Author: John Au-Yeung

Sometimes, we want to check if the time is between 7pm and 7am and do something with JavaScript.

In this article, we’ll look at how to check if the time is between 7pm and 7am and do something with JavaScript.

How to check if the time is between 7pm and 7am and do something with JavaScript?

To check if the time is between 7pm and 7am and do something with JavaScript, we can use the date’s getHours method to get the time.

For instance, we write:

const today = new Date().getHours();
if (today >= 7 && today <= 19) {
  document.body.style.background = "green";
} else {
  document.body.style.background = "blue";
}

to get the hour of the day with getHours.

Then we get if the time is between 7am and 7pm with today >= 7 && today <= 19.

And we set the background to green if it’s true.

Otherwise, we set the background to blue.

Conclusion

To check if the time is between 7pm and 7am and do something with JavaScript, we can use the date’s getHours method to get the time.

Sometimes, we want to not allow typing alphabetic characters in a number input with JavaScript.

In this article, we’ll look at how to not allow typing alphabetic characters in a number input with JavaScript.

How to not allow typing alphabetic characters in a number input with JavaScript?

To not allow typing alphabetic characters in a number input with JavaScript, we can listen for the keypress event and check which key is pressed when the event is triggered.

For instance, we write:

<input>

to add an input.

Then we write:

const input = document.querySelector('input')
input.onkeypress = (evt) => {
  const charCode = evt.keyCode;
  if (charCode != 46 && charCode > 31 &&
    (charCode < 48 || charCode > 57)) {
    evt.preventDefault()
  }
}

to select an input with querySelector.

Then we add a keypress event handler on the input by setting the input.onkeypress property to a function that checks which key is pressed.

If a number key or a period isn’t pressed, then we call evt.preventDefault to stop the character for the key from being appended to the input value.

Conclusion

To not allow typing alphabetic characters in a number input with JavaScript, we can listen for the keypress event and check which key is pressed when the event is triggered.

Sometimes, we want to find the index of the longest array in an array of arrays with JavaScript.

In this article, we’ll look at how to find the index of the longest array in an array of arrays with JavaScript.

How to find the index of the longest array in an array of arrays with JavaScript?

To find the index of the longest array in an array of arrays with JavaScript, we can use some array methods and Math.max.

For instance, we write:

const arr = [
  [1, 2, 3, 4, 5],
  [1, 2],
  [1, 1, 1, 1, 2, 2, 2, 2, 4, 4],
  [1, 2, 3, 4, 5]
];
const lengths = arr
  .map(a => a.length)
const index = lengths
  .indexOf(Math.max(...lengths));
console.log(index)

to get the lengths of each array in arr with:

const lengths = arr
  .map(a => a.length)

Then we call lengths.indexOf to return the index of the array with the longest length.

We get the longest length with Math.max(...lengths).

We spread the lengths entries as arguments for Math.max to find the longest length.

Therefore, index is 2 since the 3rd array is the longest.

Conclusion

To find the index of the longest array in an array of arrays with JavaScript, we can use some array methods and Math.max.

Sometimes, we want to check if a key exists in an object with Lodash and JavaScript.

In this article, we’ll look at how to check if a key exists in an object with Lodash and JavaScript.

How to check if a key exists in an object with Lodash and JavaScript?

To check if a key exists in an object with Lodash and JavaScript, we can use the has method.

For instance, we write:

const obj = {
  foo: 1,
  bar: 2
}
console.log(_.has(obj, 'foo'))
console.log(_.has(obj, 'baz'))

We call has with the obj we want to check if the key exists in and the key we’re searching for.

Therefore, the first console log logs true and the 2nd one logs false.

Conclusion

To check if a key exists in an object with Lodash and JavaScript, we can use the has method.

Sometimes, we want to make a promise return the resolved value instead of [object Promise] with JavaScript.

In this article, we’ll look at how to make a promise return the resolved value instead of [object Promise] with JavaScript.

How to make a promise return the resolved value instead of [object Promise] with JavaScript?

To make a promise return the resolved value instead of [object Promise] with JavaScript, we can use the await keyword.

For instance, we write:

(async () => {
  const d = await Promise.resolve(1)
  console.log(d);
})();

to call Promise.resolve with 1 to return a promise that resolves to 1.

Then we use the await keyword to get the resolved value and assign that to d.

Therefore, d is 1.

Conclusion

To make a promise return the resolved value instead of [object Promise] with JavaScript, we can use the await keyword.