Extract nested property from array

Question

Given an array of tokens:

const myArray = <const>[
  {
    tokens: ["one", "two"],
  },
  {
    tokens: ["three"],
  },
  {
    tokens: [],
  },
  {
    somethingElse: {},
  },
];

I want to get a tuple for the all the different literals that the tokens property has. In the above example the result should be "one" | "two" | "three", however all my attempts yield any, unless I remove the third array item.

This is what I tried:

type ArrayItem = typeof myArray extends (infer T)[] ? T : never;
type ArrayItemWithTokens = Extract<ArrayItem, { tokens: [string] }>;
type TokenArrays = ArrayItemWithTokens extends { tokens: infer T } ? T : never;
type Token = TokenArrays extends (infer T)[] ? T : never;

HTN · Accepted Answer · 2021-04-23 13:58:58Z

2

const myArray = [
  {
    tokens: ["one", "two"],
  },
  {
    tokens: ["three"],
  },
  {
    tokens: [],
  },
  {
    somethingElse: {},
  },
] as const;

type Token = Extract<(typeof myArray)[number], {tokens: unknown}>['tokens'][number];

answered Apr 23, 2021 at 13:58

HTN

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1 Comment

d_inevitable Over a year ago

Nice! That works. Thanks. Didn't know you could put types within the [ ] op, makes so much sense though.

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