slowli / chai-bytes Goto Github PK

The types for .equalBytes are:

equalBytes(bytes: string | ArrayLike<number>): Assertion;

I was writing a test that ensures marshalling/unmarshalling works as expected. The object being marshalled has an optional field that's a Uint8Array.

// for reasons the type is `{ foo?: Uint8Array }`
const input = createInput('some seemingly unrelated data')

const marshalled = marshal(input)
const output = unmarshal(marshalled)

// now do some assertions

What I'd like to do, and can do without types:

expect(input).to.have.property('foo').that.is.ok
expect(output).to.have.property('foo').that.equalBytes(input.foo)

What I have to do with type checking:

if (!input.foo) {
  throw new Error('input had no `foo`')
}

expect(output).to.have.property('foo').that.equalBytes(input.foo)

The reason being that the type of input.foo is Uint8Array | undefined and undefined is not compatible with string | ArrayLike<number>, so by doing if (!input.foo) and throwing we let tsc know that input.foo is a thing and therefore a Uint8Array and compatible with ArrayLike<number>.

This feels a bit weird because we're using chai to test some properties and tsc to test others, though it's really just a quirk of the type system.

Chai itself specifies values to be tested as any. If this module did the same thing it would allow much more concise tests.