JSFiddle - React, Tailwind, and code Playground

JavaScript

function removeKeyStartsWith(obj, letter) {
  Object.keys(obj).forEach(function (key) {
     if(key[0]==letter) delete obj[key];
  });
}

var map = new Object(); 
map['XKey1'] = "Value1";
map['XKey2'] = "Value2";
map['YKey3'] = "Value3";
map['YKey4'] = "Value4";
console.log(map);
removeKeyStartsWith(map, 'X');
console.log(map);