Typescript says object property possibly undefined altough is defined

Question

Maybe someone with extended ts knowledge can shed some light on me. I have an object which MAY have some keys - and I want to limit which keys.

In a function I want to initialize a key with a default value (empty array) if it does not exist and then add something to that array.

Typescript keeps complaining the property may be undefined altough thats not logically possible.

Example code:

type keys = "a" | "b" | "c";
let test: {[key in keys]?: string[]} = {};
    
function add(key: keys, value: any){
  test[key] = test[key] || [];
  test[key].push(value);
}

test.push is marked by tsc as "object possibly undefined".

This works neither:

function add(key: keys, value: any){
  test[key] = test[key] || [];
  if(test[key] !== undefined){
      test[key].push(value);
  }
}

Whatever I do - typescript insists the property might be undefined.

Playground

Answer 1

You can use the non-null assertion operator (!) to inform the compiler that the value is not undefined:

TS Playground

type Key = 'a' | 'b' | 'c';
type Test = Record<Key, string[]>;

let test: Partial<Test> = {};

function add (key: Key, value: any) {
  test[key] ??= [];
  test[key]!.push(value);
  //       ^
  // Use non-null assertion operator to inform
  // the compiler that the value is not undefined
}

---

Bonus: If you also "add" (define) all of the keys in the union, you can then use an assertion function to assert that they're all defined so afterward you no longer have to perform a check when using them:

See related: Required<Type>

TS Playground

type NonNullableValues<T> = { [K in keyof T]-?: NonNullable<T[K]> };

function assertHasValues <T>(obj: T): asserts obj is NonNullableValues<T> {
  for (const key in obj) {
    if (obj[key] == null) {
      throw new Error('Value is null or undefined');
    }
  }
}

type Key = 'a' | 'b' | 'c';
type Test = Record<Key, string[]>;

let test: Partial<Test> = {};

function add (key: Key, value: any) {
  test[key] ??= [];
  test[key]!.push(value);
  //       ^
  // Use non-null assertion operator to inform
  // the compiler that the value is not undefined
}

add('a', 1);
add('b', 2);
add('c', 3);

test.a; // string[] | undefined
test.b; // string[] | undefined
test.c; // string[] | undefined

assertHasValues(test);

test.a; // string[]
test.b; // string[]
test.c; // string[]