CODEWITHSUNDEEP
javascripttypescript2.0typechecking
prmph
prmph

Reputation: 8246

Typescript not enforcing types

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?

Upvotes: 1

Views: 650

Answers (1)

jJ'
jJ'

Reputation: 3078

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: {}; }'

Upvotes: 2

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