How to force types in Array with TypeScript’s union types

From TypeScript 1.4 we can do nice stuff with types now, with help of feature called Union Types : https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#union-types which will let you do also this sweet stuff :

ts union types

You can see this in action on this link :

http://www.typescriptlang.org/Playground

Sample snippet here :

module ArrPlayground {
    export function fn() {
        var arr: Array<boolean|string> = [];
        arr[0] = "aaa";
        arr[1] = false;
        arr[2] = 987;

        return arr;
    }
}

Basically you can now limit types that could be used in Array (all only at compile time, same rules apply as for other type checking features of TypeScript).

Enjoy.

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