AJAX: Check if a string is JSON?
JavascriptAjaxJsonValidationJavascript Problem Overview
My JavaScript sometimes crashes on this line:
var json = eval('(' + this.responseText + ')');
Crashes are caused when the argument of eval() is not JSON. Is there any way to check if the string is JSON before making this call?
I don't want to use a framework - is there any way to make this work using just eval()? (There's a good reason, I promise.)
Javascript Solutions
Solution 1 - Javascript
If you include the JSON parser from json.org, you can use its parse() function and just wrap it in a try/catch, like so:
try
{
var json = JSON.parse(this.responseText);
}
catch(e)
{
alert('invalid json');
}
Something like that would probably do what you want.
Solution 2 - Javascript
Hers's the jQuery alternative...
try
{
var jsonObject = jQuery.parseJSON(yourJsonString);
}
catch(e)
{
// handle error
}
Solution 3 - Javascript
I highly recommend you use a javascript JSON library for serializing to and from JSON. eval() is a security risk which should never be used unless you are absolutely certain that its input is sanitized and safe.
With a JSON library in place, just wrap the call to its parse() equivalent in a try/catch-block to handle non-JSON input:
try
{
var jsonObject = JSON.parse(yourJsonString);
}
catch(e)
{
// handle error
}
Solution 4 - Javascript
Maybe this helps: With this code, you can get directly your data…
<!DOCTYPE html>
<html>
<body>
<h3>Open console, please, to view result!</h3>
<p id="demo"></p>
<script>
var tryJSON = function (test) {
try {
JSON.parse(test);
}
catch(err) {
// maybe you need to escape this… (or not)
test = '"'+test.replace(/\\?"/g,'\\"')+'"';
}
eval('test = '+test);
console.debug('Try json:', test);
};
// test with string…
var test = 'bonjour "mister"';
tryJSON(test);
// test with JSON…
var test = '{"fr-FR": "<p>Ceci est un texte en français !</p>","en-GB": "<p>And here, a text in english!</p>","nl-NL": "","es-ES": ""}';
tryJSON(test);
</script>
</body>
</html>
Solution 5 - Javascript
The problem with depending on the try-catch approach is that JSON.parse('123') = 123 and it will not throw an exception. Therefore, In addition to the try-catch, we need to check the type as follows:
function isJsonStr(str) {
var parsedStr = str;
try {
parsedStr = JSON.parse(str);
} catch (e) {
return false;
}
return typeof parsedStr == 'object'
}
Solution 6 - Javascript
Why you can't just check what is the response? It is more more efficient.
var result;
if (response.headers['Content-Type'] === 'application/json')
result = JSON.parse(this.responseText);
else
result = this.responseText;
Solution 7 - Javascript
jQuery $.ajax() will add the responseJSON property to the response object, and to test if the response is JSON, you can use:
if (xhr.hasOwnProperty('responseJSON')) {}
Solution 8 - Javascript
There is a tiny library that checks JavaScript types: is.js
is.json({foo: 'bar'});
=> true
// functions are returning as false
is.json(toString);
=> false
is.not.json([]);
=> true
is.all.json({}, 1);
=> false
is.any.json({}, 2);
=> true
// 'all' and 'any' interfaces can also take array parameter
is.all.json([{}, {foo: 'bar'}]);
=> true
Actually is.js is much more then this, some honorable mentions:
var obj = document.createElement('div');
is.domNode(obj);
=> true
is.error(new Error());
=> true
is.function(toString);
=> true
is.chrome();
=> true if current browser is chrome