JSFiddle - React, Tailwind, and code Playground

JavaScript

var json = [
	{
		"city": "California"
	},
	{
		"city": "Pittsburgh"
	},
	{
		"city": "Pittsburgh"
	},
	{
		"city": "Cranberry Twp"
	}			
];

var obj = {};

for (var i = 0, j = json.length; i < j; i++) {
   if (obj[json[i]]) {
      obj[json[i]]++;
   }
   else {
      obj[json[i]] = 1;
   } 
}

console.log(obj);