It's just an identifier like any other defined in types.ts: https://github.com/MathisBullinger/froebel/blob/main/types.ts#L1 https://github.com/MathisBullinger/

It's just an identifier like any other defined in types.ts: https://github.com/MathisBullinger/froebel/blob/main/types.ts#L1 https://github.com/MathisBullinger/froebel/blob/main/types.t... It's defined as type λ<TA extends any[] = any[], TR = any> = (...args: TA) => TR; meaning λ is just any function and λ<[number], string> would be the type of a function that takes a number as its only argument and returns a string.