Why is Redux state undefined?

Why is Redux state undefined?

If the state passed to the reducer is undefined, you must explicitly return the initial state. The initial state may not be undefined. I added the logging statement and can verify that it returns an empty array. Even setting state to something like 1 will produce the same results.

What does mapStateToProps do in Redux?

The mapStateToProps function is used in the Redux pattern to reflect any updates to the Redux store and merge them into props in your component. The Redux store serves as a centralized place for the state to live in your application.

What is mapStateToProps and mapDispatchToProps in React Redux?

Overview​ The connect() function connects a React component to a Redux store. The mapStateToProps and mapDispatchToProps deals with your Redux store’s state and dispatch , respectively. state and dispatch will be supplied to your mapStateToProps or mapDispatchToProps functions as the first argument.

How many arguments can be passed in mapStateToProps ()?

Declared with two parameters const mapStateToProps = function(state, ownProps) { console.

Where is state defined in Redux?

In Redux, State means the same thing. The whole state of your app is stored in an object tree inside a single store. This means that you have a centralized placed with the whole state of your application. The only way to change the state tree is to emit an action, an object describing what happened.

What is initial state in Redux?

When Redux initializes it dispatches a “dummy” action to fill the state. So your counter reducer was called with state equal to undefined . This is exactly the case that “activates” the default argument. Therefore, state is now 0 as per the default state value ( state = 0 ). This state ( 0 ) will be returned.

Why do we need mapStateToProps?

As the first argument passed in to connect , mapStateToProps is used for selecting the part of the data from the store that the connected component needs. It’s frequently referred to as just mapState for short. It receives the entire store state, and should return an object of data this component needs.

Can we use mapStateToProps in functional component?

You can definitely use mapStateToProps with a functional component, the same way you would with a class component.

What do mapStateToProps and mapDispatchToProps actually do?

mapStateToProps is a function that you would use to provide the store data to your component, whereas mapDispatchToProps is something that you will use to provide the action creators as props to your component.

What is mapStateToProps in react?

What is ownProps in mapStateToProps?

The mapStateToProps(state, ownProps) is specified as the first argument of connect() call and its ownProps parameter receives the props object of the wrapper component. If the ownProps parameter is not present, React Redux skips calling the function at the props change.

How can the state stored in Redux be updated from a React component?

The only way to update a state inside a store is to dispatch an action and define a reducer function to perform tasks based on the given actions. Once dispatched, the action goes inside the reducer functions which performs the tasks and return the updated state to the store. This is what Redux is all about.

Is Redux state undefined in mapstatetoprops?

Redux state is undefined in mapStateToProps Ask Question Asked5 years ago Active9 months ago Viewed35k times 33 3 I am currently following thistutorial. I’ve hit a bit of a snag involving mapStateToPropsin the following code:

Why doesn’t mapstatetoprops work when two state values are identical?

If the two state values are identical by reference, then it will not re-run your mapStateToProps function, because it assumes that the rest of the store state hasn’t changed either. The Redux combineReducers utility function tries to optimize for this.

How can I improve the performance of mapstatetoprops?

If performance is a concern, ensure that these transformations are only run if the input values have changed. Much like a Redux reducer, a mapStateToProps function should always be 100\% pure and synchronous. It should only take state (and ownProps) as arguments, and return the data the component needs as props without mutating those arguments.

Should mapstatetoprops() function be used as an arrow function?

It does not matter if a mapStateToProps function is written using the function keyword ( function mapState (state) { } ) or as an arrow function ( const mapState = (state) => { } ) – it will work the same either way.