How can I set a state only if there is a condition?

I have a percentage into a text view that changes, but it can't be more than 100%. I use a function with a setstate and I want that if the percentage in the text view is more than 100% it changes in 100%. Something like this:

onPressLearnMore= () => { this.setState({

rareperc1: (this.state.rareperc2*this.state.boxes),

rareperc:parseFloat(this.state.rareperc1).toFixed(4),

if(rareperc==100){

rareperc:100,

} ... }); }

The actual code is:

onPressLearnMore= () => {

    this.setState({

            rareperc1: (this.state.rareperc2*this.state.boxes)

            rareperc:parseFloat(this.state.rareperc1).toFixed(4),

... }); }