Typescript: Generate types from an object with the interface

Question

It's the second part for my question that was asked here. I've been helped with the solution for my previous problem, but once I added the interface for the alphabet object, Typescript stopped detecting that I provided the incorrect value.

 interface ICharMap { lower: number upper: number } const alphabet: {[key: string]: ICharMap} = { 'a': {lower: 97, upper: 65}, 'b': {lower: 98, upper: 66} } type Char = keyof typeof alphabet; function printSome(char: Char){ console.log(char) } printSome('c') \\ Typescript does not detect that the incorrect value is passed. 

Answer 1

You could do something like this:

interface ICharMap { lower: number upper: number } type Letter = 'a' | 'b' const alphabet: Record<Letter, ICharMap> = { 'a': {lower: 97, upper: 65}, 'b': {lower: 98, upper: 66} } function printSome(char: Letter){ console.log(char); console.log(alphabet[char]); // no error; TS knows that char is in alphabet } printSome('c') // error 

Answer 2

It is because your alphabet has {[key: string]} so when you do keyof typeof alphabet it returns string, with that you are allowed to pass any string.

If you modify your script like this then it will tell typescript that you should pass only 'a' and 'b'

interface ICharMap { lower: number upper: number } type keys = 'a' | 'b'; const alphabet: {[key in keys]: ICharMap} = { 'a': {lower: 97, upper: 65}, 'b': {lower: 98, upper: 66} } type Char = keyof typeof alphabet; function printSome(char: Char){ console.log(char) } printSome('c')