criticise error when CEP is invalid (via CEP )

Question

Hello!

I am using the code below to get address through the CEP informed.

It works, but when typing an invalid zip code or 0000000, I get the return {"erro":true}. How can I turn this return into an Alert with the code below:

	console.log("webservice cep");
	
	$("#numero").attr("readOnly", true);
	$("#complemento").attr("readOnly", true);
	//var lastCepCheck = '';
	var ultimo_cep = '';
	document.getElementById('cep').addEventListener('keyup', function() {
	    //Impede inserir algo alem de Números
	    //this.value = this.value.replace(/[^0-9]/, "");

	    //Pega apenas os números
	    var cep = this.value.replace(/[^0-9]/, "");

	    //Só pesquisa se tiver 8 caracteres e o ultimo cep pesquisado seja diferente do atual.
	    if (cep.length != 8 || ultimo_cep == cep) {
	        return false;			
	    }
	    ultimo_cep = cep;

	    ajax = new XMLHttpRequest();

	    var url = "http://viacep.com.br/ws/" + cep + "/json/";
	    ajax.open('GET', url, true);
	    ajax.send();

	    ajax.onreadystatechange = function() {
			//console.log(ajax.status);
	        if (ajax.readyState == 4 && ajax.status == 200) {
	            var json = JSON.parse(ajax.responseText);				
	            console.log(ajax.responseText);
				if (json == "{erro: true}"){
					alert("erro");
				}
				else {
				$("#logradouro").val(json.logradouro);
				$("#bairro").val(json.bairro);
				$("#cidade").val(json.localidade);
				$("#uf").val(json.uf);
				$("#numero").attr("readOnly", false);
				$("#complemento").attr("readOnly", false);
				$("#numero").focus();
	            }
	        } 
	    }
	});

Answer 1

There may be other, more simplified methods, but in this way it also works. Just convert to String the return of the error that is in JSON and do the check:

if (JSON.stringify({erro: true}) == '{"erro":true}')
  alert("erro");

Edited:

if (JSON.stringify(json) == '{"erro":true}')
  {alert("erro");}

Answer 2

You can use the function hasOwnProperty() to check if there is the "error" Indice in the value returned by API;

var json = JSON.parse(ajax.responseText);
if (json.hasOwnProperty('error')) {
    console.log(json.error);
}
...