JSFiddle - React, Tailwind, and code Playground

JavaScript

$(document).ready(function(){
    
var obj1 = [{"POST":"Developer","USERNAME":["User1","User2"]},{"POST":"Tester","USERNAME":["User3"]}];
var obj2 = [{"USERNAME":"User1","USER_PASSWORD":"test1"},{"USERNAME":"User2","USER_PASSWORD":"test2"},{"USERNAME":"User3","USER_PASSWORD":"test3"}];

    $.each(obj2,function(i,val){
       $.each(obj1,function(index,innerVal){
            
            var post=innerVal.POST;
            $.each(innerVal.USERNAME,function(i,user){
                if(user==val.USERNAME){
                    val.POST=post;
                }
            });
        });
    });
alert(JSON.stringify(obj2));
    
});