How do I create a type from a string array in TypeScript?

Since Typescript 3.4, you could use as const and generate a union type from an array as follows

const colors = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet'] as const;
export type Color = typeof colors[number]; // 'red'|'orange'|'yellow'|'green'|'blue'|'indigo'|'violet'