javascript includes() case insensitive

You can create a RegExp from filterstrings first

var filterstrings = ['firststring','secondstring','thridstring'];
var regex = new RegExp( filterstrings.join( "|" ), "i");

then test if the passedinstring is there

var isAvailable = regex.test( passedinstring ); 

You can simply convert the passedinstring to lowercase.

var passedinstring = localStorage.getItem("passedinstring").toLowerCase();

ES6 array method filter() can simply the solution in single line, use includes() method to determines whether an array includes a certain value among its entries in conjunction to the toLowerCase() method to convert it to lowercase.

var filterstrings = ['firststring','secondstring','thridstring'];
var passedinstring = localStorage.getItem("passedinstring");

// convert each string from filterstrings and passedinstring to lowercase
// to avoid case sensitive issue.
filteredStrings = filterstrings.filter((str) => str.toLowerCase().includes(passedinstring.toLowerCase())

convert filterstring and passedinstring to lowercase and compare

var filterstrings = ['firststring','secondstring','thridstring'];
var passedinstring = 
localStorage.getItem("passedinstring").toLowerCase();

for (i = 0; i < filterstrings.lines.length; i++) {
   if (passedinstring.includes(filterstrings[i].toLowerCase())) {
       alert("string detected");
   }
}

Fixed case sensitivity issue using toLowerCase(). It turns all the string to lower case while comparing.

var product=productList.filter((x) => x.Name.toLowerCase().includes(("Active").toLowerCase()))