TypeScript 联合类型推断失败

Question

似乎有时 TypeScript(最新版本)无法缩小联合类型，即使存在类型保护。此行为是错误还是功能:

前言:

// some config
interface Config {
    name: string;
    option1?: number;
    option2?: boolean;
}

// arbitrary type
interface Entity {
    a: number;
    b: number;
}

// name aware type guard for entity property-to-config map
// some default config may be replaced with a property name
type TConfigSet<TData> = {
    [P in keyof TData]: (Config & { name: P }) | P;
}

// example of TConfigSet usage
const EntityConfigs: TConfigSet<Entity> = {
    a: {
        name: 'a',
        option2: true
    },
    b: 'b'
}

问题:

// this function compiles
function TypeLooseFieldToName(name: string | Config): string {
    if (typeof name === 'string') return name;
    else return name.name;
}

// this one doesn't
function TypeStrictFieldToName<TData>(name: keyof TData | { name: keyof TData }): keyof TData {
    if (typeof name === 'string') return name;
    else return name.name; // still keyof TData | { name: keyof TData }, should be shrinked to { name: keyof TData }
}

Answer 1

这似乎是类型检查器中的错误，因为 TypeScript 手册说“keyof T 类型被认为是 string 的子类型。”

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html

作为解决方法，可以反转类型保护以首先排除自定义类型:

function hasName(obj: string | { name: string }): obj is { name: string } {
    return typeof obj.name === 'string';
}

function getName<TData>(name: keyof TData | { name: keyof TData }): keyof TData {
    if (hasName(name)) return name.name;
    else return name;
}

// compiles with valid keys
getName<Entity>('a');
getName<Entity>({ name: 'a' });

// doesn't compile with invalid keys
getName<Entity>('z');
getName<Entity>({ name: 'z' });

您可以在 GitHub 中搜索 TypeScript Issues 并提交一个新问题(如果之前没有解决):

https://github.com/Microsoft/TypeScript/issues