Q:

window.URLSearchParams broken on Edge

I'm trying something like the following; 
let formUrl = new window.URLSearchParams(
        new window.FormData(this.searchForm)
      ).toString()

But I'm getting '0: Invalid argument' 
This works on every browser apart from edge. 
Has anyone run into this issue before

A:

The problem in Edge is that they do not support destructuring of FormData object. (for ... of in linked table).
Indeed, you are relying on this behavior from other browsers in your code, because URLSearchParams( init ) constructor accepts a record as init parameter.  
The specs read

Otherwise, if init is a record, then for each name → value in init, append a new name-value pair whose name is name and value is value, to query’s list. 

So this means that your code is actually doing

const formData = new FormData( searchForm );

const urlParams = new URLSearchParams();
for ( let [ name, value ] of formData ) {
  urlParams.append( name, value );
}
let formUrl = urlParams.toString();

console.log( formUrl );
<form id="searchForm">
  <input name="foo" value="bar">
  <input name="baz" value="bla">
</form>

But since, once again, Edge doesn't support destructuring of FormData, your code breaks. 
However, according to MDN, they do support for ... of FormData.entries(), so maybe this would work (no Edge to test though so let me know).

let formUrl = new window.URLSearchParams(
  new window.FormData(this.searchForm)
    .entries() // explicitely call .entries() to get the iterator
).toString()

console.log( formUrl );
<form id="searchForm">
  <input name="foo" value="bar">
  <input name="baz" value="bla">
</form>

