This is due to this.delta
not being bound to this
.
In order to bind set this.delta = this.delta.bind(this)
in the constructor:
constructor(props) {
super(props);
this.state = {
count : 1
};
this.delta = this.delta.bind(this);
}
Currently, you are calling bind. But bind returns a bound function. You need to set the function to its bound value.