JSFiddle - React, Tailwind, and code Playground

JavaScript

var obj = [
    { "pa":"va1", "pb":"vb1" },
    { "pa":"va2", "pb":"vb2" },
    { "pa":"va3", "pb":"vb3" },
    { "pa":"va4", "pb":"vb4" }
];
var myString = "va4";
var filtered = obj.filter(function(element) { 
    return element.pa === myString;
});

if (filtered.length > 0) {
   alert(myString + " found");
}