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.

Also, it's bool for booleans. I skimmed the docs and had the exact same problem until I went back and read them carefully.

https://facebook.github.io/react/docs/typechecking-with-proptypes.html

e.g.

React.PropTypes.sting
React.PropTypes.text

React could also tell the user: "The PropType 'sting' you defined is undefined. Did you misspell it?. Available PropTypes are: string, number, bool, etc."

In my case it happened when I declared Component.propTypes and below that, instead of writing Component.defaultProps I wrote again Component.propTypes assigning the default values.

just copy & paste, unless you can make sure that you would not type some typo!

> I also had this problem, and it wastes my much time to fix it!

So, just make a note!

VS code & plugins

> in case a typo!

    TestModal.propTypes = {
        title: PropTypes.string,
        //badHideModal: PropTypes.func.required,
        hideModal: PropTypes.func.isRequired,
    };

The easy way to solve the problem in 3 steps:

• step 1: install NPM prop-types package (npm prop-types install);
• step 2: import the package in your file.

Code: import PropTypes from 'prop-types';

• step 3: Remove the React keyword from the statement (e.g: React.PropTypes.string.isRequired to PropTypes.string.isRequired depends on your statement, so just remove the React keyword, because you are now importing from prop-types, not from the React library.

Something similar to isRequred instead of isRequired happened to me. I was trying to define the default props, but copy-pasted the propTypes and never changed the name, so I ended up with something like this:

import PropTypes from 'prop-types'

const Main = ({ title }) => // whatever

Main.propTypes = {
  title: PropTypes.string
}

Main.propTypes = { // this should have been Main.defaultProps! 🤦‍♂️
  title: 'This is the default title!'
}

So PropTypes was complaining about the title's prop type not being the right thing. It said something like "it must be a function, usually from React.PropTypes but it was a string".

I thought I was going crazy. I was looking at the first Main.propTypes and trying to find typo or anything that could be wrong, but as you can see it was completely fine. I opened multiple files that had prop types defined but weren't throwing the error.

I was obviously looking at the wrong place but eventually found it out of luck. I was pretty lucky to actually find it. These facepalming bugs are quite hard to figure out because they're like a magician moving one hand to attract all the eyes while preparing the trick with the other one, usually in plain sight.