React setState inside of a function this is undefined

Use arrow function (() => {}) which keeps the last scope (this) as it is .

MyAction: function(){
    this.doFetch().then((response) => {
        this.setState({
            the_message: response.message
        });
    });
},

Your inner promise resolution function won't have this context. One way to fix it:

MyAction: function(){
    this.doFetch().then(function(response){
        this.setState({
            the_message: response.message
        });
    }.bind(this))
},

Read more about that on this StackOverflow question.

A simple .bind(this) would solve the issue:

render: function() {
return (
<div>
<div>{this.props.title}</div><br></br>
<div>{this.props.price}</div><br></br>
<div onClick={this.MyAction.bind(this)}>{this.props.qty}</div>
</div>
);

By adding .bind(this) you keep the scope within your function.