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StypeScript 无法推断类型,在符合逻辑或逻辑的 或
原标题：TypeScript failing to infer type is not undefined after type guard with logical or
  • 时间：2024-07-24 12:18:35
  •  标签：
  • typescript
In the code below, in the else block, it is clear that b cannot be undefined (a||b is truthy and a is falsy so b must be truthy). Why does Typescript produce the error b is possibly undefined , and how can I avoid this error? function test(a: string | undefined, b: string | undefined) { if (a || b) { if (a) { const x = a.length } else { const y = b.length } } }
问题回答
You can debug it like this: function test(a: string | undefined, b: string | undefined) { type A1 = typeof a; // string | undefined type B1 = typeof b; // string | undefined if (a || b) { type A2 = typeof a; // string | undefined type B2 = typeof b; // string | undefined if (a) { type A3 = typeof a; // string type B3 = typeof b; // string | undefined const x = a.length } else { type A4 = typeof a; // string | undefined type B4 = typeof b; // string | undefined const y = b.length } } } Basically, (a || b) is not able to narrow down the types. The conditional flow analysis looks for for !== undefined and !== null checks, you should re-write it as follows: function test(a: string | undefined, b: string | undefined) { if (a !== undefined ) { const x = a.length; // number } else if(b !== undefined){ const y = b.length; // number } }
时间：2024-07-24 13:00:33


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