react.dev/tips/10-props-in-getInitialState-as-anti-pattern.md

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props-in-getInitialState-as-anti-pattern	Using state to cache calculations is an antipattern	tips	props-in-getInitialState-as-anti-pattern.html	componentWillReceiveProps-not-triggered-after-mounting.html	dom-event-listeners.html

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 state to cache values calculated from props (for example 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:

/** @jsx React.DOM */

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

React.renderComponent(<MessageBox name="Rogers"/>, mountNode);

Better:

/** @jsx React.DOM */

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

React.renderComponent(<MessageBox name="Rogers"/>, mountNode);

For more complex logic:

/** @jsx React.DOM */

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

React.renderComponent(<MessageBox name="Rogers"/>, mountNode);