Support computed properties in class property declarations

[JsonFreeman]

We can emit computed properties class properties, using the following scheme:

class C {
    ["hello"] = 0;
    static ["bye"] = 0;
}

emits as:

var C = (function() {
    var __t1 = "hello";
    function C() {
       this[__t1] = 0;
    }
    C["bye"] = 0;
    return C;
})();

[JsonFreeman]

We would only do this for initialized properties. For uninitialized properties, we would have to give an error.

[DanielRosenwasser]

So if you had something like

var x = 10;

class C {
  [x++] = x;
}

I'm assuming that every time the constructor is invoked, the x++ expression gets evaluated.

[JsonFreeman]

No, that is not the semantics I had in mind. It would get evaluated when the class is evaluated, which happens only once.

[DanielRosenwasser]

Why? We don't cache the initializers anyway.

TypeScript input:

var x = 0;
function f(): number {
    return x++;
}

class Greeter {
    id: number = f();
    constructor() {
    }
}

JavaScript output:

var x = 0;
function f() {
    return x++;
}
var Greeter = (function () {
    function Greeter() {
        this.id = f();
    }
    return Greeter;
})();

[JsonFreeman]

Yeah but we would cache the property name. It feels different from the initializer. But I see what you're saying, we could move the name into the constructor too, unless anyone has objections to that.

[DanielRosenwasser]

As per our design meeting last week, we've decided that we just won't support this because the semantic implications can be counterintuitive.