Javascript Events

One of the ways to run Javascript code is in response to actions taken by the user on the page. Javascript supports a number of event handlers that are triggered by user interaction. Examples are the click event handler which can be added to any element to trigger a fragment of code to run when the user clicks on that element.

The easiest way to add a handler for an event is to use a HTML attribute inline in the page. The attributes for each event are prefixed by the word 'on', so to set a click event handler I would add an onClick attribute to an element:

<img src='foo.jpg' onClick="alert('this is a test');">

In this case the fragment of javascript code would run if the user clicked on this image. Any javascript can be included in an attribute value but it is best to keep this brief and just call a handler function that is defined in a script block elsewhere on the page.

Another way to add handlers is via the addEventListener method for a DOM element. This has the advantage that it can be done from within a code block running elsewhere in the page and that it can add multiple handlers for a single event to an element. Here's an example from the Mozilla Developer Network on using this method:

// Function to change the content of t2
function modifyText(new_text) {
  let t2 = document.getElementById("t2");
  t2.firstChild.nodeValue = new_text;    
}
// Function to add event listener to table
let el = document.getElementById("outside");
el.addEventListener("click", function(){modifyText("four")}, false);

The sample code defines a handler function modifyText which is then attached to the element with id "outside". Note that the second argument given to addEventListener is an anonymous function that calls the handler with a given argument and returns false.

There's a good list of the different events and the browsers that support them on this MDN page.

this in Event Listeners

The variable this in Javascript is used in object methods to refer to the object instance that the method was called on. It behaves like the self argument used in Python but it is defined implicitly rather than having to be included as a parameter. We can also use this in the context of an event handler because of the way that events are managed in the DOM.
In an event handler this refers to the element that captured or triggered the event that the handler is responding to. Generally this is the element that the event listener has been added to. Here's an example:

let el = document.getElementById("outside")
el.addEventListener("click", function(){
    this.css("color", "red")
});

In the click handler defined here as an anonymous function, this will refer to whatever element received the click event. As the handler was bound to the element with id outside, then this will refer to that element.

One note of caution here. You may have seen the alternate form of functions (arrow functions) mentioned in the Javacript chapter used in some examples. This is a more compact way of writing functions and is often useful in callback functions like this. However, one significant difference between these and regular functions is that arrow functions do not get a special this variable. There's a reason for this which we'll discuss below, but it means you need to be aware of this when you write a callback. So the above example would not work as an arrow function:

let el = document.getElementById("outside")
el.addEventListener("click", () => { this.css("color", "red") });   // bad - there is no this variable

If you try this, you'll get an error message saying this.css is not a function.

The reason for this behaviour is because of a common pattern of use where we create an eventhandler within another eventhandler (there are also other places where this is useful). Here's an example. We want to create an event handler for the button primary which, as part of it's action, creates a new event handler on another element secondary. The job of this new event handler is to change the color of both the primary and secondary elements. Here's the example:

function clickAction() {

  // here `this` refers to the element that was clicked
  // what ee do is add a new event handler onto another
  // element
  this.css('color', 'green');

  let sec = document.getElementById("secondary");
  sec.css('color', 'green');

  sec.addEventListener("click", function() {
    sec.css('color', 'red')
    this.css('color', 'red');
  });
}

let prim = document.getElementById("primary");
prim.addEventListener("click", clickAction);

Inside the main handler (clickAction) the value of this will be the primary element that was clicked to trigger the action. We change the color of the element, then find another element with id secondary and change it's color to green too. We then add a new event listener to the secondary element so that when it is clicked, the color of both elements are changed to red. However, the value of this inside the new event handler will refer to the secondary element, not the primary one. Remember, this function won't be run until later when the secondary button is clicked. So at that point, there's no way to find the primary element - the event handler won't work.

Instead, we use an arrow function for the second event handler:

function clickAction() {

  // here `this` refers to the element that was clicked
  // what ee do is add a new event handler onto another
  // element
  this.css('color', 'green');

  let sec = document.getElementById("secondary");
  sec.css('color', 'green');

  sec.addEventListener("click", () => {
    sec.css('color', 'red')
    this.css('color', 'red');
  });
}

Since we've used an arrow function there is no new this created when it runs. This means that Javascript needs to find a value for this by looking outside the function. Here we're creating what is called a closure

  • the function we create here is closed over the values of the variables that exist at the time it was created. So, the value of this is the value it had inside clickAction when the arrow function was defined.

Closures are a useful tool in building event handlers in Javascript. They can be confusing if you are new to this idea. Understanding it involves thinking about the time that the function is created and the time it is executed.

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