JSFiddle - React, Tailwind, and code Playground

HTML

<input class="functionName" type="text">
<button class="btn">Try</button>
    <p>The following functions are declared:
        <ul>
            <li>test1</li>
            <li>test2</li>
            <li>test3</li>
            <li>test4</li>
            <li>test5</li>
            <li>test6</li>
        </ul>
    </p>

JavaScript

function test1() {}
function test2() {}
function test3() {}
function test4() {}
function test5() {}
function test6() {}

$(document).on("ready", function () {
    $(".btn").on("click", function () {
    
        var functionName = $(".functionName").val().trim();
        console.log(functionName, typeof window["test1"]);
        if (typeof window[functionName] === 'function'){
            alert("The function exists");
        } else {
            alert("The function doesn't exist. Declare it first.");
        }
    });
});