Union types and array.concat problem

[NikitaEgorov]

var a = (): number | Array<number> => {
    return Math.random() > 0.5? [2,3] : 3;
};

var b = [1];

b.concat(a());

http://www.typescriptlang.org/Playground#src=var%20a%20%3D%20()%3A%20number%20%7C%20Array%3Cnumber%3E%20%3D%3E%20%7B%0A%09return%20Math.random()%20%3E%200.5%3F%20%5B2%2C3%5D%20%3A%203%3B%0A%7D%3B%0A%0Avar%20b%20%3D%20%5B1%5D%3B%0A%0Ab.concat(a())%3B

[zpdDG4gta8XKpMCd]

in the left hand you have apples, in the right hand you have apples and oranges, now you are confused: when you put your hands together those oranges that you hold in the right hand don't turn into apples, why is that?

[darrylring]

Which is the slightly unhelpful way of saying that union types aren't currently compatible with overloaded functions (see #1805). Neither overload of concat accepts number | number[].

From lib.d.ts:

concat<U extends T[]>(...items: U[]): T[];
concat(...items: T[]): T[];

[zpdDG4gta8XKpMCd]

Pardon me, didn't know concat is designed so loose.

[darrylring]

I'd say it's the opposite of loose. In JavaScript you could do:

b.concat(2, [3,4], 5, [6]);

The way concat is defined in lib.d.ts forbids this. You can give it one or more T or T[], but not both.

[zpdDG4gta8XKpMCd]

that's what I meant by saying it's designed loose in JavaScript, didn't expect it to be this way, the original observation is valid, technically libs.d.ts should be revised, although personally I'd keep it the way it is, but it's a question of what you believe in

[darrylring]

Ah, gotcha. I also think lib.d.ts is probably correct.

[weswigham]

You can retype the original code like so:

declare type RandomResult = number | Array<number>;

var a = (): RandomResult => {
    return Math.random() > 0.5? [2,3] : 3;
};

var b: RandomResult[] = [1];

b.concat(a());

And it passes the typechecker. Possibly not necessarily for the expected reasons.

[mhegazy]

Looks like the original issue has been answered. please reactivate if you are running into other issues.

[pmowrer]

Here's an active thread on this problem: #6594