Typescript not enforcing types

Question

I have this very simple code:

let viewsDictionary: { [key: string]: Object } = {
    abc: {}
};
type ViewName = keyof typeof viewsDictionary;

let result: ViewName;

result = "category";

TypeScript version 2.2.2 does not complain that result can only have the value "abc". Why?

Are you sure you mean Object with an uppercase O?

user663031
– user663031

2017-05-04 05:00:41 +00:00
Commented May 4, 2017 at 5:00 — user663031
– user663031, Commented May 4, 2017 at 5:00

jJ' · Accepted Answer · 2017-05-06 03:48:00Z

2

You are explicitly giving viewsDictionary type { [key: string]: Object }. The fact that you are assigning a compatible value does not change its type and so typeof viewsDictionary stays { [key: string]: Object } and keyof are any string.

You can verify that by assigning

viewsDictionary = { category: {} };

which works fine as well.

Just remove the explicit type declaration, so TS infers the type itself and it will work as expected:

let viewsDictionary = {
    abc: {}
};
type ViewName = keyof typeof viewsDictionary;

let result: ViewName;

result = "category"; error

now complains that Type '"category"' is not assignable to type '"abc"'.

Update:

You can also specify the type explicitly (from comments):

let viewsDictionary: {abc: {}} = {
    abc: {},
    def: {}, // error 
};
type ViewName = keyof typeof viewsDictionary;

let result: ViewName;

result = "def"; // still error

This will complain when you add another key to viewsDictionary that Type '{ abc: {}; def: {}; }' is not assignable to type '{ abc: {}; }'

edited May 6, 2017 at 3:48

answered May 4, 2017 at 4:45

jJ'

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1 Comment

prmph Over a year ago

I thought as much; the technical term for this is type widening. But if I do as you say, problem now is that viewsDictionary itself can have values of any type. I want to ensure that the values of viewsDictionary are all of a specific type, and also create a type off of its keys.

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