TypeScript generic type and potential absence of a value

Question

TypeScript generic type and potential absence of a value

Recently, I've been delving into Facebook's immutable library and exploring their TypeScript bindings.

Consider this snippet of code:

const list: User[] = ...;
list.map(user => ...)

The lambda parameter user correctly has the type of User.

However, when I incorporate immutable's List and wrap my array like this:

import {Map, List} from "immutable";
List(list).map(user => ...)

To my surprise and confusion, the lambda parameter user is now inferred as User | undefined. Even trying to specify the type with List<User>(list) doesn't resolve the issue.

Looking at the library's .d.ts file, we see the following definition:

export function List<T>(array: Array<T>): List<T>;

So what could be causing this unexpected behavior?

generics typescript immutable.js

Answer 1

Answer №1

The reasoning behind this change can be found in the What's new in TypeScript 2.0 section under the Optional parameters and properties part:

Optional parameters and properties now automatically include undefined in their types, even without explicitly annotating it. This means that the two following types are equivalent:

// Compiled with --strictNullChecks
type T1 = (x?: number) => string;              // x has type number | undefined
type T2 = (x?: number | undefined) => string;  // x has type number | undefined

Edit

The definition file for immutable.js has been updated. If you check the signature of the map method, it now appears as follows:

map<M>(
    mapper: (value: V, key: K, iter: /*this*/Iterable<K, V>) => M,
    context?: any
): /*this*/Iterable<K, M>;

Prior to this change, if you look at an earlier commit, the signature looked like this:

map<M>(
    mapper: (value?: V, key?: K, iter?: /*this*/Iterable<K, V>) => M,
    context?: any
): /*this*/Iterable<K, M>;

In this version, the parameter value is optional.

To resolve this issue, updating the definition file is necessary.

Answer 2

The reasoning behind this change can be found in the What's new in TypeScript 2.0 section under the Optional parameters and properties part:

Optional parameters and properties now automatically include undefined in their types, even without explicitly annotating it. This means that the two following types are equivalent:

// Compiled with --strictNullChecks
type T1 = (x?: number) => string;              // x has type number | undefined
type T2 = (x?: number | undefined) => string;  // x has type number | undefined

Edit

The definition file for immutable.js has been updated. If you check the signature of the map method, it now appears as follows:

map<M>(
    mapper: (value: V, key: K, iter: /*this*/Iterable<K, V>) => M,
    context?: any
): /*this*/Iterable<K, M>;

Prior to this change, if you look at an earlier commit, the signature looked like this:

map<M>(
    mapper: (value?: V, key?: K, iter?: /*this*/Iterable<K, V>) => M,
    context?: any
): /*this*/Iterable<K, M>;

In this version, the parameter value is optional.

To resolve this issue, updating the definition file is necessary.

TypeScript generic type and potential absence of a value

Answer №1

Edit

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