ReferenceError: "Set" is not defined

Hi, I'm trying to declare a Set and get the error ReferenceError: "Set" is not defined. I'm new to and have googled stackoverflow and these boards but don't find anything specific to my issue. I commented all code after the declaration as well and it still gives the error, it it's definitely that first line.

var dBSet = new Set();
....some code cleaning up my data
if (!dBSet.has(sfrobj5.items[i].title)) {
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