Simple counter jsx component
First, you need to bind this to the notice function as well

this:

if(this.state.number &lt;= 0) {
  return true;
} else {
  return false;
}


Can be written as: 

this.state.number &lt;= 0


as It will return those booleans anyway

in which case you could write: 

notice() {
  const { number } = this.state

  if(number &gt;= 10) {
    return 'You\'ve reached N° 10.';
  }

  if(number &lt;= 0) {
    return 'You can\'t decrement. You\'re at N° 0';
  }

  return 'You can do both.';
}


Note the destructuring pattern:

const { number } = this.state
// is the same as
const number = this.state.number