Mosh React tutorial (part 2)

ref –

Handling Events

All those standard DOM events are available via the JSX.

We’ll use onClick and have it respond to our function handleIncrement.
However, when we execute the function, we notice that the this object is undefined.

Its because we need to bind the event handler.

But why is it that in an event handler function, the this object is undefined?

Depending on how a function is called, the ‘this’ changes.

The this reference will always return the calling object.

However, if the function is called as standalone, it will return window object.
If the standalone function is called within ‘strict mode’, it will return undefined.

And that is the reason, our event handler function does not have access to ‘this’.

Notice that its a handleIncrement REFERENCE. We didn’t execute the function yet. If the function was executed right away, then it will correctly call the function, and the function will be able to display this.

But for event handlers, since we pass the handleIncrement function reference, when it is used, it will be used in another function of a different environment. It will not be able have class Counter as its surrounding environment. It will be used in its own environment, namely, as a standalone function. Hence standalone functions (in a ‘strict mode’) will have undefined as the this object.

use ‘bind’ in the constructor

We implement a constructor function. Make sure you call super to initiate its parent class.
Then we bind the current this object to that function. So that when the reference of that function gets passed around, the this will always return to this object, and subsequently, the properties in this classCounter.

Use custom constructor to bind your event handlers to class Counter

Now, this.state.count will be valid.

use an arrow function

Updating the State

full code

What happens when State Changes

Let’s see what happens under the hood.

At this point, we call setState to tell React that the state of this component will change.

React will then schedule an async call to the render method. We don’t know when it happens, but it will happen in the future.

The render function will return a new React element. That element comes from the JSX code.

Hence, our virtual DOM is a div element, with span and button as its children.
We also have old virtual DOM. React will put them side by side and figure out what elements in the virtual DOM are modified.

It will notice that our span is modified because that’s where it has used formatCount().
So it will reach out to the real DOM, and update that span, in order to match what we have in the virtual DOM.

Go to the web page, Inspect Elements, and click on the ‘Increment’ button.
You’ll see that as you push the increment button, it calls

which updates the span. It updates the span because it depending on this.state.count it will update the class name to badge-primary or badge-info, and also update the count integer. Thus, in your Inspect Element, nothing else in the DOM is affected. Only that span element.

Passing Event Arguments