How to check if a component prop is a function in React?

class MyComponent extends React.Component {
    render() {
        return <h1>if (typeof this.props.showName === 'function') {
            //call this.props.showName(d)
        }
    }
}

Your above code seems fine. But your approach is wrong.

You need to validate props using propTypes

MyComponent .propTypes = {
showName : React.PropTypes.func
};

Here is the other checks,

propArray: React.PropTypes.array.isRequired,
   propBool: React.PropTypes.bool.isRequired,
   propFunc: React.PropTypes.func,
   propNumber: React.PropTypes.number,
   propString: React.PropTypes.string,
   propObject: React.PropTypes.object

---

React.PropTypes has moved into a different package since React v15.5. Please use the prop-types library instead.

import PropTypes from 'prop-types';

class Greeting extends React.Component {
  render() {
    return (
      <h1>Hello, {this.props.name}</h1>
    );
  }
}

Greeting.propTypes = {
  name: PropTypes.string
};

Write it like this:

checkMethod(){
    if(typeof(this.props.showName) === 'function') {
        //call this.props.showName(d);
        return null;
    }else{
        return this.props.showName;
    }    
}

class MyComponent extends React.Component {
    render() {
        return 
            <h1>
                {this.checkMethod()}
            </h1>
        }
    }
}