for-in statement

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);
}

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"