proposal: "is" keyword

[alidorosty1234]

I'd like to suggest a language feature. The "is" keyword would check if an object is of a specific type (using duck typing).

interface myInterface {
    myVar1: number;
    myVar2: number;
}
var a;
console.log(a is myInterface);

translates into:

var a;
console.log(typeof(a.myVar1) != "undefined" && typeof(a.myVar2) != "undefined");

[fdecampredon]

In my opinion this is very hard to implement feature, and I doubt it's feasible.

Firstly it has been discussed in ECMAProposal about an is operator that would have the same behavior than Object.is so TypeScript should really not introduce an operator with this name in case it lands in ES7.
Also I doubt it's feasible. firstly in your example what if a was { myVar1: 'hello'; myVar2: 'world' } your check with 'undefined' would not work. also what about that kind of interface :

interface MyIface {
  new (): something;
  (a: stringng): somethingElse
}

Or very complex interface with union types etc etc.
The generated type check would take hundred of lines of code and could completely kill performance (especially if used in a loop).

[RyanCavanaugh]

fdecampredon nailed it -- this varies from "way too much code" to "simply impossible". TypeScript intentionally avoids adding runtime type information overhead.

[danquirk]

Note that we would be able to do something like this for classes with instanceof and type guards (#1283)

[jbondc]

Could be supported via a user-defined 'is function':

interface myInterface {
    myVar1: number;
    myVar2: number;
} is<T> => T.myVar1 !== undefined && T.myVar2 !== undefined
var a;
console.log(a is myInterface);

Emits

var a;
console.log(a.myVar1 !== undefined && a.myVar2 !== undefined);