for-in statement

Question

TypeScript docs say nothing about loop like for or for-in. From playing with the language it seems that only any or string variables are supported in for loop.

Why has this decision been made?

Why not use the type information and have strongly-typed iteration variable?

score 157 · Answer 1 · edited May 31 '23 at 14:34

157

In Typescript 1.5 and later, you can use for..of as opposed to for..in

var numbers = [1, 2, 3];

for (var number of numbers) {
    console.log(number);
}

edited May 31 '23 at 14:34

hsusanoo

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answered Dec 09 '15 at 14:47

brianbruff

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for...in iterates the keys of the objects in the collection, while for...of is the correct approach for iterating the objects themselves – plyawn Feb 22 '17 at 23:47
for..of does not give you an iterator. So, if you're doing something like comparing consecutive values in an array, for..of is not the correct solution, for..in is. TypeScript is WRONG for artificially constraining how these constructs might be used. – martinp999 Feb 13 '19 at 19:35

Ryan Cavanaugh · Answer 2 · 2013-01-07T17:15:35.153

74

TypeScript isn't giving you a gun to shoot yourself in the foot with.

The iterator variable is a string because it is a string, full stop. Observe:

var obj = {};
obj['0'] = 'quote zero quote';
obj[0.0] = 'zero point zero';
obj['[object Object]'] = 'literal string "[object Object]"';
obj[<any>obj] = 'this obj'
obj[<any>undefined] = 'undefined';
obj[<any>"undefined"] = 'the literal string "undefined"';

for(var key in obj) {
    console.log('Type: ' + typeof key);
    console.log(key + ' => ' + obj[key]);
}

How many key/value pairs are in obj now? 6, more or less? No, 3, and all of the keys are strings:

Type: string
0 => zero point zero
Type: string
[object Object] => this obj; 
Type: string
undefined => the literal string "undefined"

edited Jan 07 '13 at 17:15

answered Oct 19 '12 at 21:03

Ryan Cavanaugh

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OK... but a string is not as precise as "a key of this specific object". So how does one type keys of an object when iterating with a for in loop? – Romain Vincent Sep 14 '21 at 16:18
use for...of instead of for...in – Chrispher Nov 04 '22 at 10:50

score 34 · Accepted Answer · edited May 23 '17 at 12:34

34

The for-in statement is really there to enumerate over object properties, which is how it is implemented in TypeScript. There are some issues with using it on arrays.

I can't speak on behalf of the TypeScript team, but I believe this is the reason for the implementation in the language.

edited May 23 '17 at 12:34

Community

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answered Oct 18 '12 at 09:11

Fenton

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citykid · Answer 4 · 2018-06-30T10:07:31.457

edit 2018: This is outdated, js and typescript now have for..of loops.
http://www.typescriptlang.org/docs/handbook/iterators-and-generators.html

The book "TypeScript Revealed" says

"You can iterate through the items in an array by using either for or for..in loops as demonstrated here:

// standard for loop
for (var i = 0; i < actors.length; i++)
{
  console.log(actors[i]);
}

// for..in loop
for (var actor in actors)
{
  console.log(actor);
}

"

Turns out, the second loop does not pass the actors in the loop. So would say this is plain wrong. Sadly it is as above, loops are untouched by typescript.

map and forEach often help me and are due to typescripts enhancements on function definitions more approachable, lke at the very moment:

this.notes = arr.map(state => new Note(state));

My wish list to TypeScript;

Generic collections
Iterators (IEnumerable, IEnumerator interfaces would be best)

for-in statement

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