Script works in chrome but not in firefox or IE

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Javascript

Firefox

script

chrome

The below script works perfectly fine on Google Chrome only and doesn't work on Firefox or IE

//I created the script at the bottom of the body to change select dropdown based on currently selected mainheading value.
//old method tried to use scriptlets mixed with javascript, but it's easier to do this with javascript.
const urlParams = new URLSearchParams(window.location.search);
const myParam = urlParams.get('mainHeading'); 
//if mainHeading parameter is null or equal to  Operations show  Operations dropdown
if(myParam === ' Operations' || myParam === null) {
	document.getElementById('mainHeading').value = ' Operations';
}else{
	document.getElementById('mainHeading').value = ' Future Plans';
}

What I have tried:

If I change the order of the if statement to the below, then the only option available in the drop down list is "Operation" and the second option "Future Plans" never shows up

var urlParams = new URLSearchParams(window.location.search);
var myParam = urlParams.get('mainHeading'); 
//if mainHeading parameter is null or equal to Operations show Operations dropdown
if(myParam ==='Future Plans'){
	document.getElementById('mainHeading').value = 'Future Plans';
}else{
	document.getElementById('mainHeading').value = 'Operations';