it must be a function, usually from React.PropTypes

I want to pass string from Main to Header. It succeeds but warning. I'm a beginner of React so I can not figure out what it must be a function means.

Anyone knows how to solve this warning?

The warning is:

And my code is below:

Main.js

import React from 'react';

import Header from './Header';
import AppList from './AppList/AppList';
import Footer from './Footer';

const propTypes = {
  mainInfo: React.PropTypes.shape({
    title: React.PropTypes.string.isRequired,
    apps: React.PropTypes.array.isRequired,
  }),
};

class Main extends React.Component {
  static methodsAreOk() {
    return true;
  }

  render() {
    return (
      <div>
        <Header title={this.props.mainInfo.title} />
        <AppList apps={this.props.mainInfo.apps} />
        <Footer />
      </div>
    );
  }
}

Main.propTypes = propTypes;

export default Main;

Header.js

import React from 'react';

const propTypes = {
  title: React.PropTypes.string.isRequred,
};

class Header extends React.Component {
  static methodsAreOk() {
    return true;
  }

  render() {
    return (
      <div className="header">
        <h1>{this.props.title}</h1>
      </div>
    );
  }
}

Header.propTypes = propTypes;

export default Header;

You have an error: React.PropTypes.string.isRequred. Spell isRequired correctly, and it should be ok.

This happens when your PropType is actually undefined.

In my case I had specified a propType of PropTypes.integer, which is not one of the list of proptypes. That literally turns into undefined. Instead I should have used PropTypes.number.

Watch out for similar mistakes with PropTypes.bool vs PropTypes.boolean, as well as PropTypes.func vs PropTypes.function.