How to check variable is set and exists from javascript?

Question

I am getting below json data:

{
  "ID": [
    "The i d field is required."
  ],
  "terms_condition": [
    "The terms condition field is required."
  ]
}

and stored in variable:

var DataJson = data.responseText;
var Json = JSON.parse(DataJson);

var IdError = Json.ID[0];
var TermsConditionError = Json.terms_condition[0];

Now I am getting this error Uncaught TypeError: Cannot read property '0' of undefined when ID is not exists

I have tried with this example to prevent error handling.

if (typeof Json.ID[0] !== 'undefined') {
    alert('validation message found');
} else {
    alert('validation message not found');
}

but this is not working any idea what i am doing wrong?

Thanks.

Possible duplicate of [JavaScript check if variable exists (is defined/initialized)](http://stackoverflow.com/questions/5113374/javascript-check-if-variable-exists-is-defined-initialized) — Alexander Elgin, Dec 25 '15 at 11:21

Dhiraj · Answer 1 · 2015-12-25T12:19:05.540

2

Try this.

 if (Json !=undefined && typeof Json.ID[0] != undefined ) {
        alert('validation message found');
    } else {
        alert('validation message not found');
    }

TypeError: Cannot read property '0' of undefined

This error occurs when main json object is either empty or undefined. This happens when there is no data in json object.

edited Dec 25 '15 at 12:19

answered Dec 25 '15 at 11:12

Dhiraj

1,302
9
17

Hi I have tried you example but every time I am getting alert('validation message found') – Mr.Happy Dec 25 '15 at 12:00
Please check revised answer. You have to put && operator instead of || – Dhiraj Dec 25 '15 at 12:03
try to use undefined instead of 'undefined' – Dhiraj Dec 25 '15 at 12:18

Alexander Elgin · Accepted Answer · 2015-12-25T12:11:47.773

To check whether a variable or a field is defined you can compare it with undefined

if ((Json !== undefined) && (Json.ID !== undefined) && (Json.ID[0] !== undefined)) {
    alert('validation message found');
} else {
    alert('validation message not found');
}

I don't see any issue in your code. See the snippet below.

var Json = JSON.parse('{"ID": ["The i d field is required."], "terms_condition": ["The terms condition field is required."]}');

var IdError = Json.ID[0];
var TermsConditionError = Json.terms_condition[0];

document.writeln(IdError + '<br>');
document.writeln(TermsConditionError + '<br>');
document.writeln('==================<br>');

function isDefined(Json) {
  return (Json !== undefined) && (Json.ID !== undefined) && (Json.ID[0] !== undefined);
}
  
var inputData = [
  undefined,
  {},
  {ID: ''},
  {ID: ['value']}
];

inputData.forEach(function(inputDaten) {
  document.writeln(JSON.stringify(inputDaten) + ': ' + isDefined(inputDaten) + '<br>');
});

Probably the issue is with data.responseText

Hi I have tried you example but every time I am getting `alert('validation message found')`. — Mr.Happy, Dec 25 '15 at 11:59
I've updated the snippet. You can check the result on a variety of data — Alexander Elgin, Dec 25 '15 at 12:12

How to check variable is set and exists from javascript?

2 Answers2