JSFiddle - React, Tailwind, and code Playground

JavaScript

var users = [
  {
    firstname : "jetlag",
    lastname: "son"
  },
  {
    firstname : "becky",
    lastname: "love"
  },
  { 
    firstname : "just",
    lastname: "me"
  }
];
    function matchName(name) {
      var validUsers = [];                        //the array of users that pass the logic
      for (var i = 0; i < users.length; i++) {  
          if(users[i].firstname.lastIndexOf(name) === 0) { //this is "starts with" logic
               validUsers.push(users[i]);         //add the element if it's valid
          } 
      }
      //I moved the logic of the outside the loop
      if (validUsers.length === 0) 
          console.log('No matching users found')
      return validUsers;
    }

console.log(matchName("j"));