Assert that for every value of a const array, an object has a key

Question

const supported = ["a", "b", "c"];

const someObject = {a: 2, b: 3}; // I want TS to throw: error, has no 'c'!

I've got a const array of strings. How do I assert that an object's keys cover all the values?

What should happen for const someObject = {a: 2, b: 3, c: undefined};? — mjwills
– mjwills, Commented Oct 26, 2020 at 9:55
You could do: const allKeysExist = supported.forEach(key => someObject.hasOwnProperty(key)); // true or false — Joel
– Joel, Commented Oct 26, 2020 at 10:01
@zerkms Perfect, thank you. Please post this as a solution, so I can accept it. And out of pure curiosity, is it doable without helper types? — John Smith
– John Smith, Commented Oct 26, 2020 at 10:06

zerkms · Accepted Answer · 2020-10-26 10:19:40Z

3

You could do

const supported = ["a", "b", "c"] as const;

const someObject: Record<typeof supported[number], number> = {a: 2, b: 3};

It enforces all values to be numbers and the object to have all the keys from supported.

You surely can relax the type of the values to be unknown or any.

If you for some reason don't want to use the built-in Record<K, V> type you can always expand it to { [P in typeof supported[number]]: number }.

Important note: the supported value must be known compile time.

Playground: https://www.typescriptlang.org/play?ssl=1&ssc=38&pln=1&pc=43#code/MYewdgzgLgBBCuAHRIBOUCmATGBeGA2gEQCGRANDEQEYVXBEC6MJEMokUA3ALABQ-DtDggAthgDy1AFYZgUAFwwASnLRYAPFACeiDCABmcJCnTYCYeKOoZUjSpeu2AfHhgBvEkoBMlakoBmAF8uGAB6MJgASRgAdxIwWAAVAGUYKBB0gAtUEFilW1zUSizWGDBMgHJgSoBCIA

Update:

a solution to have some keys optional:

const supported = ["a", "b", "c", "d", "e"] as const;

type Keys = typeof supported[number];
type OptionalKeys = "d" | "e";
type MandatoryKeys = Exclude<Keys, OptionalKeys>

const someObject: Record<MandatoryKeys, number> & Partial<Record<OptionalKeys, number>>  = {a: 2, b: 3, c: 42};

https://www.typescriptlang.org/play?#code/MYewdgzgLgBBCuAHRIBOUCmATGBeGA2gEQCGRANDEQEYVXB1FaMZEC6MJEMokUA3AFgAUCKgBPRBhgBpDOO74JUkADM4SFOmwEw8ALbUMqNkOHLpAeURQAluBIAbOQrxVmMAD5VWZizABZEjAsEig0cRdFGABRAA9gR3gsDAAeKMprOwdneQgAPhERXmg4EH0MS2oAKwxgKAAuGAAlOrQsVKCQsIiMmD1DY3yYADIYAAUSdFsnVNbQVA6s+zAnPoGjVHzhtwBvEiaAJkpqJoBmSmAmgBZDgF9+IA

edited Oct 26, 2020 at 10:19

answered Oct 26, 2020 at 10:04

zerkms

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1 Comment

John Smith Over a year ago

What if I wanted all of the keys be present in the array, but not the entire array should be covered? I.e. adding d would throw, but not adding c is fine.

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