typescript: define the type of an object that behaves like a map

Question

typescript: define the type of an object that behaves like a map

My current approach involves utilizing an object to store a map, where keys are strings and values are of a fixed type T.

Upon looking up a key in the object, the type inference automatically assigns it the type T. However, there is a possibility that it might be undefined.
In the given scenario, I anticipate the variable 'entry' to have the type number|undefined. Strangely though, Typescript infers it as number, which appears to be incorrect:

const data: {[index:string]: number} = {
    "aa34da": 1,
    "basd23": 2,
    "as34sf": 5
};

const entry = data["doesn't exist"];
console.log(entry);

Could this be possibly attributed to a bug within the type inference system?

I am acquainted with the ES6 Map that offers a get() method matching the exact signature I expect. Nevertheless, the Map structure does not integrate well with JSON serialization. As a preference, I would rather stick with using objects.

javascript typescript serialization

Answer 1

Answer №1

It's clear that there is no issue with type inference since the type of data is explicitly defined as "object containing numbers at any string key". To ensure TypeScript recognizes the possibility of the value being undefined, we specify it in the type declaration:

declare const data: { [index: string]: number|undefined };

For a more concise way to define this type, you can utilize the Record utility:

declare const data: Record<string, number | undefined>;

Record<K, T> Constructs a type with a set of properties K of type T.

Answer 2

It's clear that there is no issue with type inference since the type of data is explicitly defined as "object containing numbers at any string key". To ensure TypeScript recognizes the possibility of the value being undefined, we specify it in the type declaration:

declare const data: { [index: string]: number|undefined };

For a more concise way to define this type, you can utilize the Record utility:

declare const data: Record<string, number | undefined>;

Record<K, T> Constructs a type with a set of properties K of type T.

Answer 3

Answer №2

Through experimentation with the code, I have determined the correct method for declaring a map-like object in which certain keys may not point to values is as follows:

const table: {[key:string]: number|undefined} = {
    "abc123": 6,
    "def456": 9,
    "xyz789": undefined
}

const result = table["unknown key"];

Answer 4

Through experimentation with the code, I have determined the correct method for declaring a map-like object in which certain keys may not point to values is as follows:

const table: {[key:string]: number|undefined} = {
    "abc123": 6,
    "def456": 9,
    "xyz789": undefined
}

const result = table["unknown key"];

typescript: define the type of an object that behaves like a map

Answer №1

Answer №2

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