Typescript：爲什麼Array <number>與[number，number，number]是一樣的？

Question

我在打字稿這個非常簡單的代碼：

type SomeType = [number, number, number]; // Should be an array of exactly 3 numbers 

interface IThing { 
    someThing: SomeType 
} 

abstract class SomeClass { 
    abstract getThing(): IThing; 
} 

class ConcreteClass extends SomeClass { 
    getThing() { 
     return { 
      someThing: [4, 2, 2]; 
     } 
    } 
} 

在具體的類，當我分配到someThing[4, 2, 2]，打字稿抱怨Type number[] is not assignable to type [number, number, number]。爲什麼會這樣，我怎麼才能確保someThing是一個只有3個數字的數組？

Answer 1

如果你把類型標註在返回正常工作：

type SomeType = [number, number, number]; // Should be an array of exactly 3 numbers 

interface IThing { 
    someThing: SomeType 
} 

abstract class SomeClass { 
    abstract getThing(): IThing; 
} 

class ConcreteClass extends SomeClass { 
    getThing(): IThing { // annotation here 
     return { 
      someThing: [4, 2, 2] // no error here 
     } 
    } 
} 

在沒有類型註釋的你在下面的情況：

type SomeType = [number, number, number]; // Should be an array of exactly 3 numbers 

let x: SomeType; 
// Okay 
x = [1,2,3]; 
// Not okay 
let y = [1,2,3]; // inferred `number[]` 
x = y; 

即推斷回報類型與所需3tupple類型不兼容。