How find variable/array if I don't know full variable name in javascript

I am trying to get additional information from site and there is variable/array that defined randomly. Like this:

var firstvar_numbers= "already exist"
var secondvar_numbers= ["a","b","c","d"]

numbers is random value that given by site. In firefox DOM when I wrote half of secondvar_ it immediately find that I wanted because there is only one variable that starts with second. My question is how I can get value/array of variable in userscript/javascript by knowing part of the variable. Example if you don't understood: Html

//Variable that I need from server
<div id="variables" class="container">

<script type="text/javascript">
var exists_73647286="hello world"

var array_636353=[62,96,11,28]
</script>
</div>

Javascript

//Code that will get array
alert("exists_"+seconpartofvar())
function seconpartofvar(){your code}

alert(autocomplate.exists_)

Here I can write alert(exists_) in Firefox console and it will recommend autocomplate.

Solution

Since it's declared with var on the top level, you can iterate over the properties of the window and find one which startsWith what you're looking for:

// Example site code:
var array_636353 = [62, 96, 11, 28];

// Userscript code:
const prop = Object.keys(window).find(key => key.startsWith('array_'));
console.log(prop, window[prop]);