Auto-initialisation of string literal types in some circumstances

[danielearwicker]

Regarding issue #1003. Given:

interface A { P: "P"; }

All of the following are valid:

var v1: A = { P: undefined };
var v2: A = { P: null };
var v3: A = { P: <any>5 };
var v4: A = { P: "P" };

But (consistent with TS 1.3) this is not, because it doesn't explicitly initialise P:

var v5: A = { };

I suggest the compiler behave as if P had been initialised, as for v4. This is the overwhelmingly likely option and avoids the redundancy of both stating the type A and repeating the definition of P.

For classes:

class C1 implements A { }
class C2 { P: "P" }
class C3 { P: "P" = "P" }

I suggest for C1 and C2 the compiler behave as for C3, for a similar reason: that's almost certain to be what I'd have to write out by hand.

[RyanCavanaugh]

This is kind of scary. What would the behavior for this code be, for example?

interface A { P: "P"; }
interface B { P: "Q"; }
interface C { X: "Y"; }
var x: A|B = {};
var y: A|C = {};

[danielearwicker]

Behaviour will be exactly as in the prototype of @Nevor. i.e. this suggestion makes no difference to those examples. As {} does not satisfy any of those types, you'll get a compilation error.

The point of this suggestion is purely to avoid redundancy when the user has already definitely chosen A, B or C, because they've restricting the type that the object literal is being assigned to, and therefore it would be pure ceremony for them to have to specify any implied literal constant(s).

In your examples, the context doesn't provide any such certainty, the compiler can't do anything to fill in any blanks, and so would not.

[mhegazy]

This looks like a lot of magic. it is not clear when this would happen and when it would not. how can users opt-out of this auto-initialization? what if the object literal has some property declarations, does the auto-initialization still occur? what about optional properties? what about properties that does not have default values...

[saschanaz]

I think an auto-completion list will be better here.

[mhegazy]

+1 for @saschanaz proposal.

[saschanaz]

#606 discusses auto-completion of string literals so maybe we can talk more there.