The code:
const str = 'QXByaWwncyBGb29sIQ==';
function decodeStr(str) {
const result = atob(str);
console.log(result)
return result;
}
...
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It seems good
If you are not able to see the console.log, it can be related to an error when calling the code before.
So you can try to add a console.log in the first line of your function (just to be sure you are able to call your method) and then you should be able to see if everything goes right