TypeScript closure scope error: The name does not exist in the current scope

Question

does anyone know why test1 fails to compile?

class Y { public myMethod: any; };
class QQ { public test(name, fun: () => any) { } }

var qq = new QQ();
qq.test("Run test1", () => {

        var outer = 10;

        Y.prototype.myMethod = () => {

          // Error: The name 'outer' does not exist in the current scope
            outer = 11;
        }
});

But the following works:

   qq.test("Run test2", () => {

            var outer = 10;
            var fun = ()=> { outer = 11; };

            Y.prototype.myMethod = fun;
    });

The JavaScript version of the required code would look look like this:

qq.test("Run test1", function () {
    var outer = 10;

    Y.prototype.myMethod = function () {
        outer = 11;
    };
});

The outer function declares a variable "outer" within its closure, which should naturally be visible to the inner function.

Fenton · Accepted Answer · 2012-10-29 13:03:02Z

1

Shortened to just the salient points:

This is the JavaScript I think you are expecting.

var Y = (function () {
    function Y() { }
    Y.prototype.myMethod = function () {
    };
    return Y;
})();
var QQ = (function () {
    function QQ() { }
    QQ.prototype.test = function (name, fun) {
        fun();
    };
    return QQ;
})();
var qq = new QQ();
qq.test("Run test1", function () {
    var _this = this;
    _this.outer = 10;
    Y.prototype.myMethod = function () {
        alert(_this.outer);
    };
});
var y = new Y();
y.myMethod();

You need to change your TypeScript to get this output:

class Y { 
    public myMethod() {

    }
}

class QQ {
    public test(name, fun: () => any) { // updated signature
        fun(); // call the function
    }
}

var qq = new QQ();

qq.test("Run test1", () => {
        this.outer = 10; // use this.
        Y.prototype.myMethod = () => {
            alert(this.outer);
        }
});

var y = new Y();
y.myMethod();

And yes, TypeScript believes this.outer to be a problem in the alert statement, but compiles the correct JavaScript anyway. You can raise that as a bug at http://typescript.codeplex.com.

edited Oct 29, 2012 at 13:03

answered Oct 29, 2012 at 9:04

Fenton

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3 Comments

Zephyr Over a year ago

Sohnee, I don't quite follow. There are two lambda expressions here. The outer lambda defines a variable "outer" and then assigns the inner lambda to some object's prototype. The outer lambda defines a closure, which means that all objects within the outer closure should be visible within the inner closure. This is very standard JavaScript - nothing at all funky about it. I believe this is a bug in the TypeScript compiler.

Fenton Over a year ago

@NoelAbrahams clarification needed - I have posted a JavaScript example and questioned what difference you expect.

Zephyr Over a year ago

'@Sohnee, I've edited the question to show the expected result. The solution you have proposed does not seem natural to me, a "this" in a lambda seems strange. I'll raise this as a bug, if there is nothing more. Thanks.

Zephyr · Accepted Answer · 2013-02-01 09:31:11Z

0

This was a bug in TypeScript until verion 0.8.2 that has been fixed since then.

answered Feb 1, 2013 at 9:31

Zephyr

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