JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" value="Hypoteneuse";" />
A:<input type="text" id="leg1" size="2" />
B:<input type="text" id="leg2" size="2" />
Hypotenuse:<input type="text" placeholder="0" id="result" size="2" />
<button id="submit">Calculate</button>

JavaScript

var leg1 = document.getElementById('leg1');
var leg2 = document.getElementById('leg2');

function hypotenuse(a,b){
     return Math.sqrt(a*a + b*b);
}

document.getElementById('submit').addEventListener('click', function() {
    if (leg1.value !== '' && leg2.value !== '') {
         document.getElementById('result').value = hypotenuse(parseInt(leg1.value),parseInt(leg2.value));    
    } else {
         console.log('Please fill out both fields');   
    }
});