For-In loop in Javascript (Node.js) unexpectedly iterates through array indexes instead of elements

Question

I have a Node.js script that reads and parses the content of a JSON file and iterates through the elements of its root array; when I try to inspect the objects of such iteration I am faced with some unexpected result; the object being iterated seems to be the index of the element being iterated. Here is the script:

if (process.argv.length != 3) {
    console.log("Usage: node chooseAdAmongAds.js /path/to/ads.json");
    process.exit(9);
}

fs = require("fs");
var content = fs.readFileSync(process.argv[2], "utf8")

var ads = JSON.parse(content);
for (ad in ads) {
    console.log(ad);
}

The JSON file:

[
    {
        "impressions": 131, 
        "ID": 514, 
        "investment": 2
    }, 
    {
        "impressions": 880, 
        "ID": 451, 
        "investment": 5
    }, 
    {
        "impressions": 135, 
        "ID": 198, 
        "investment": 9
    }, 
    {
        "impressions": 744, 
        "ID": 262, 
        "investment": 8
    }, 
    {
        "impressions": 234, 
        "ID": 954, 
        "investment": 19
    }, 
    {
        "impressions": 673, 
        "ID": 274, 
        "investment": 12
    }, 
    {
        "impressions": 442, 
        "ID": 734, 
        "investment": 6
    }, 
    {
        "impressions": 453, 
        "ID": 982, 
        "investment": 5
    }, 
    {
        "impressions": 55, 
        "ID": 275, 
        "investment": 5
    }, 
    {
        "impressions": 628, 
        "ID": 524, 
        "investment": 1
    }
]

And the console output:

iMac-di-Michele:Node.js michele$ node chooseAdAmongAds.js example.json 
0
1
2
3
4
5
6
7
8
9
iMac-di-Michele:Node.js michele$

Why does this happen?

Explosion Pills · Accepted Answer · 2015-02-17 22:21:14Z

2

for...in loops over enumerable properties of an Object and arrays are objects. You should not use for...in for array iteration

Instead, use Array constructs for iteration such as .forEach, or just use a for loop and access the array element by its index.

ads.forEach(function (ad) {
    console.log(ad)
})

answered Feb 17, 2015 at 22:21

Explosion Pills

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Comments

Ezra Morse · Accepted Answer · 2015-02-17 22:21:40Z

2

You are just outputting the index. Try

console.log(ads[ad]);

answered Feb 17, 2015 at 22:21

Ezra Morse

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