typescript: indexing a class instance

Question

Where there is no error at this line

            if (!t[1]) {
                t[1] = [];
            }

            t[1].push(key);

t is of type Test, which is not directly indexible. But, it has a member, data, which can be indexed. So why there is no error in these lines.

ITestData models a nested object, with integer and string key respectively.

{ 
  1 : {
        "hello" : {}
  }
  2 : { ..
}

demo

Code:

export interface ITestData {
    [idx:number] : {[prop:string]:any}
}

export interface ITest {
    data:ITestData
}

export class Test implements ITest {
        data:ITestData = {};
}

class Greeter {
    data:ITestData = {};

    private del(t:Test, key:string) {
        if (!t[1]) {
            t[1] = [];
        }

        t[1].push(key);
   }
}

basarat · Accepted Answer · 2014-04-12 09:26:59Z

1

Everything in JavaScript can be indexed via a string (and number using number.toString()). TypeScript will allow indexing by string (and number).

A simpler version of what you are experiencing :

var foo = {data:123};
var bar = foo[0]; // Valid : bar is of type any

You can prevent what can be assigned to an indexed member e.g. only Foo can be assigned in the following case when indexed:

var bas: {[index:string]:Foo};

But you cannot disable indexing all together (i.e. you can restrict the type of read / write but not the fact that you can read / write).

edited Apr 12, 2014 at 9:26

answered Apr 12, 2014 at 3:05

basarat

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3 Comments

bsr Over a year ago

thanks. How to prevent it (how to use index signature with a class?). Or any better way to represent this datastructure?

basarat Over a year ago

@bsr I had poor choice of words. Clarified

bsr Over a year ago

thanks basarat. I see that typescript is a super set of JS, but class is a concept introduced by TS. IT doesn't make sense to support indexing it. Will be it supported in native ES6 classes, I don't know. But, if it doesn't make logical sense, at least at TS level, the compiler can warn the usage. Do you think it is possible, desired, should be a feature? I also not sure your usage of foo, bas, an object, equivalent to mine, t:Test, a class. Again, may be compile down to a function or whatever, but considering other languages and expectations, it is an odd behavior.

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