Argument of type {stringVariable} not assignable to object of type {string literal}

Question

I have the following code, which takes an options parameter:

const getCat = function (options: { format: "decimal" }) {
    return null
}
const options = { format: "decimal" }
const cat = getCat(options)

However, the const cat = getCat(options) runs into an error:

Argument of type '{ format: string; }' is not assignable to parameter of type '{ format: "decimal"; }'.
  Types of property 'format' are incompatible.
    Type 'string' is not assignable to type '"decimal"'.

How can I cast my options to be of the type TypeScript is looking for?

Directly pass it into the call: getCat({ format: "decimal" }) or declare an actual type for the parameter. — VLAZ
– VLAZ, Commented Mar 15, 2022 at 16:46
Because then the type doesn't get widened, because there's no reference through which that value could be changed. options is infered as { format: string } unless you add as const (either to the object or the string) [related: stackoverflow.com/a/62652353/3001761]. — jonrsharpe
– jonrsharpe, Commented Mar 15, 2022 at 16:49
@PatrickCollins Because then TS knows the type is { format: "decimal" } and not just any string. If you declare a type you can also do const options: MyType = /* ... */ to get the same effect. — VLAZ
– VLAZ, Commented Mar 15, 2022 at 16:50
Well that worked splended! So typescript won't just detect the typing works? — Patrick Collins
– Patrick Collins, Commented Mar 15, 2022 at 16:53

jonrsharpe · Accepted Answer · 2022-03-15 17:04:24Z

0

You have 2 choices:

Send the options right to the function:

const cat = getCat({ format: "decimal" })

Declare a type and have options be that type

type MyType = { format: "decimal" }
const options: MyType = { format: "decimal" }
const cat = getCat(options)

edited Mar 15, 2022 at 17:04

jonrsharpe

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answered Mar 15, 2022 at 16:54

Patrick Collins

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3 Comments

jonrsharpe Over a year ago

You have at least 3, because there are various uses of const assertions that work too.

Patrick Collins Over a year ago

Could you add the third?

jonrsharpe Over a year ago

I explained why this happens and various options (including these two) in the post I linked to above

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