const T = t.type({ a: t.unknown })
assert.strictEqual(T.is({}), true) // property 'a' is missing, but the type check still passes

const T = t.type({ a: t.unknown })

// property 'a' is missing but must be present
assert.strictEqual(T.is({}), false) 

// property 'a' is present, and undefined is a valid value for unknown
assert.strictEqual(T.is({ a: undefined }), true) 

// property 'a' is present, and null is a valid value for unknown
assert.strictEqual(T.is({ a: null }), true) 

type T = { a: unknown }

// won't compile
const T1: T = { } 

// good
const T2: T = { a: undefined } 

// good
const T3: T = { a: null }