Author: John Au-Yeung

Sometimes, we want to detect if JavaScript object is a FormData instance.

In this article, we’ll look at how to detect if JavaScript object is a FormData instance.

How to detect if JavaScript object is a FormData instance?

To detect if JavaScript object is a FormData instance, we can use the instanceof operator.

For instance, we write:

const formData = new FormData()
console.log(formData instanceof FormData)

to create a new FormData instance and assign it to formData.

Next, we check if formData is an instance of FormData with the instanceof operator.

The console log should log true since formData is a FormData instance.

Conclusion

To detect if JavaScript object is a FormData instance, we can use the instanceof operator.

Sometimes, we want to use Promise.race() with JavaScript.

In this article, we’ll look at how to use Promise.race() with JavaScript.

How to use Promise.race() with JavaScript?

To use Promise.race() with JavaScript, we can call it with an array of promises.

For instance, we write:

const p1 = new Promise((resolve, reject) => {
  setTimeout(resolve, 500, 'one');
});
const p2 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'two');
});

(async () => {
  const value = await Promise.race([p1, p2])
  console.log(value);
})()

We call Promise.race with [p1, p2] which will return the promise that is settled first.

Therefore, value should be 'two' since the setTimeout callback is run in 100 seconds after the callback is queued for running.

Conclusion

To use Promise.race() with JavaScript, we can call it with an array of promises.

Sometimes, we want to reset a GIF animation with JavaScript.

In this article, we’ll look at how to reset a GIF animation with JavaScript.

How to reset a GIF animation with JavaScript?

To reset a GIF animation with JavaScript, we can add a random query string to the end of the image URL.

For instance, we write:

<img src='https://media4.giphy.com/avatars/JulietGlock/S2TkRZ9GyF91.GIF' style='height: 100px'>

to add an animate GIF img element.

Then we write:

const img = document.querySelector('img')
setTimeout(() => {
  img.src = `${img.src.replace(/\?.*$/,"")}?x=${Math.random()}`;
}, 2000)

to select the img element with querySelector.

Then we set the img.src property to the img.src plus a new query string that had the x parameter set to a random number.

We have that in the setTimeout callback so src will be changed with a delay.

The animated GIF will start from the beginning when src is changed.

Conclusion

To reset a GIF animation with JavaScript, we can add a random query string to the end of the image URL.

Sometimes, we want to get a list of duplicate objects in an array of objects with JavaScript.

In this article, we’ll look at how to get a list of duplicate objects in an array of objects with JavaScript.

How to get a list of duplicate objects in an array of objects with JavaScript?

To get a list of duplicate objects in an array of objects with JavaScript, we can use the JavaScript array’s reduce method.

For instance, we write:

const values = [{
  id: 10,
  name: 'someName1'
}, {
  id: 10,
  name: 'someName2'
}, {
  id: 11,
  name: 'someName3'
}, {
  id: 12,
  name: 'someName4'
}];

const lookup = values.reduce((a, e) => {
  if (a[e.id]) {
    return {
      ...a,
      [e.id]: a[e.id] + 1
    }
  }
  return {
    ...a,
    [e.id]: 1
  }
}, {});

const dups = values.filter(e => lookup[e.id] > 1)
console.log(dups);

We call values.reduce with a callback that returns an object with the counts of the object give in values given the value of e.id.

If a[e.id] is defined, then we increment it by 1.

Otherwise, we set [e.id] to 1.

Then we get the id‘s of the objects in values with value bigger than 1.

As a result, dups is:

[
  {
    "id": 10,
    "name": "someName1"
  },
  {
    "id": 10,
    "name": "someName2"
  }
]

Conclusion

To get a list of duplicate objects in an array of objects with JavaScript, we can use the JavaScript array’s reduce method.

Sometimes, we want to insert or remove HTML content between div tags with JavaScript.

In this article, we’ll look at how to insert or remove HTML content between div tags with JavaScript.

How to insert or remove HTML content between div tags with JavaScript?

To insert or remove HTML content between div tags with JavaScript, we can set the innerHTML property of the element.

For instance, we write:

<div>
  hello world
</div>

to add a div.

Then we write:

const div = document.querySelector('div')
div.innerHTML = '<span>Something</span>';

to select the div with querySelector.

Then we set div.innerHTML to a string with a span to replace ‘hello world’ with the span.

Therefore, we should see ‘Something’ instead of ‘hello world’ displayed.

Conclusion

To insert or remove HTML content between div tags with JavaScript, we can set the innerHTML property of the element.