I'm currently studying Dan Abramov's Redux tutorials on Egghead. At the part where he constructs the Redux Store from scratch, there is this code snippet (around 1:28 - ):
const dispatch = (action) => {
state = reducer(state, action);
listeners.forEach(listener => listener());
};
This piece of code goes through each element in the listener array, and while I grasp the need to update each listener, I am puzzled by what listener() is accomplishing. Where is this function originating from?
In the createStore method, there is a variable called listeners. This variable is an array of functions that will be executed whenever the store is updated.
To add your own function to the listeners, you can use the subscribe method of the store.
Here is an example:
const store = createStore(reducer);
store.subscribe(state => console.log(state)); // add function to listeners
store.dispatch(action);
The function with console.log will be triggered after the state has been changed.
The new syntax referred to as an arrow function is a feature of ECMAScript 6. In its absence, the code would appear as follows:
listeners.forEach(function(listener){
return listener();
});
Within this code snippet, the variable listeners represents an array of functions. The Array.prototype.forEach function is utilized to iterate over each element in the array by passing a function as an argument.
When invoking your function with the Array.prototype.forEach method, the current item from the iteration is passed into it. In this case, the item corresponds to a function and is executed immediately.
In essence, the purpose of this code is to loop through an array of functions and execute each one consecutively.
Understanding the documentation for the Array.prototype.forEach function can provide additional clarity. Here is the link to access the information:
Array.prototype.forEach
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach
To explore more about Arrow Functions, visit the following reference guide:
Arrow Functions
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions
When you call the Listen method on the store, it requires a handler as an argument. This handler is essentially a function that triggers the DOM to render:
store.listen(() => {
ReactDOM.render(
<div>{JSON.stringify(store.getState(), null, 2)}</div>,
document.getElementById('root')
);
});From my understanding, the listeners array contains the render functions of various React components that are linked to the Redux store.
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