using string index for array avoids type safety?

Question

I'm just learning typescript, and I noticed it doesn't have type safety for arrays when indexing by strings. It works as expected when indexing by number. Anyone explain this? I doubt its a bug but I can't find any information about this behavior. Thanks

let x : string[] = [];
x[0] = 'test'; // OK, as expected
x[0] = 123; // as expected, error TS2322: Type 'number' is not assignable to type 'string'.
x['hi'] = 123; // OK??  Expected error TS2322 as above

David Sherret · Accepted Answer · 2016-06-25 20:13:54Z

3

It's because you don't have the --noImplicitAny compiler option turned on.

Turn it on and it will throw a compile error:

answered Jun 25, 2016 at 20:13

David Sherret

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2 Comments

quetzalcoatl Over a year ago

That option doesn't seem to cause TS2322 to show up, it seems to be a different check. What would happen if you try x['hi'] = 'bar'? Would it show up the same error as on your screenshot? If so, then it's a generic error/warning related to accessing an array with non-numeric index (~trying to use array as a hash) and completely not about a type mismatch between the array's element type string and assigning 123 -- and from what I understood the OP and TS2322 are about that.

David Sherret Over a year ago

@quetzalcoatl doing x['hi'] = 'bar'; would be the same error. Also, yes, it's an error with --noImplicitAny because the object's type does not have an index signature defined with [key: string]: number;. Arrays have an index signature of [key: number]: T;... in this case T being string.

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