Showing posts from October, 2018

The TypeScript "extends" Keyword Doesn't Extend Types

I was looking through the built in TypeScript definition file "lib.es5.d.ts" and found the definition "Pick". //From T pick a set of properties K type Pick<T, K extends keyof T> = {     [P in K]: T[P]; }; In more words: Pick<T,K> is a new type of object whose keys are the set K , which has to be a subset of the keys in T . But when I tried to understand what it was from the code itself, I had some trouble, and the source of that trouble is simply the extends  keyword. We usually use ' A extends B ' to mean  A  has all the keys in  B  and maybe some more, which the English also suggests. But in reality  K must be a subset of T 's keys. What is going on? What is a Type? A type is a set of values based on some rule. e.g. Colour = { red orange yellow ... } If A is a subtype of B, values that match A are a subset of values that match B. The definition of the subtype therefore has to be a narrower version of the