Support computed properties in class property declarations

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;
})();

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

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.

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

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;
})();

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.

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

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Support computed properties in class property declarations #1841

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Support computed properties in class property declarations #1841

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JsonFreeman commented on Jan 28, 2015

DanielRosenwasser commented on Feb 5, 2015

JsonFreeman commented on Feb 5, 2015

DanielRosenwasser commented on Feb 5, 2015

JsonFreeman commented on Feb 5, 2015

DanielRosenwasser commented on Feb 18, 2015

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