an Object in an Array From a Given Object

To find the index of an object in an array from a given object, we can use the JavaScript array’s and Object.entries methods and also the JavaScript array’s every method.

JavaScript array’s entries method returns an array of arrayt with the index and entry of the array.

The Object.entries method returns an array of array of key-value pairs of an object.

And the JavaScript array’s every method returns whether every entry of an array matches a given condition.

For instance, we can write:

const indexOf = (arr, obj) => {
  for (const [index, aObj] of arr.entries()) {
    const allEqual = Object
      .entries(aObj)
      .every(([key, val]) => {
        return obj[key] === val
      })
    if (allEqual) {
      return index;
    }
  }
  return -1;
}

const obj = {
  x: 1,
  y: 2
};
const arr = [{
    x: 1,
    y: 2
  },
  {
    x: 3,
    y: 4
  }
]
const index = indexOf(arr, obj)
console.log(index)

We create the indexOf function that takes an arr array and obj object.

In it, we have a for-of loop to loop through each entry of the arr array.

Then we get the key-value pairs of each entry with the Object.entries method.

Next, we call every with the key and val key-value pair and check if they obj[key] is the same as the val value.

And we assign that value to allEqual .

If that’s true , then we return the index ,

If we loop through all the entries and found nothing, then we return -1.

Next, we call indexOf with arr and obj and get that index is 0.

Conclusion

To find the index of an object in an array from a given object, we can use the JavaScript array’s and Object.entries methods and also the JavaScript array’s every method.

JavaScript array’s entries method returns an array of arrayt with the index and entry of the array.

The Object.entries method returns an array of array of key-value pairs of an object.

And the JavaScript array’s every method returns whether every entry of an array matches a given condition.

Sometimes, we want to preload images using JavaScript.

In this article, we’ll look at how to preload images using JavaScript.

Preload Images Using JavaScript

To preload images using JavaScript, we can create Image instances for each image URL.

For instance, we can write:

const preload = (sources) => {
  const images = [];
  for (const s of sources) {
    const image = new Image();
    image.src = s
    images.push(image)
  }
}

preload([
  'https://picsum.photos/id/0/5616/3744',
  'https://picsum.photos/id/10/2500/1667'
])

to create the preload function that takes the sources array which has an array of image URLs.

In the function, we create the images array to store the Image instances.

Then we loop through the sources with the for-of loop.

In the loop body, we create a new Image instance.

And we set the src property of it to the source entry s .

Finally, we call images.push to add the entry to images .

Conclusion

To preload images using JavaScript, we can create Image instances for each image URL.

Sometimes, we want to listen for all events on an element with JavaScript.

In this article, we’ll look at how to listen for all events on an element with JavaScript.

Listen for All Events on an Element with JavaScript

To listen for all events on an element with JavaScript, we can loop through the keys of the window object to see which property starts with on .

Then we can take the part of the key after on as the event name and call addEventListener with it to listen to the event.

For instance, we can write:

Object.keys(window).forEach(key => {  
  if (/^on/.test(key)) {  
    window.addEventListener(key.slice(2), event => {  
      console.log(event);  
    });  
  }  
});

to get the keys from window with Object.keys .

Then we call forEach with a callback that takes the key .

And if the key starts with on , then we call window.addEventListener on it with the key without the on part, which we get with key.slice(2) .

The 2nd argument for window.addEventListener is the event handler callback.

Now when we do anything to the browser window, we’ll see the event object logged.

Conclusion

To listen for all events on an element with JavaScript, we can loop through the keys of the window object to see which property starts with on .

Then we can take the part of the key after on as the event name and call addEventListener with it to listen to the event.

Sometimes, we may run into the JavaScript .replaceAll() is not a function error in some browsers.

In this article, we’ll look at how to fix the JavaScript .replaceAll() is not a function error in some browsers.

Fix the JavaScript .replaceAll() is not a function Error

To fix the JavaScript .replaceAll() is not a function error in some browsers, we can add a polyfill to make the method available in all browsers.

For instance, we write:

if (typeof String.prototype.replaceAll == "undefined") {  
  String.prototype.replaceAll = function(match, replace) {  
    return this.replace(new RegExp(match, 'g'), () => replace);  
  }  
}

to add the code for the JavaScript string replaceAll method.

It adds an instance method that calls replace with a global regex to match all the substrings that matches the given regex.

Then the callback in the 2nd argument of replace is called to replace all substring matches with replace .

Most newer browsers should have this method built into the browser, so this is only needed for supporting older browsers.

Conclusion

To fix the JavaScript .replaceAll() is not a function error in some browsers, we can add a polyfill to make the method available in all browsers.

Sometimes, we may run into the JavaScript ‘document.querySelector(…) is null’ error when we try to select an element that’s loading in our JavaScript app.

In this article, we’ll look at how to fix the JavaScript ‘document.querySelector(…) is null’ error when we try to select an element that’s loading in our JavaScript app.

Fix the JavaScript ‘document.querySelector(…) is null’ Error

To fix the JavaScript ‘document.querySelector(…) is null’ error when we try to select an element that’s loading in our JavaScript app, we should make sure the element is present when we’re trying to select it.

To make sure this is the case, we can use document.querySelector in the DOMContentLoaded event listener.

For instance, if we have:

<input id="tags">

then we can write:

document.addEventListener("DOMContentLoaded", () => {  
  document.querySelector('input')  
    .addEventListener('change', (event) => {  
      //...  
    })  
});

to select the input in the DOMContentLoaded event handler.

We add the event handler with the document.addEventListener method to listen to the event emitted by the html element.

Then we make sure that the input element is already loaded by putting the document.querySelector call inside the DOMContentLoaded event handler.

Conclusion

To fix the JavaScript ‘document.querySelector(…) is null’ error when we try to select an element that’s loading in our JavaScript app, we should make sure the element is present when we’re trying to select it.

To make sure this is the case, we can use document.querySelector in the DOMContentLoaded event listener.