typescript complaining about valid javascript

Question

This pretty much standard way of defining a javascript object (or namespace of functions) fails in typescript, saying that publicFunction does not exist on type {}. Is this by design?

var MyObject = function (){
  var that = {};

  var privateFunction = function () {};
  that.publicFunction = function () {};

  return that;
}();

PS: Found a "fix" to it by changing the first line to var that = <any>{}; but I was wondering why is this necessary.

TypeScript has internal and external modules, you don't necessarily need to use this "standard way". — user1233508
– user1233508, Commented Feb 14, 2015 at 16:31
Yes, I'm aware of those but I already have tons of valid javascript in my existing project. — Pablo Fernandez
– Pablo Fernandez, Commented Feb 14, 2015 at 23:32

André Werlang · Accepted Answer · 2015-02-14 16:33:17Z

2

TypeScript infers that to have type {}, an empty object. Probably it doesn't make much sense to have an object with any properties, though.

var that = {};

You've have worked around by assert it to have type any. Another way to do it is:

var that;
that = {};

This way TypeScript doesn't infer variable type.

To take profit of TS advantages, you're better at declaring all object members at once. If you can't, you could use an interface:

interface MyObjectInterface {
    publicFunction();
}

var MyObject = function (){
  var that = <MyObjectInterface>{};

  var privateFunction = function () {};
  that.publicFunction = function () {};

  return that;
}();

answered Feb 14, 2015 at 16:33

André Werlang

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Comments

Niet the Dark Absol · Accepted Answer · 2015-02-14 16:21:54Z

1

Knowing nothing about typescript, I'm going to hazard a guess that, judging by its name, it is strictly-typed. This means you can't define properties on objects that weren't there when the object was created.

<any>{} overrides this restriction, giving you an object that you can define arbitrary properties on.

The "correct" way would probably be:

var that = {
    publicFunction: function () {}
};

answered Feb 14, 2015 at 16:21

Niet the Dark Absol

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1 Comment

user1233508 Over a year ago

Your guesses are all correct. One more thing to mention is that returning an <any>{} will make it impossible for the compiler to check further usages of that returned object...

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