Review a simple jQuery example to loop over a JavaScript array object.

var json = [
	{"id":"1","tagName":"apple"},
	{"id":"2","tagName":"orange"},
	{"id":"3","tagName":"banana"},
	{"id":"4","tagName":"watermelon"},
	{"id":"5","tagName":"pineapple"}
];
$.each(json, function(idx, obj) {
	alert(obj.tagName);
});

Above code snippet is working fine, prompts the “apple”, “orange” … as expected.

Problem : JSON string

Review below example, declares a JSON string (enclosed with single or double quotes) directly.

var json = '[{"id":"1","tagName":"apple"},{"id":"2","tagName":"orange"},
{"id":"3","tagName":"banana"},{"id":"4","tagName":"watermelon"},
{"id":"5","tagName":"pineapple"}]';
$.each(json, function(idx, obj) {
	alert(obj.tagName);
});

In Chrome, it shows following errors in console :

Uncaught TypeError: Cannot use 'in' operator to search for '156' 
in [{"id":"1","tagName":"apple"}...

Solution : Convert JSON string to JavaScript object

To fix it, converts it to Javascript object via standard JSON.parse() or jQuery $.parseJSON.

var json = '[{"id":"1","tagName":"apple"},{"id":"2","tagName":"orange"},
{"id":"3","tagName":"banana"},{"id":"4","tagName":"watermelon"},
{"id":"5","tagName":"pineapple"}]';
$.each(JSON.parse(json), function(idx, obj) {
	alert(obj.tagName);
});
//or 
$.each($.parseJSON(json), function(idx, obj) {
	alert(obj.tagName);
});

References

1. jQuery $each() example
2. Wikipedia : JSON