I'm writing a simple for...in loop in javascript, and wondering why the key is a string and not a number?

Why is it so, and can I change that to be a number?

var array = ["a", "b", "c"];

for (var key in array) {
   console.log(typeof key); //string
   console.log(key + 1); //expected output : 01, 11, 21...
}

I'm writing a simple for...in loop in javascript, and wondering why the key is a string and not a number?

Why is it so, and can I change that to be a number?

var array = ["a", "b", "c"];

for (var key in array) {
   console.log(typeof key); //string
   console.log(key + 1); //expected output : 01, 11, 21...
}

• javascript
• for-loop

• It's probably because for y in x treats the x as an object so you can iterate through its properties – Sebastian Oleński Commented Jan 28, 2019 at 11:47
• Using a foreach with arrays is a bit expensive if you want to access array elements as you'd have to parse key on to an int, is best to use foreach loops for objects ({}) and not arrays ([]). The standard loops for arrays are either for(let i=..; i < ....; i++) {} or array.forEach(function(val, i) { }). – Bargros Commented Jan 28, 2019 at 11:53

3 Answers 3

It's a string because standard arrays in JavaScript aren't really arrays at all¹, they're objects with properties for the array entries, and object property names (keys) are strings, Symbols, or (soonish) private names.

You can't make it a number by default in a for-in, but you can convert it to a number, or use other forms such as a standard for or a forEach call:

for (var key = 0; key < array.length; ++k) {
    // ...
}
// ..or
array.forEach((entry, key) => {
    // ...
});

Using for-in to loop through an array is almost always an anti-pattern. See my answer here for a thorough rundown of your various options for looping through arrays.

¹ That's a post on my anemic little blog.

Objects and properties

Please note that all keys in the square bracket notation are converted to String type, since objects in JavaScript can only have String type as key type.

Object property names are always strings.

Use +, parseInt, JSON.parse or any other standard method to convert a string to a number if you want a number.

var array = ["a", "b", "c"];

for (var key in array) {
  console.log(+key + 1);
}