[英]TypeScript type guard function infers wrong type when using array reduce
Returning the result of the reduce function gives a type error, but saving the result in a variable ( isValid
) and then returning the variable doesn't give any error.返回 reduce function 的结果会出现类型错误,但将结果保存在变量 (
isValid
) 中然后返回该变量不会出现任何错误。
Since both methods are semantically equivalent I wonder if this is a bug in TS type inference system.由于这两种方法在语义上是等价的,我想知道这是否是 TS 类型推理系统中的错误。
TypeScript Playground TypeScript 游乐场
interface Params {
a: string;
b: number;
}
const paramKeys = ["a", "b"] as const;
// Doesn't work -> Type 'string' is not assignable to type 'boolean'.
const isHydrationParamsValid = (params: any): params is Params => {
return paramKeys.reduce((_, curr) => {
if (!params[curr]) {
console.warn(`Hydration param ${curr} is missing`);
return false;
}
return true;
}, true);
};
// Works!
const isHydrationParamsValid2 = (params: any): params is Params => {
const isValid = paramKeys.reduce((_, curr) => {
if (!params[curr]) {
console.warn(`Hydration param ${curr} is missing`);
return false;
}
return true;
}, true);
return isValid;
};
There are already a lot of issues regarding the type inference for reduce in TS Github, the one collecting them all is Array method definition revamp: Use case collection #36554在 TS Github 中已经有很多关于 reduce 的类型推断的问题,将它们全部收集起来的是Array method definition revamp: Use case collection #36554
I believe your method falls into the same error category as the one in comment :我相信您的方法与评论中的方法属于同一错误类别:
// the return type here is correctly inferred as `boolean`
function works() {
return new Array('foo', 'bar').reduce(
(previousValue, currentValue) => !!(previousValue && currentValue),
true
)
}
function broken(): boolean {
return new Array('foo', 'bar').reduce(
(previousValue, currentValue) => !!(previousValue && currentValue),
true
)
}
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