typescript '属性不存在于类型'从不'
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'Property does not exist on type 'never'
提问by Ray Booysen
This is similar to #40796374but that is around types, while I am using interfaces.
这与#40796374类似，但这是关于类型的，而我使用的是接口。
Given the code below:
鉴于以下代码：
interface Foo {
name: string;
}
function go() {
let instance: Foo | null = null;
let mutator = () => {
instance = {
name: 'string'
};
};
mutator();
if (instance == null) {
console.log('Instance is null or undefined');
} else {
console.log(instance.name);
}
}I have an error saying 'Property 'name' does not exist on type 'never'.
我有一个错误，说“属性”名称在“从不”类型上不存在。
I don't understand how instance could ever be a 'never'. Can anyone shed some light on this?
我不明白实例怎么可能是“从不”。任何人都可以对此有所了解吗？
Thanks in advance.
提前致谢。
采纳答案by Saravana
Because you are assigning instanceto null. The compiler infers that it can never be anything other than null. So it assumes that the else block should never be executed so instanceis typed as neverin the else block.
因为您正在分配instance给null. 编译器推断它永远不会是null. 所以它假设 else 块永远不应该被执行，所以instance就像never在 else 块中一样输入。
Now if you don't declare it as the literal value null, and get it by any other means (ex: let instance: Foo | null = getFoo();), you will see that instancewill be nullinside the if block and Fooinside the else block.
现在，如果你不声明为文本值null，和通过任何其他方式得到它（例如：let instance: Foo | null = getFoo();），你会看到，instance会null的，如果块内Foo的其他块内。
Never type documentation: https://www.typescriptlang.org/docs/handbook/basic-types.html#never
永远不要输入文档：https: //www.typescriptlang.org/docs/handbook/basic-types.html#never
Edit:
编辑：
The issue in the updated example is actually an open issue with the compiler. See:
更新示例中的问题实际上是编译器的未解决问题。看：
https://github.com/Microsoft/TypeScript/issues/11498https://github.com/Microsoft/TypeScript/issues/12176
https://github.com/Microsoft/TypeScript/issues/11498 https://github.com/Microsoft/TypeScript/issues/12176
回答by Nitzan Tomer
This seems to be similar to this issue: False "Property does not exist on type 'never'" when changing value inside callback with strictNullChecks, which is closed as a duplicate of this issue (discussion): Trade-offs in Control Flow Analysis.
这似乎与此问题类似：False "Property does not exist on type 'never'" 当使用 strictNullChecks 更改回调内的值时，该问题已作为此问题的副本（讨论）关闭：控制流分析中的权衡。
That discussion is pretty long, if you can't find a good solution there you can this:
那个讨论很长，如果你找不到一个好的解决方案，你可以这样做：
if (instance == null) {
console.log('Instance is null or undefined');
} else {
console.log(instance!.name); // ok now
}
回答by Eric Grotke
I had the same error and replaced the dot notation with bracket notation to suppress it.
我有同样的错误，并用括号表示法替换了点表示法以抑制它。
e.g.: obj.name -> obj['name']
例如：obj.name -> obj['name']
回答by blue-hope
if you write Component as React.FC, and using useState(),
如果你把组件写成 React.FC，并使用 useState()，
just write like this would be helpful:
像这样写会有所帮助：
const [arr, setArr] = useState<any[]>([])
