The following patterns are considered warnings: var Hello = createReactClass({ render: function() { return <div>Hello {this.props.name}</div>; } }); var Hello = createReactClass({ propTypes: { firstname: PropTypes.string.isRequired }, render: function() { return <div>Hello {this.props.firstname} {this.props.lastname}</div>; // lastname type is not defined in propTypes } }); function Hello({ name }) { return <div>Hello {name}</div>; }