Condition type if array includes all keys of interface

Question

interface IYears {
  one: string;
  two: string;
  three: string;
}

function transformYears(years: Array<keyof IYears>): [if all items of "years" includes in keyof IYears] ? IYears : Partial<IYears> {
  return years.reduce((acc, year) => ({
    ...acc,
    [year]: 'foo'
  }), {})
}

const yearsFirst = transformYears(['one', 'two']) // type of yearsFirst is Partial<IYears>

const yearsSecond = transformYears(['one', 'two', 'three']) // type of yearsFirst is IYears

How modified transformYears to match type of yearsFirst and yearsSecond? And possible check condition "if all items of "years" includes in keyof IYears" in ts?

Tobias S. · Accepted Answer · 2022-06-29T17:23:25.273

0

You need to make the function generic to get this to work.

function transformYears<
  Y extends (keyof IYears)[]
>(years: [...Y]): Exclude<keyof IYears, Y[number]> extends never 
  ? IYears 
  : Partial<IYears> 
{
  return years.reduce((acc, year) => ({
    ...acc,
    [year]: 'foo'
  }), {}) as any
}

If we store the passed array in the generic type Y, we can use Exclude to compute the difference between keyof IYears and Y[number]. If Y[number] contains all keys of IYears, this evaluates to never which we can check for in the condition.

Playground

After thinking about it, we can make it even a bit shorter:

function transformYears<
  Y extends (keyof IYears)[]
>(years: [...Y]): keyof IYears extends Y[number] ? IYears : Partial<IYears> {
  return years.reduce((acc, year) => ({
    ...acc,
    [year]: 'foo'
  }), {}) as any
}

Playground

edited Jun 29 '22 at 17:23

answered Jun 29 '22 at 17:12

Tobias S.

21,159
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Thank for cool ideas! Based on your answer make [improvements](https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgJIE0JygZ2QbwChkTkB7ECALmRzClAHMBuY0sAdzJroZBbYkwACygRqtek1YBfQoRgBXEAjDAKyenBA4YZKAFtM2HAB5ByACrIIAD0ggAJngxZcAGgvob9iE7wAglBQcACepgDWEKFkMFYAfJ6kyACyPg7OyFExcdZ2GXjoANogigYARtAAusgA-FbINAAK2GpwADamlvGE8QAUoW44NEUAdOPoVQCUNGlEyWJgilAgyIMmo2KOikh9Fsl9iAjua25TyAC88ch988n3JOOjR0kP90XrUFU0AOR6ZD99qQZFNXg98DJkHA8CkLFNZPIEBQ6KcTAAxYC4MCXTQhHR6QzGXB9Io-CgQH4nH6cAHTZjIAD0DM0oQADihYqjcBischgHgWlA2p1XCYeoQkTpsZ8cABlCCSxw4rT4-RGIYksmUSnIalcHXU0TiH50xnMsBsjlxGU8lH8tBEnBAA) – RocketArtyom Jun 29 '22 at 22:01
Мove types to the header is good idea? Also do not understand how work construction ```keyof IYears extends Y[number]```, especially what mean ```Y[number]``` for ts, like instead of ```number``` put index 0, 1, 2,...length of ```years``` and result compared with keyof IYears, where to look to find out more? – RocketArtyom Jun 29 '22 at 22:34
`Y` is a generic type which holds the array you pass as the argument. We can now call `Y[number]` to get a union of all the elements inside of `Y` (see [this answer](https://stackoverflow.com/questions/45251664/typescript-derive-union-type-from-tuple-array-values)). When we use `keyof IYears extends Y[number]` we essentially check if every key in `IYears` is also in the union of the elements inside the array. If there are more strings in `keyof IYears` than in `Y[number]` the first one would not extend the second one anymore. – Tobias S. Jun 29 '22 at 23:04
Thanks for help! I started to understand the syntax better – RocketArtyom Jun 30 '22 at 07:19

Condition type if array includes all keys of interface

1 Answers1