CODEWITHSUNDEEP
typescript
Dace Zarina
Dace Zarina

Reputation: 6872

This in TypeScript arrow definitions?

I wrote such object literal in TypeScript:

var object = {
    message: "Say",
    say: () => {
        return this.message;
     }
};

And I got such generated JavaScript:

var object = {
    message: "Say",
    say: function () {
        return _this.message;
    }
};

Shouldn't there be such line before return statement:

 var _that = this;

as I am using arrow function expression?

Upvotes: 2

Views: 958

Answers (2)

Ryan Cavanaugh
Ryan Cavanaugh

Reputation: 221212

=> is a bit of a danger here because, critically, it binds this to the enclosing scope's this. In the correct code generation, the var _this = this; line goes above the object literal and your say function simply returns undefined.

You only ever want to use this in a => expression when you actually want to refer to this that would be present in the enclosing scope. In this particular case, you don't (you want the interior scope this, i.e. the object literal itself).

Upvotes: 2

Murat Sutunc
Murat Sutunc

Reputation: 953

You're correct. It's missing the :

var _this = this;

This is discovered after the release(0.8) and is currently fixed on the developer branch.

Note: Also I think you wanted to write 

var object = {
    message: "Say",
    say: function () {
        return () => this.message;
    }
};

which will actualy print Say when it's run. See: http://wiki.ecmascript.org/doku.php?id=harmony:arrow_function_syntax

Upvotes: 2

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