打字稿歧视联合类型安全不适用于子对象？

Question

我正在使用 Typescript 4 尝试创建一个带有属性的对象，该对象使用区分联合，但类型安全似乎不起作用？

export enum StageType {
    PULL = 'pull',
    FILTER = 'filter',
}

export enum StageMetricProperty {
    IMPORTED = 'imported',
    SKIPPED = 'skipped',
    PIPED = 'piped',
    ERRORS = 'errors',
    PROCESSED = 'processed',
    SENT = 'sent',
    SAVED = 'saved',
}

// Pull stage metrics
export interface PullMetrics {
    [StageMetricProperty.IMPORTED]?: number;
    [StageMetricProperty.PIPED]?: number;
    [StageMetricProperty.ERRORS]?: number;
}
export interface PullReportStage { 
    type: StageType.PULL;
    name: string;
    args: string[];
    status: StageStatus;
    errors: string[];
    metrics: PullMetrics;
}

// Filter Stage Metrics
export interface FilterMetrics {
    [StageMetricProperty.SKIPPED]?: number;
    [StageMetricProperty.PIPED]?: number;
    [StageMetricProperty.ERRORS]?: number;
}
export interface FilterReportStage {
    type: StageType.FILTER;
    name: string;
    args: string[];
    status: StageStatus;
    errors: string[];
    metrics: FilterMetrics;
}

export type ReportStage =
    | PullReportStage
    | FilterReportStage;

根据上面的定义，我尝试创建我的 FilterReportStage 对象（如下），但是 Typescript 从不报告 FilterReportStage.metrics 属性中对象的任何错误，尽管它确实在顶部正确报告了错误级别 metric: FilterMetrics 对象？

const metric: FilterMetrics = {
    [StageMetricProperty.IMPORTED]: 123, // <-- Correctly parsed as invalid (as expected).
    [StageMetricProperty.PIPED]: 123, // <-- Valid
};

const sample = {
    type: StageType.FILTER,
    name: 'utils:command',
    args: [],
    errors: [],
    metrics: {
        [StageMetricProperty.IMPORTED]: 123, // <-- Invalid, Typescript does not report aby errors.
        [StageMetricProperty.PIPED]: 123, // <-- Valid
    } as FilterMetrics,
    status: StageStatus.PENDING
} as FilterReportStage;

有谁能指导我为什么会发生这种情况？

Answer 1

当你使用 type assertions（例如，as FilterMetrics）时，你是在告诉打字稿“我比你知道的多，所以不要在这里检查我的工作。假设它是类型我说是”。所以既然你告诉打字稿不要检查你的工作，它就不会。

如果你想要类型检查，那么使用标准类型（例如，: FilterReportStage），而不是类型断言：

const sample: FilterReportStage = {
    type: StageType.FILTER,
    name: 'utils:command',
    args: [],
    errors: [],
    metrics: {
        [StageMetricProperty.IMPORTED]: 123, // <--- it points out the error here
        [StageMetricProperty.PIPED]: 123,
    },
    status: StageStatus.PENDING
};

Playground link

打字稿歧视联合类型安全不适用于子对象？

Typescript Discriminated Unions typesafety not working on child object?

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