Props in getInitialState Is an Anti-Pattern

Note:

This isn't really a React-specific tip, as such anti-patterns often occur in code in general; in this case, React simply points them out more clearly.

Using props, passed down from parent, to generate state in getInitialState often leads to duplication of "source of truth", i.e. where the real data is. Whenever possible, compute values on-the-fly to ensure that they don't get out of sync later on and cause maintenance trouble.

Bad example:

var MessageBox = React.createClass({
  getInitialState: function() {
    return {nameWithQualifier: 'Mr. ' + this.props.name};
  },

  render: function() {
    return <div>{this.state.nameWithQualifier}</div>;
  }
});

ReactDOM.render(<MessageBox name="Rogers"/>, mountNode);

Better:

var MessageBox = React.createClass({
  render: function() {
    return <div>{'Mr. ' + this.props.name}</div>;
  }
});

ReactDOM.render(<MessageBox name="Rogers"/>, mountNode);

(For more complex logic, simply isolate the computation in a method.)

However, it's not an anti-pattern if you make it clear that synchronization's not the goal here:

var Counter = React.createClass({
  getInitialState: function() {
    // naming it initialX clearly indicates that the only purpose
    // of the passed down prop is to initialize something internally
    return {count: this.props.initialCount};
  },

  handleClick: function() {
    this.setState({count: this.state.count + 1});
  },

  render: function() {
    return <div onClick={this.handleClick}>{this.state.count}</div>;
  }
});

ReactDOM.render(<Counter initialCount={7}/>, mountNode);

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