Category: JavaScript Answers

Sometimes, we want to check if a year is a leap year in JavaScript.

In this article, we’ll look at how to check if a year is a leap year in JavaScript.

Check if a Year is a Leap Year in JavaScript

To check if a year is a leap year in JavaScript, we can check if the year is evenly divisible by 4 and it isn’t evenly divisible by 100, or the year is evenly divisible by 400.

For instance, we can write:

const leapYear = (year) => {  
  return ((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0);  
}  
console.log(leapYear(1900))  
console.log(leapYear(2016))

to create the leapYear function that does the checks.

We check if year is evenly divisible by 4 and it isn’t evenly divisible by 100 with:

(year % 4 === 0) && (year % 100 !== 0)

And we check if year is evenly divisible by 400 with:

year % 400 === 0

We join both expressions together with the OR operator.

Therefore, the first console log should log false since it’s evenly divisble by 4 but not evenly divisble by 100, and it’s not evenly divisible by 400.

On the other hand, the 2nd console log should log true , since it’s evenly divisible by 4 and it’s not evenly divisible by 100.

Conclusion

To check if a year is a leap year in JavaScript, we can check if the year is evenly divisible by 4 and it isn’t evenly divisible by 100, or the year is evenly divisible by 400.

Sometimes, we want to delete duplicate elements from an array with JavaScript.

In this article, we’ll look at how to delete duplicate elements from an array with JavaScript.

Delete Duplicate Elements From an Array with JavaScript

To delete duplicate elements from an array with JavaScript, we can use the array filter method or the Set constructor with the spread operator.

For instance, we can use the array filter method by writing:

const arr = [1, 2, 2, 3, 4, 5, 5, 5, 6, 7, 7, 8, 9, 10, 10]
const unique = arr.filter((elem, index, self) => {
  return index === self.indexOf(elem);
})
console.log(unique)

We call filter with a callback that checks if index is the same as the index returned by indexOf called on the self array with elem .

If it’s not the first instance of elem , then they’ll be different.

self is the same as arr .

Therefore, duplicate instances of elem won’t be present in arr .

So unique is:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Likewise, we can use the Set constructor with the spread operator to remove the duplicates.

For instance, we can write:

const arr = [1, 2, 2, 3, 4, 5, 5, 5, 6, 7, 7, 8, 9, 10, 10]
const unique = [...new Set(arr)]
console.log(unique)

We pass arr into the Set constructor to convert arr to a set to remove the duplicate elements.

Then we spread that back into an array to convert the set back into an array.

And therefore, unique is the same result as before.

Conclusion

To delete duplicate elements from an array with JavaScript, we can use the array filter method or the Set constructor with the spread operator.

Sometimes, we want to automatically add properties to an object that is undefined with JavaScript.

In this article, we’ll look at how to automatically add properties to an object that is undefined with JavaScript.

Automatically Add Properties to an Object that is Undefined with JavaScript

To automatically add properties to an object that is undefined with JavaScript, we can use the hasOwnProperty method to check if a property exists.

It’ll return true if it exists and false otherwise.

If it returns false , then we can set the property value to the value we want.

For instance, we can write:

const test = {}  
if (!test.hasOwnProperty('hello')) {  
  test.hello = {};  
}  
test.hello.world = "Hello World!"

We have the test object that we want to add the hello property to if it doesn’t exist.

We check that it doesn’t exist with !test.hasOwnProperty(‘hello’) .

And if that’s true , we set test.hello to an empty object.

Then we set test.hello.world to “Hello World!” .

The hasOwnProperty method can be overridden easily since it’s inherited from the Object constructor.

Therefore to make sure we always call the right hasOwnProperty method, we can write:

const test = {}  
if (!Object.prototype.hasOwnProperty.call(test, 'hello')) {  
  test.hello = {};  
}  
test.hello.world = "Hello World!"

We call Object.prototype.hasOwnProperty.call with test to do the same thing as test.hasOwnProperty , but we make sure that we always call the right one from the Object constructor.

Conclusion

To automatically add properties to an object that is undefined with JavaScript, we can use the hasOwnProperty method to check if a property exists.

It’ll return true if it exists and false otherwise.

If it returns false , then we can set the property value to the value we want.

Sometimes, we want to tell setInterval to only fire x amount of times with JavaScript.

In this article, we’ll look at how to tell setInterval to only fire x amount of times with JavaScript.

Tell setInterval to Only Fire x Amount of Times with JavaScript

To tell setInterval to only fire x amount of times with JavaScript, we can keep track of the number of times the setInterval callback is called.

Then when the limit is reached, we can call clearInterval to stop the setInterval timer.

For instance, we can write:

let x = 0;  
const intervalID = setInterval(() => {  
  if (++x === 5) {  
    window.clearInterval(intervalID);  
  }  
  console.log('hello')  
}, 200);

to keep track of the number of times the setInterval callback is called with x .

Then we call setInterval with a callback that increments x with ++ in front to return the value of x after incrementing by 1.

Next, we check if x is 5 to top the timer with clearInterval when the setInterval callback has been called 5 times.

clearInterval is called with the intervalID to stop the timer.

And then we call console log to log some message.

200 is the number of milliseconds between each setInterval call.

As a result, the setInterval callback will be called 5 times only.

Conclusion

To tell setInterval to only fire x amount of times with JavaScript, we can keep track of the number of times the setInterval callback is called.

Then when the limit is reached, we can call clearInterval to stop the setInterval timer.

Sometimes, we want to disable submit button on form submission with JavaScript.

In this article, we’ll look at how to disable submit button on form submission with JavaScript.

Disable the Submit Button on Form Submission with JavaScript

To disable submit button on form submission with JavaScript, we can listen to the submit event of the form.

Then we can set the disabled attribute of the submit button to true when we submit the form.

For instance, if we have the following HTML form:

<form>
  <input>
  <input type='submit'>
</form>

Then we can write the following JavaScript to disable the submit button when we submit the form with jQuery by writing:

$('form').submit(function() {
  $(this).find(':input[type=submit]').prop('disabled', true);
});

We get the form with $(‘form’) .

Then we call submit on it with a callback to listen to the submit event of the form.

The callback we pass into submit get the submit button of the form with $(this).find(‘:input[type=submit]’) .

Then we call prop with 'disabled' and true to disable the form’s submit button.

Conclusion

To disable submit button on form submission with JavaScript, we can listen to the submit event of the form.

Then we can set the disabled attribute of the submit button to true when we submit the form.