Unable to access FOR...IN in Javascript Array

Question

Hi,

I want to access the object in [] JSON literal as an array using FOR...IN. But iterating through FOR...IN gives the object x undefined. Please see the code below.

var myJSONObject = [
    {"ircEvent": "PRIVMSG", "method": "newURI", "regex": "^http://.*"},
    {"ircEvent": "PRIVMSG", "method": "deleteURI", "regex": "^delete.*"},
    {"ircEvent": "PRIVMSG", "method": "randomURI", "regex": "^random.*"}
];

for (var x in myJSONObject) {
    alert(x['method']);
}

You may test the code online here @ JSBin

Regards,
Munim

Answer 1

Try

for (var x in myJSONObject) {
    alert( myJSONObject[x]['method'] );
}

Answer 2

The for .. in syntax gives you the keys of the iterated object, not the values. Thus, you have to do the following:

for (var idx in myJSONObject) {
    alert(myJSONObject[idx]['method']);
}

Answer 3

You shouldn't iterate over an array using for … in:

Although it may be tempting to use this as a way to iterate over an Array, this is a bad idea. The for...in statement iterates over user-defined properties in addition to the array elements, so if you modify the array's non-integer or non-positive properties (e.g. by adding a "foo" property to it or even by adding a method or property to Array.prototype), the for...in statement will return the name of your user-defined properties in addition to the numeric indexes. Also, because order of iteration is arbitrary, iterating over an array may not visit elements in numeric order. Thus it is better to use a traditional for loop with a numeric index when iterating over arrays.

Just loop through your array this way:

for (var i = 0, length = myJSONObject.length; i < length; i++) {
    alert(myJSONObject[i].method);
}

See JSBin.