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I'm having difficulty trying to understand this:

There are two types of supported index types: string and number. It is possible to support both types of index, with the restriction that the type returned from the numeric index must be a subtype of the type returned from the string index.

While index signatures are a powerful way to describe the array and 'dictionary' pattern, they also enforce that all properties match their return type. In this example, the property does not match the more general index, and the type-checker gives an error:

interface Dictionary {
    [index: string]: string;
    length: number;    // error, the type of 'length' is not a subtype of the indexer
}

source:TypeScript Handbook's interface

I have tried 4 cases, but still cannot understand what is happening. Would anyone explain why only [index: string]: string; will has error TS2411? enter image description here

Another case:

another case

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3
  • You missed one codio@compact-guide. I don't see a reason to hide it anyways Commented Nov 20, 2015 at 3:17
  • miss two exactly ..... Commented Nov 20, 2015 at 4:22
  • Nice. Hiding it made me want to find out more :) Commented Nov 20, 2015 at 5:16

3 Answers 3

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6

if you have [index: string]: string; all properties must be a string. Hence: 

interface Dictionary {
    [index: string]: string;
    length: number;    // error, length is not a string.
}
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6 Comments

thanks, I am sorry for late reply. I am too foolish. I include one more test case in your answer, would you help me.
Sorry, but I am unclear which code sample you have a question about
sorry, I just add it.
That case is an error too if you try in the playground. You have an older version of tsc I suppose
it will has error only if I add one more line -->var foo:Dictionary=['peter','john']; <-- is it what you mean ^_^
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2

A workaround with TypeScript 2.4 is

interface Dictionary {
    [index: string]: string | number;
    length: number;
}

However, you may need to cast explicitly when using

let v = dict.key as string
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-1

It is possible to support both types of index, with the restriction that the type returned from the numeric index must be a subtype of the type returned from the string index.

Also there is no question mark, the answer is No. This Array wont work in any way:

interface Dictionary {
    [index: string]: string;
    length: number;  // error
}

because even this will work:

interface Dictionary {
    [index: string]: string;
}

var dic: Dictionary = {}
dic[1] = "123"

Note that we are indexing the string array with a number.

interface Dictionary { [index: string]: MYCLASS; } is meant to support you to define the value allowed, but not the type of the Index, sorry

What you are doing is not about interfaces but arrays: http://www.typescriptlang.org/Handbook#interfaces-array-types

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