Given a typescript array, I would expect it to limit my options to only adding elements of the value type with an integer index. But this is not really the case and I cannot find a good reason why.
var test: string[] = [];
// OK as expected
test[1] = "42";
// Error as expected
test[1] = 42;
// Error as expected
test.test = 42;
// Unexpected OK, why?
test['1'] = 42;
Does anyone know why it is like this?
This is because of the way the TypeScript is designed. You are allowed to access any object by string indexers and by default it assumes the type any. This is for convinience of porting over existing code.
For stuff that is defined a string indexing leads to valid type checking, consider:
var test: string[] = [];
// Ok
test['push']('123');
// Error
test['push'](123);
Because at JavaScript:
var a = [];
a[1] = 123;
a['1'] = 456;
alert(a[1]); // 456
Update 1.
Possibly TypeScript compiler could not trace that at square brackets there are not a integer (because of only compile-time checks and no runtime overhead)?
For example, how to check a situation at a compile time?
var idx = getSomething();
var arr: string[] = [];
arr[idx] = 'Abcdef';
Update 2.
First, square brackets is not only used for accessing array items, so it is possible to has a attribute with such name.
Second, there is possible a difficult situation to trace.
Example.
var idx = {}
idx.toString = (): number => { return 2; };
var arr:number[] = [10, 11, 12, 13];
alert(arr[idx]); // 12
It is dynamically correct, but statically... Yes or no?
So, i dont have an answer to your question, but i think that it is just a difficult situation for compiler and it just pass here.
