TypeScript报错：类型'S'不满足'JSONSchema'约束，求解决方案

问题重现 #

你定义的TypeScript类型代码如下：

import type {
  APIGatewayProxyEvent,
  APIGatewayProxyResult,
  Handler
} from 'aws-lambda'
import type { FromSchema } from 'json-schema-to-ts'

export type ValidatedAPIGatewayProxyEvent<S> = Omit<APIGatewayProxyEvent,'body'> & { body: FromSchema<S> }
export type ValidatedEventAPIGatewayProxyEvent<S> = Handler<ValidatedAPIGatewayProxyEvent<S>,APIGatewayProxyResult>

export const formatJSONResponse = (
  statusCode: number,
  response: Record<string, unknown>
) => {
  return {
    statusCode: statusCode,
    body: JSON.stringify(response, null, 2)
  }
}

编译时触发错误：

Type 'S' does not satisfy the constraint 'JSONSchema'.
  Type 'S' is not assignable to type '{ readonly type?: ReadonlyDeep<JSONSchema6TypeName | JSONSchema6TypeName[] | undefined>; readonly const?: ReadonlyDeep<unknown> | undefined; ... 35 more ...; readonly format?: ReadonlyDeep<...>; }'.

8 export type ValidatedAPIGatewayProxyEvent<S> = Omit<APIGatewayProxyEvent,'body'> & { body: FromSchema<S> }

错误原因 #

json-schema-to-ts库的FromSchema类型要求传入的泛型参数必须是合法的JSONSchema类型，但你定义的两个泛型类型中，S没有添加任何类型约束，TypeScript无法确认S符合JSONSchema的结构要求，因此抛出约束不满足的错误。

修复步骤 #

1. 从json-schema-to-ts导入JSONSchema类型
2. 为泛型参数S添加extends JSONSchema的约束，明确限定S必须是合法的JSONSchema结构

修复后的完整代码 #

import type {
  APIGatewayProxyEvent,
  APIGatewayProxyResult,
  Handler
} from 'aws-lambda'
// 新增导入JSONSchema类型
import type { FromSchema, JSONSchema } from 'json-schema-to-ts'

// 为S添加extends JSONSchema约束
export type ValidatedAPIGatewayProxyEvent<S extends JSONSchema> = Omit<APIGatewayProxyEvent,'body'> & { body: FromSchema<S> }
// 同样给这里的S添加约束
export type ValidatedEventAPIGatewayProxyEvent<S extends JSONSchema> = Handler<ValidatedAPIGatewayProxyEvent<S>,APIGatewayProxyResult>

export const formatJSONResponse = (
  statusCode: number,
  response: Record<string, unknown>
) => {
  return {
    statusCode: statusCode,
    body: JSON.stringify(response, null, 2)
  }
}

这样修改后，TypeScript就能确认S是符合要求的JSONSchema类型，错误会被消除。

内容的提问来源于stack exchange，提问作者Heisenberg