Typescript restrict type with no properties from accepting strings or arrays

Question

Is it possible to restrict param not to accept strings, arrays etc.?

interface foo {
    a?: number;
    b?: string;
}

function baz(param: foo) {
}

baz("hello");

Why do you care? If it fits your interface, why do you mind whether it's a string or an object? — Madara's Ghost
– Madara's Ghost, Commented Mar 3, 2017 at 16:47
We had a bug where we were passing the object's property instead of the object, and typescript compiled fine. — MichaelAttard
– MichaelAttard, Commented Mar 3, 2017 at 17:01

Saravana · Accepted Answer · 2017-03-04 02:38:25Z

5

You can do something like this to make baz accept at least an object:

interface foo {
    a?: number;
    b?: string;
}

interface notAnArray {
    forEach?: void
}

type fooType = foo & object & notAnArray;

function baz(param: fooType) {
}

baz("hello"); // Throws error
baz([]); // Throws error

fooType here is an Intersection Type.

edited Mar 4, 2017 at 2:38

answered Mar 3, 2017 at 16:24

Saravana

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6 Comments

Murat Karagöz Over a year ago

Isn't that cheating a bit? You are forcing that the parameter has to be an object.

T.J. Crowder Over a year ago

@MuratK.: Isn't that what the OP wants? But I would lean toward not making it a type, just function baz(param: foo & object)

Madara's Ghost Over a year ago

Worth noting that the object type is very new, you'll need TypeScript 2.2 to use it.

MichaelAttard Over a year ago

Nice one, is there a type that allows objects apart from array?

artem Over a year ago

@user3233089 yes, for some definition of array: interface NotAnArray { forEach?: void }

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