I'm having trouble with iterating and getting the values within a collection of arrays (an array of arrays I guess)

I'd hope that the code below would display an alert showing the 3 values of each array in turn (eg "infant", "0" and then "2") but the alert just displays "0" "undefined", undefined".

What am I missing?

Declare the array:

var ageGroups = [["infant", 0, 2], ["child", 3, 18], ["child", 3, 17], ["adult1", 18, 64], ["adult2", 65, 74], ["adult3", 75, 79]];

Iterate the array

for (var item in ageGroups) {
    alert(item[0]);
    alert(item[1]);
    alert(item[2]);
}

I'm having trouble with iterating and getting the values within a collection of arrays (an array of arrays I guess)

I'd hope that the code below would display an alert showing the 3 values of each array in turn (eg "infant", "0" and then "2") but the alert just displays "0" "undefined", undefined".

What am I missing?

Declare the array:

var ageGroups = [["infant", 0, 2], ["child", 3, 18], ["child", 3, 17], ["adult1", 18, 64], ["adult2", 65, 74], ["adult3", 75, 79]];

Iterate the array

for (var item in ageGroups) {
    alert(item[0]);
    alert(item[1]);
    alert(item[2]);
}

• javascript

• anyway what you need to know is the for in loop returns item as a string , the name of the current property the loop is looking up , not an object or an array , so for in is not a foreach you can find in other langages , there is a forEach function for arrays in ES5 though – mpm Commented Apr 4, 2012 at 13:07

7 Answers 7

use console.log instead of alert, Alert will show just [Object ], if variable is a object but in console you can see what kind of object and you can debug further

for (var item in ageGroups) { 
    console.log(ageGroups[item][0]); 
    console.log(ageGroups[item][1]); 
    console.log(ageGroups[item][2]); 
}

for (var item in ageGroups) {
    alert(ageGroups[item][0]);
    alert(ageGroups[item][1]);
    alert(ageGroups[item][2]);
}

your porblem is that item is the key of your array

try this:

for (var item in ageGroups) {
    alert(ageGroups[item][0]);
    alert(ageGroups[item][1]);
    alert(ageGroups[item][2]);
}

Use the damn forEach! :-) Not cross-browser though, but the shim is easy to implement.

// Call forEach and define the callback function
ageGroups.forEach(loopArray)

// Now let's work with the array!
function loopArray(ageGroup) {
    console.log(ageGroup[0])
    console.log(ageGroup[1])
    console.log(ageGroup[2])
}

You should do

for (i = 0; i <ageGroups.length; i++) {
    var item = ageGroups[i];
    alert(item[0]);
    alert(item[1]);
    alert(item[2]);
}

for..in in javascript is used to iterate over the properties of an object

Here's an optimized for loop, I'm storing the length here so it doesn't evaluate at every iteration:

 for (var i = 0, l = ageGroups.length; i < l; i++){
    alert(ageGroups[i][0]);
    alert(ageGroups[i][1]);  
    alert(ageGroups[i][2]);      
 }

to make it exactly like your example you can store the ageGroup's iteration in a variable:

for (var i = 0, l = ageGroups.length, item; i < l; i++){
    item = ageGroups[i];  
    alert(item[0]);
    alert(item[1]);
    alert(item[2]);
}

Don't use for in to iterate arrays in JavaScript. Its purpose is to iterate over object properties. Instead use an incremental for loop..

for (var i=0; i<ageGroups.length; i++) {
  for (var j=0; j<ageGroups[i].length; j++) {
    console.log(ageGroups[i][j]);
  }

  // Or instead of an inner loop access them individually
  console.log(ageGroups[i][0]);
  console.log(ageGroups[i][1]);
  console.log(ageGroups[i][2]);
}

See it in action...

Using the for-in construct on an array can produce drastically different results than an incremental loop if, for example, you defined only one array item like myArr[3] = 123;. In that case, JavaScript will allocate items 0-2, and the for loop will iterate them, but the for-in won't. More importantly, external scripts and frameworks may extend the Array prototype and add properties which will suddenly be included in your for-in iterator when you really just wanted the array elements.