You can see a working example here:

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I am trying to create an array of objects in javascript. When I attempt to access the objects I am getting "undefined" output. Here is some sample code:

var dates = [];

var beginDate1 = new Date("01 / 01 / 01");
var endDate1 = new Date("02 / 02 / 02");
var beginDate2 = new Date("03 / 03 / 03");
var endDate2 = new Date("04 / 04 / 04");

// this outputs the correct dates
alert("before: " + beginDate1 + "--" + endDate1);
alert("before: " + beginDate2 + "--" + endDate2);

dates.push({
    "beginDate": beginDate1,
    "endDate": endDate1
}, {
    "beginDate": beginDate2,
    "endDate": endDate2
});

var date;
for (date in dates) {
    // this outputs "date: undefined--undefined"
    // why would that be?
    alert("after: " + date.beginDate + "--" + date.endDate);
}

You can see a working example here:

http://jsfiddle/bwhitney/ZDHp4/1/

I am trying to create an array of objects in javascript. When I attempt to access the objects I am getting "undefined" output. Here is some sample code:

var dates = [];

var beginDate1 = new Date("01 / 01 / 01");
var endDate1 = new Date("02 / 02 / 02");
var beginDate2 = new Date("03 / 03 / 03");
var endDate2 = new Date("04 / 04 / 04");

// this outputs the correct dates
alert("before: " + beginDate1 + "--" + endDate1);
alert("before: " + beginDate2 + "--" + endDate2);

dates.push({
    "beginDate": beginDate1,
    "endDate": endDate1
}, {
    "beginDate": beginDate2,
    "endDate": endDate2
});

var date;
for (date in dates) {
    // this outputs "date: undefined--undefined"
    // why would that be?
    alert("after: " + date.beginDate + "--" + date.endDate);
}

• javascript

4 Answers 4

The for ... in loop in JavaScript gives you the keys in the object, not the values.

You really should use a numeric index however:

for (var date = 0; date < dates.length; ++date) {
  alert("date " + date + " is: " + dates[date]);
}

Iterating over keys with for ... in will not pick up only the numerically-indexed array elements; it operates on arrays as if they are plain ordinary objects. Other properties will be picked up too, plus you're not even guaranteed it'll go in ascending numeric order!

Common mistake in for each loop. date is index of dates. You should write: dates[date].beginDate.

When using a for..in loop, the variable gets assigned the key, not the value!

for (date in dates) {
    alert('after: ' + dates[date].beginDate ...);
}

for ... in doesn't work like that with arrays, you can use a regular for like this fiddle: http://jsfiddle/bwhitney/ZDHp4/1/

Reference: Why is using "for...in" with array iteration a bad idea?