add decrypted javascript inside of javascript

I am trying to add decrypt javascript and it is not working

    var decrypedjs = "alert(\"test\")" //for example;
    function start() {
    decrypedjs
    };

and the html

<button onclick="start()">started</button>

i have looked at so many sources and what i am finding is not really helpful or not working.

var newScript = document.createElement("script");
var inlineScript = document.createTextNode("alert('Hello World!');");
newScript.appendChild(inlineScript); 
target.appendChild(newScript);

>Solution :

I’m not going to comment on any security point other than to not use eval with external sourced strings (post-, get parameters, etc), but you’re looking for the eval method:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval

    var decrypedjs = "alert(\"test\")" //for example;
    
    function start() {
      eval(decrypedjs);
    };
    
    start();