React - How to use keys to avoid using getDerivedStateFromProps

Background

Until today I used key in react only when react dev threw warnings inside the console. Other than that I never cared about keys and never invested time to understand them. This is me being brutally honest to you all readers.

Problem

Have a component which display person name form based on different name and id passed to it. So using that form user can edit person details.

import React, { Component } from "react";
import ReactDOM from "react-dom";

class NameForm extends Component {
    constructor(props) {
        super(props);
        this.state = {
            userId: props.userId || 0,
            name: props.name || ""
        };
    }

    handleChange = event => {
        this.setState({ name: event.target.value });
    };

    handleSubmit = event => {
        console.log("A name was submitted: " + this.state.name);
        event.preventDefault();
    };

    render() {
        return (
            <form onSubmit={this.handleSubmit}>
                <label>
                    Name:
                    <input
                        type="text"
                        value={this.state.name}
                        onChange={this.handleChange}
                    />
                </label>
                <input type="submit" value="Submit" />
            </form>
        );
    }
}

class App extends Component {
    constructor(props) {
        super(props);
        this.state = {
            users: {
                1: "A",
                2: "B",
                3: "C"
            },
            editUserId: "new"
        };
    }

    onChange = event => {
        const value = event.target.value;

        this.setState(() => ({
            editUserId: value
        }));
    };

    render() {
        const { editUserId, users } = this.state;

        return (
            <div>
                <span>Select Edit userId: </span>
                <select onChange={this.onChange}>
                    <option value="new">New User</option>
                    {Object.entries(users).map(([userId, name]) => (
                        <option value={userId}>{name}</option>
                    ))}
                </select>
                <NameForm userId={editUserId} name={users[editUserId] || ""} />
            </div>
        );
    }
}

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

Observations

• Form name field displays no data since by default its set to new user.
• Now when you select user A from the select list, it still shows name as empty.
• Since react thinks its the same component so it will not call the constructor again.
• At this point you will be thinking that I need to change state of NameForm component based on prop changes.
  • This is called syncing state with props.
• And now you will be convinced that you need to use getDerivedStateFromProps static method because you need to change state based on prop change.
• This is where you went wrong.

Solution

Let me quote a line from React documentation:

If you want to “reset” some state when a prop changes, consider either making a component fully controlled or fully uncontrolled with a key instead.

Make NameForm component controlled by using key for React elements.

That's it.

import React, { Component } from "react";
import ReactDOM from "react-dom";

class NameForm extends Component {
    constructor(props) {
        super(props);
        this.state = {
            userId: props.userId || 0,
            name: props.name || ""
        };
    }

    handleChange = event => {
        this.setState({ name: event.target.value });
    };

    handleSubmit = event => {
        console.log("A name was submitted: " + this.state.name);
        event.preventDefault();
    };

    render() {
        return (
            <form onSubmit={this.handleSubmit}>
                <label>
                    Name:
                    <input
                        type="text"
                        value={this.state.name}
                        onChange={this.handleChange}
                    />
                </label>
                <input type="submit" value="Submit" />
            </form>
        );
    }
}

class App extends Component {
    constructor(props) {
        super(props);
        this.state = {
            users: {
                1: "A",
                2: "B",
                3: "C"
            },
            editUserId: "new"
        };
    }

    onChange = event => {
        const value = event.target.value;

        this.setState(() => ({
            editUserId: value
        }));
    };

    render() {
        const { editUserId, users } = this.state;

        return (
            <div>
                <span>Select Edit userId: </span>
                <select onChange={this.onChange}>
                    <option value="new">New User</option>
                    {Object.entries(users).map(([userId, name]) => (
                        <option value={userId}>{name}</option>
                    ))}
                </select>
                <NameForm key={editUserId} userId={editUserId} name={users[editUserId] || ""} />
            </div>
        );
    }
}

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

Observations

• We added key to the NameForm component.
  • This tells react to create a new instance of NameForm based on the key.
  • With that, component constructor will get called and the state is maintained.
• Now when you change values from the dropdown the form values will change.

Conclusion

• When you think about using getDerivedStateFromProps, take a step back and think through the problem and see if you can use key.
• That's why you will see people say that you should avoid or use getDerivedStateFromProps rarely.
• This was an eye opener for me when I actually used it. While reading docs I didn’t get the whole point.

Liquid error: internal

Comments

[batu]

Nice information!

I only use key for mapping, I’ve never thought it’s about instance. Now, enlightened