Category: JavaScript Answers

Sometimes, we want to programmatically trigger a change event on an input element with JavaScript.

In this article, we’ll look at how to programmatically trigger a change event on an input element with JavaScript.

Use the dispatchEvent Method

We can use the dispatchEvent method on an element to trigger an event programmatically.

For instance, if we have the following HTML:

<input>

We can write:

const element = document.querySelector('input');  
element.addEventListener('change', () => console.log('change'))  
const event = new Event('change');  
element.dispatchEvent(event);

to trigger the change event on an input.

We get the input with document.querySelector .

Then we add a change event listener with the addEventListener method.

Then we create a new event with the Event constructor.

'change' sets it to create the change event.

And then we call element.dispatchEvent to trigger the event we created.

Therefore, we should see 'change' logged from the change event listener.

Conclusion

We can trigger the change event programmatically on an input element with the Event constructor.

Then we can call the dispatchEvent method to dispatch the event.

Sometimes, we want to trigger a mouse click programmatically with JavaScript.

In this article, we’ll look at how to simulate a mouse click with JavaScript.

Use the MouseEvent Constructor

We can trigger a mouse click event by using the MouseEvent constructor.

For instance, we can write:

document.body.addEventListener('click', () => console.log('clicked'))

const evt = new MouseEvent("click", {
  view: window,
  bubbles: true,
  cancelable: true,
  clientX: 20,
});
document.body.dispatchEvent(evt);

We add a click event on document.body with the addEventListener method.

Then we create a MouseEvent instance with 'click' as the first argument to set the type of mouse event to click.

The 2nd argument is an object that sets some properties for the mouse event.

view is set to window so that we trigger the click event on an element within window .

bubbles is set to true to make it propagate from child to parent.

clientX is the x-coordinate of the mouse click.

Then we call document.body.dispatchEvent to trigger the click event on the body element.

Now when dispatch is run, we should see console log log 'clicked' .

Conclusion

We can simulate a mouse click with JavaScript by creating a MouseEvent instance.

Then we call dispatchEvent on the element that we want to trigger the click from to click on the element programmatically.

Sometimes, we want to add getters and setters dynamically to a JavaScript object.

In this article, we’ll look at how to implement dynamically getters and setters with JavaScript.

Use a Proxy

JavaScript comes with the Proxy constructor to let us create an object that is a facade on another object.

For instance, we can add a dynamic getter to it by writing:

const original = {
  "foo": "bar"
};
const proxy = new Proxy(original, {
  get(target, name, receiver) {
    let rv = Reflect.get(target, name, receiver);
    if (typeof rv === "string") {
      rv = rv.toUpperCase();
    }
    return rv;
  }
});
console.log(original.foo)
console.log(proxy.foo);

We create the original object with the foo property.

Then we use the Proxy constructor to create a proxy with the original object.

And we pass it an object with the get method to get the property we want to modify the return value for with the Reflect.get method.

Ir returns the value from the original object with the property name set to name .

And we return rv as upper case if the rv return value is a string.

Therefore, the first console log logs 'bar' , but the second one logs 'BAR' .

We can add a setter by adding the set method into the object we pass in as the 2nd argument of the Proxy constructor.

For instance, we can write:

const original = {
  "foo": "bar"
};
const proxy = new Proxy(original, {
  get(target, name, receiver) {
    let rv = Reflect.get(target, name, receiver);
    if (typeof rv === "string") {
      rv = rv.toUpperCase();
    }
    return rv;
  },
  set(target, name, value, receiver) {
    if (!Reflect.has(target, name)) {
      console.log(`Setting non-existent property '${name}', initial value: ${value}`);
    }
    return Reflect.set(target, name, value, receiver);
  }
});
proxy.baz = 'abc'
console.log(original.foo)
console.log(proxy.foo);

We add the set method that lets us modify how the properties of the original object is modified.

We call the Reflect.has method to check if the original object has the property name .

If it doesn’t, we log a message.

Otherwise, we call Reflect.set to set the name property to the given value and return the result.

Therefore, when we try to set the proxy.baz property, we get:

'Setting non-existent property ‘baz’, initial value: abc’

And the rest of the console logs are the same as before.

Conclusion

We can use JavaScript proxies to modify getters and setters the way we want.

Sometimes, we want to format dates and subtract the days with moment.js.

In this article, we’ll look at how to subtract days from a date and format the result with moment.js.

Use the subtract and format Methods

We can use the moment.js’ subtract method to subtract days.

And then we can use the format method to format the resulting date.

For instance, we can write:

const startdate = moment('2020-01-01');  
const result = startdate  
  .subtract(1, "days")  
  .format("DD-MM-YYYY");  
console.log(result)

We defined the startdate variable and assign the moment object with the date we want to subtract to it.

Then we call subtract to subtract 1 day from startdate by calling it with 1 and 'days' .

And then we call format to format the date with a formatting string.

Therefore, result is '31–12–2019' .

Conclusion

We can call moment.js’ subtract method to subtract a day from a date.

And then we can call format to format the date into the format we want.

Sometimes, we want to trim a specific character from a string with JavaScript.

In this article, we’ll look at how to trim a specific character from a string with JavaScript.

Use the String.prototype.replace Method

We can use the JavaScript string’s replace method to remove all instances of a given character.

To use it, we write:

const x = '|f|oo||';
const y = x.replace(/^|+||+$/g, '');
console.log(y);

We have the string x that we want to remove the || substrings from,

Then we call x.replace with a regex that matches the || substring in x to replace them.

The g flag matches all instances of a given pattern.

The 2nd argument we pass into replace is an empty string so we remove all instances of the substring that matches the pattern in the first argument.

Therefore, y is 'f|oo’ .

Use the String.prototype.replaceAll Method

We can do the same thing with the JavaScript string replaceAll method that comes with ES2021.

For instance, we can write:

const x = '|f|oo||';
const y = x.replaceAll(/^|+||+$/g, '');
console.log(y);

to replace replace with replaceAll .

The regex we pass in must have the g flag or we’ll get an error since it can only replace all substrings that match the given pattern.

And so we get the same result as the previous example.

Conclusion

We can use the JavaScript string replace or replaceAll method to remove all substrings that match the given pattern.