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How to Simulate a Keypress Event Programmatically with JavaScript?

Sometimes, we may want to simulate a keypress event programmatically with JavaScript.

In this article, we’ll look at how to trigger a keypress event programmatically with JavaScript.

dispatchEvent Method

We can call the dispatchEvent method to trigger an event of our choice.

For instance, we can write:

window.addEventListener('keydown', (e) => {
  console.log(e)
})

window.dispatchEvent(new KeyboardEvent('keydown', {
  'key': 'a'
}));

to listen to the keydown event and trigger it.

We call addEventListener with 'keydown' to listen to the keydown event.

Then to trigger the keydown event, we call window.dispatchEvent with a KeyboardEvent instance.

We pass in the type of keyboard event to trigger into the first argument of the constructor.

And we pass in an object with the options to set in the event object into the 2nd argument.

Therefore, after dispatchEvent is run, then e.key should be 'a' when logged in the callback.

We can set more options in the object.

For instance, we can write:

window.addEventListener('keydown', (e) => {
  console.log(e)
})

window.dispatchEvent(new KeyboardEvent('keydown', {
  key: "e",
  keyCode: 69,
  code: "KeyE",
  which: 69,
  shiftKey: false,
  ctrlKey: false,
  metaKey: false
}));

to set more options in the object.

keyCode is the numeric code of the key that we want to set.

code has the name of the key.

which has the keyboard key number.

shiftKey sets whether we want to press the shift key in addition to the key we’re pressing.

ctrlKey sets whether we want to press the Ctrl key in addition to the key we’re pressing.

metaKey sets whether we want to press the meta key in addition to the key we’re pressing.

The meta key is the Windows key on PC keyboards and the command key on Mac keyboards.

When the event is triggered, we should see all the options logged in the addEventListener callback.

They should be in the same properties in the e object as in the options object we pass into the KeyboardEvent constructor.

We can write the same code with the keyup event.

We just replace 'keydown' with 'keyup' everywhere.

Conclusion

To simulate keypress events with JavaScript, we can use the dispatch method with an event object.

We can use the KeyboardEvent constructor to create a keyboard event with various options.

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How to Check That a Number is NaN in JavaScript?

Sometimes, we may want to check that a number if NaN in JavaScript.

In this article, we’ll look at ways we can use to check that a number is NaN in JavaScript.

The isNaN Function

We can use the isNaN function to check whether a number is NaN .

For instance, we can write:

const nan = isNaN(parseFloat("abc"))  
console.log(nan)

isNaN will try to parse the argument we pass into it automatically to a number.

But to be safe, we can parse it ourselves with parseFloat first to make sure we get the expected result.

We parseFloat should return NaN since we passed in a non-numeric string into it.

So isNaN should return NaN .

Therefore, nan is true .

Check Whether a Value Equals Itself

Since NaN is the only value in JavaScript which doesn’t equal itself, we can check whether a variable equals itself to check if the variable’s value is NaN .

For instance, we can write:

const nan = NaN  
console.log(nan !== nan)

Then the console log will show true since NaN doesn’t equal itself.

If nan has any other value, the console log would log false .

So if we have:

const nan = true  
console.log(nan !== nan)

Then the console log would show false .

The Number.isNaN Method

The Number.isNaN method is another method that lets us check whether a value we pass into it is NaN .

The difference between isNaN and Number.isNaN is that Number.isNaN doesn’t try to convert a value to a number before it’s checked.

For instance, we can write:

const nan = Number.isNaN('abc');  
console.log(nan)

Then console log shows false since we didn’t pass in NaN .

We just passed in something that’s not a number.

But if we write:

const nan = Number.isNaN(NaN);  
console.log(nan)

Then nan is true .

Object.is

Another method we can use to check for NaN is the Object.is method.

Object.is takes 2 arguments with the values that we want to compare.

If both values are NaN , then it returns true instead of false like we have with === .

For instance, we can write:

const a = NaN  
const nan = Object.is(a, NaN);  
console.log(nan)

Then nan is true according to the console log.

Conclusion

We can check for NaN with various functions and methods provided by JavaScript.

Also, we can use the === operator to check if a value doesn’t equal itself.

If it doesn’t, then we know the value must be NaN since NaN is the only value that doesn’t equal itself in JavaScript.

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How to Get the Number of Days Between Two Dates in JavaScript?

We often have to get the number of days between 2 dates in our JavaScript apps.

In this article, we’ll look at how to get the number of days between 2 dates with JavaScript.

Using String and Date Methods

We can calculate the number of days between 2 dates with JavaScript by using native string and date methods.

For instance, we can write:

const parseDate = (str) => {
  const [month, day, year] = str.split('/');
  return new Date(year, month - 1, day);
}

const datediff = (first, second) => {
  return Math.round((second - first) / (1000 * 60 * 60 * 24));
}

const diff = datediff(parseDate("1/1/2000"), parseDate("1/1/2001"))
console.log(diff)

We have the parseDate function that takes str date string in MM/DD/YYYY format.

We parse it by splitting the date string with '/' as the separator.

Then we get the year , month and day by destructuring.

And we pass all that into the Date constructor to return a date object.

We’ve to subtract month by 1 to get the correct JavaScript month.

Then we calculate the date difference with the dateDiff method by subtracting the second by first .

When we subtract 2 dates, both dates will be converted to timestamps automatically before subtraction.

So we can subtract them directly.

And then we divide this by 1 day in milliseconds.

Finally, we round the division result with Math.round .

Now we can call all the functions we created to parse and get the date difference from the parsed dates.

And so we get diff is 366.

moment.js

We can use moment.js to get the difference between 2 dates easily.

For instance, we can write:

const start = moment("2000-11-03");
const end = moment("2001-11-04");
const diff = end.diff(start, "days")
console.log(diff)

We just pass the date strings into the moment function.

Then we call diff to get the difference between the moment date it’s called on and the moment date object we passed in.

The 2nd argument is the unit of the difference we want to return.

So diff is also 366 since the 2 dates differ by 366 days.

Conclusion

We can use native JavaScript string and date methods to compute the difference between 2 dates.

Also, we can use the moment.js library to make our lives easier.

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JavaScript Answers

How to Get a String in YYYYMMDD Format From a JavaScript Date Object?

Sometimes, we may want to convert a JavaScript date object to a string in YYYYMMDD format.

In this article, we’ll look at how to format a JavaScript date into a string in YYYYMMDD format.

Using Native JavaScript Date and String Methods

One way to get a JavaScript date into a string in YYYYMMDD format is to use native JavaScript date and string methods.

For instance, we can write:

const date = new Date(2021, 1, 1)
const year = date.getFullYear()
const month = ('0' + (date.getMonth() + 1)).substr(-2)
const day = ('0' + date.getDate()).substr(-2)
const dateStr = [year, month, day].join('')
console.log(dateStr)

We create a date object with the Date constructor with the year, month, and day.

The month’s value is from 0 to 11, where 0 is for January, 1 for February, etc.

Then we can get the year, month, and day from the date.

We call getFullYear to get the 4 digit year.

And we call getMonth to get the month plus 1 to get a human-readable month.

Then we attach string 0 before it and call substr -2 to get the last 2 characters of the string.

And we call getDate to get the date and format it the same way with substr .

Finally, we join the year , month , and day together with join .

Therefore, dateStr is '20210201' .

Date.prototype.toISOString

We can call toISOString to get the date string from a JavaScript date object.

Then we can use string methods to extract the year, month, and date parts and remove the dashes from that part.

For instance, we can write:

const date = new Date(2021, 1, 1)
const dateStr = date.toISOString().slice(0, 10).replace(/-/g, "");
console.log(dateStr)

We call toISOString to get the date string in ISO8601 format.

Then we call slice with 0 and 10 to extract the first part, which has the year, month, and day.

And then we call replace to replace all the dashes with empty strings to remove them.

Therefore, we get the same result for dateStr .

Also, we can replace slice with substring :

const date = new Date(2021, 1, 1)
const dateStr = date.toISOString().substring(0, 10).replace(/-/g, "");
console.log(dateStr)

moment.js

We can also use the moment.js library to format a date easily.

For instance, we can write:

const date = new Date(2021, 1, 1)
const dateStr = moment(date).format('YYYYMMDD');
console.log(dateStr)

We pass in our date to the moment function to create a moment object withn the date.

Then we call format with the format string to format the item.

And so we get the same value for dateStr .

Conclusion

We can format a native JavaScript date into a string with native JavaScript date and string methods.

Or we can use moment.js to format a date.

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How to Format a Floating Point Number in JavaScript?

Oftentimes, we have to format a floating-point number into the format we want in JavaScript.

In this article, we’ll look at how to format a floating-point number into the format we want with JavaScript.

Math.round

We can use Math.round to round a number into the number of decimal places we want by multiplying the original number 10 to the power of the number of decimal places we want to round to.

Then we pass that number into Math.round , and we divide the rounded number by the same number we multiplied the original number with.

For instance, we can write:

const original = 123.456
const result = Math.round(original * 100) / 100;
console.log(result)

We multiply original by 100, which is 10 to the power of 2.

So we round to 2 decimal places.

And we divide by 100.

Then result is 123.46.

This also works if we also round to other numbers of decimal places.

For instance, we can write:

const original = 123.45678
const result = Math.round(original * 1000) / 1000;
console.log(result)

And result is 123.457.

Number.prototype.toFixed

We can call the toFixed method to return a string with the number rounded to the given number of decimal places.

For instance, we can write:

const original = 123.45678
const result = original.toFixed(3)
console.log(result)

Then result is ‘123.457’ .

3 is the number of decimal places to round to.

result is a string instead of number in the previous example.

This is the easier way to format a number to the number of decimal places we want.

Conclusion

We can format a floating-point number into the number of decimal places we want with Math.round or the number’s toFixed method.