英文:
How to create a guard in Typescript for keys in a nested object?
问题
以下是代码的中文翻译部分：
type PossibleKeys = 'a' | 'b' | 'c';type MappedObject = {test: {[key in PossibleKeys]?: { foo: boolean };}}const mappedObject: MappedObject = {test: {a: { foo: true },b: { foo: false },}}function mappedObjectHasKeyDefined(possibleKey: PossibleKeys): possibleKey is keyof MappedObject['test'] {return possibleKey in mappedObject.test;}const c: PossibleKeys = 'c';const array: PossibleKeys[] = ['a', 'b', 'c'];array.forEach((newT: PossibleKeys) => {// 所有这些类型保护似乎都不起作用if(mappedObject.test[newT] !== undefined&& newT in mappedObject.test&& mappedObject.test&& mappedObject.test[newT]&& Object.prototype.hasOwnProperty.call(mappedObject.test, newT)&& mappedObjectHasKeyDefined(newT)) {// 对象可能为 'undefined'。const a = mappedObject.test[newT].foo;}})
英文:
I have a nested object that contains an object with multiple optional properties. I would like to create a typeguard to prevent the Object is possibly undefined. I do not know if this is possible or that I should cast it.
type PossibleKeys = 'a' | 'b' | 'c';type MappedObject = {test: {[key in PossibleKeys]?: { foo: boolean };}}const mappedObject: MappedObject = {test: {a: { foo: true },b: { foo: false },}}function mappedObjectHasKeyDefined(possibleKey: PossibleKeys): possibleKey is keyof MappedObject['test'] {return possibleKey in mappedObject.test;}const c: PossibleKeys = 'c';const array: PossibleKeys[] = ['a', 'b', 'c'];array.forEach((newT: PossibleKeys) => {// all of these guards somehow do not seem to workif(mappedObject.test[newT] !== undefined&& newT in mappedObject.test&& mappedObject.test&& mappedObject.test[newT]&& Object.prototype.hasOwnProperty.call(mappedObject.test, newT)&& mappedObjectHasKeyDefined(newT)) {// Object is possibly 'undefined'.const a = mappedObject.test[newT].foo;}})
答案1
得分: 0
TypeScript当前不会在obj[key]形式的属性访问的外观类型上进行缩小，当key不是单一字符串字面类型时。如果key只是string类型，或者如果它是字符串字面类型的联合类型（如PossibleKeys），或者如果它是泛型类型，编译器将不会缩小属性的类型。这仍然是一个待解决的功能请求，跟踪在microsoft/TypeScript#10530。
目前的解决方法是将属性复制到一个新变量中，因为变量可以通过检查来缩小类型。以下是一种方法：
array.forEach((newT: PossibleKeys) => {const t = mappedObject.test; // 可行const u = t[newT]; // 可行if (u) {const a = u.foo // 可行}})
英文:
TypeScript doesn't currently narrow the apparent types of property accesses of the form obj[key] when key is not of a single string literal type. If key is just of type string, or if it's a union of string literal types (like PossibleKeys) or if it's a generic type, the compiler will not narrow the type of the property. This is still an open feature request, tracked at microsoft/TypeScript#10530.
Currently the workaround is to copy the property into a new variable, since variables can be narrowed via checks. Here's one way to do it:
array.forEach((newT: PossibleKeys) => {const t = mappedObject.test; // okayconst u = t[newT]; // okayif (u) {const a = u.foo // okay}})
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