JSFiddle - React, Tailwind, and code Playground

HTML

<select id="selects">
		<option value="sqrrt" selected>Squared Root</option>
		<option value="cbdrt">Cubed Root</option>
	</select>
	<input type="text" placeholder="value" id="inp">
	<button type="button" id="calculate">Calculate!</button>

JavaScript

var button = document.getElementById("calculate"),
			selects = document.getElementById("selects"),
			option = selects.options[selects.selectedIndex].value,
			inp = document.getElementById("inp");

		function calca(glg){
			switch(glg){
				case "sqrrt":
					for(var i = 0; i < inp.value; i += 0.01){
						var check = i * i;
						check = check.toFixed(2);

						console.log(i.toFixed(2));

						if(check == inp.value){
							alert(i);
							break;
						} else if(check > inp.value){
							alert("Value could not be found.");
							break;
						}
					}
			}	
		}

		window.onkeyup = function(e){
			var code = (e.keyCode ? e.keyCode : e.which);

			if(code == 13){
				if(inp.value != ""){
				    calca(option);
				} else {
					alert("No value entered!");
				}
			}
		}

		button.onclick = function(){
			if(inp.value != ""){
				calca(option);
			} else {
				alert("No value entered!");
			}
		}