Typescript type inference inside for loop

Question

This looks like a problem with the typescript compiler, but not sure, maybe I do not understand typescript type inference. Can anybody explain this problem:

I have this simple code to do something on a list of "TypeAs".. the variable "item" is getting type "Any" when it is very clear that the list is of type "TypeA[]". So, to have type safety and intellisense I have to cast the type.

    var list: TypeA[] = this.getListOfTypeAs();
    for (var item in list) {
        var typedItem = (<TypeA> item); //clearly a problem with typescript

Why?

Community · Accepted Answer · 2017-05-23 11:51:09Z

11

As pointed out by Zev and stated in the documentation:

The for..in statement iterates over the enumerable properties of an object, in arbitrary order.

In TypeScript 1.5+ you can use for...of to iterate over the elements in an array:

var list: TypeA[] = this.getListOfTypeAs();
for (let item of list) {
    // item is an element of list in here
}

While in previous versions you can use a standard for loop:

var list: TypeA[] = this.getListOfTypeAs();
for (var i = 0; i < list.length; i++) {
    var typedItem = item[i];
}

edited May 23, 2017 at 11:51

CommunityBot

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answered Jun 6, 2015 at 20:33

David Sherret

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Comments

Community · Accepted Answer · 2017-05-23 11:43:39Z

7

Because Javascript's for...in iterates over the property names of an object/array, not the values of the properties.

edited May 23, 2017 at 11:43

CommunityBot

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answered Jun 6, 2015 at 19:55

user20416

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2 Comments

pabloelustondo Over a year ago

Thanks. I see. So, if I understood, still using "in", my code can be fixed using: for (var i in list) { var typedItem = list[i];

David Sherret Over a year ago

@pabloelustondo it's not recommended. Read this.

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