JSFiddle - React, Tailwind, and code Playground

HTML

<div id="out1"></div>
<div id="out2"></div>
<div id="out3"></div>
<div id="out4"></div>

JavaScript

var callback = function(userInput) {
    var str = userInput.indexOf('return') != -1 ? '(function() {'+userInput+'})();' : userInput;
    return eval(str);
}

out1.innerHTML = callback('var x = 8; x;');
out2.innerHTML = callback('var x = 10; return x;');
out3.innerHTML = callback('var x = 10; if(x === 10) return "yes"; "no";');
out4.innerHTML = callback('var x = 8; if(x === 10) return "yes"; "no";');