ES6 Map of Flowtype

Question

什麼是適當的方式處理ecmascript-6Mapflowtype中的對象？

const animals:Map<id, Animal> = new Map(); 

function feedAnimal(cageNumber:number) { 
    const animal:Animal = animals.get(cageNumber); 

    ... 
} 

錯誤

const animal:Animal = animals.get(cageNumber); 
         ^^^^^^^^^^^^^^^^^^^^^^^^ call of method `get` 

const animal:Animal = animals.get(cageNumber); 
         ^^^^^^^^^^^^^^^^^^^^^^^^ undefined. This type is incompatible with 
const animal:Animal = animals.get(cageNumber); 
         ^^^^^^^ Animal 

Flowtype Map declaration

Answer 1

的animals.get(cageNumber)類型爲?Animal，不Animal。你需要檢查，它不是未定義：

function feedAnimal(cageNumber:number) { 
    const animal = animals.get(cageNumber); 

    if (!animal) { 
    return; 
    } 
    // ... 
}