TypeScript 中的“从不”类型不存在属性

TypeScript 中类型“never”上不存在属性

Property does not exist on type ‘never’ in TypeScript

never当我们尝试访问类型值的属性时，或者当 TypeScript 在分析我们的代码时感到困惑时，就会出现错误“属性在‘never’类型上不存在” 。要解决该错误，请使用方括号访问属性，例如employee['salary'].

下面是错误如何发生的示例。

索引.ts


type Employee = {
  salary: number;
};

let employee: Employee | null = null;

function setEmployee() {
  employee = { salary: 100 };
}

setEmployee();

// 👉️ employee.salary is equal to 100 here
// but TypeScript doesn't know

if (employee == null) {
  console.log('employee is nullish');
} else {
  // ⛔️ Error: Property 'salary' does not
  // exist on type 'never'.ts(2339)
  console.log(employee.salary);
}

错误的原因是
TypeScript
在分析我们的代码时感到困惑。

要解决该错误，请使用方括号而不是点符号来访问该属性。

索引.ts


type Employee = {
  salary: number;
};

let employee: Employee | null = null;

function setEmployee() {
  employee = { salary: 100 };
}

setEmployee();

// 👉️ employee.salary is equal to 100 here
// but TypeScript doesn't know

if (employee == null) {
  console.log('employee is nullish');
} else {
  // ✅ Works fine now (Use bracket notation)
  console.log(employee['salary']);
}

We used bracket notation instead of dot notation, which solved the error.

In short, do obj['myProperty'], instead of obj.myProperty.

Another cause of the error is declaring an empty array without assigning a type
to it.

index.ts


const obj = {
  years: [],
};

// 👇️ never[]
console.log(obj.years);

If you declare an empty array, make sure to type it explicitly to avoid any
confusing errors.

index.ts


type Example = {
  years: number[];
};

const obj: Example = {
  years: [],
};

// ✅ number[]
console.log(obj.years);

The three most common sources of the
never
type in TypeScript are:

having a conditional block that is never going to run, because it is
impossible for the condition to be met
the return type of a function that throws an error
TypeScript getting confused when analyzing our code

Conclusion #

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