Typescript区分联合类型安全不适用于子对象

Question

我使用Typescript 4试图创建一个具有使用区分并集的属性的对象，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;

使用上面的定义，我尝试创建FilterReportStage对象(如下(，但Typescript从不报告FilterReportStage.metrics属性内对象的任何错误，尽管它确实正确地报告了顶级metric: FilterMetrics对象的错误？

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;

有人能告诉我为什么会发生这种事吗？

Answer 1

当您使用类型断言(例如as FilterMetrics(时，您会告诉typescript"我比你更清楚，所以不要在这里检查我的工作。假设它就是我所说的类型；。所以，既然你已经告诉打字稿不要检查你的作品，它就不会。

如果您想要类型检查，则使用标准类型(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
};

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