[英]How to return null instead of undefined when using arr.map(elem => dict[elem]) in typescript?
I wrote a simple function to replace values in an array according to a look-up dictionary object:我写了一个简单的 function 来根据查找字典 object 替换数组中的值:
// typescript
function recode(arr: any[], dict: Record<string, string> ) {
return arr.map(el => dict[el])
}
It works as expected.它按预期工作。 However, I want the function to return null
when array values have no match in the look-up dictionary.但是,当数组值在查找字典中不匹配时,我希望 function 返回null
。
So right now if I do:所以现在如果我这样做:
// array input
const myArr: string[] = ['eggplant', 'tomato', 'carrot', 'cabbage'];
// look-up dictionary
const myDictionary: Record<string, string> = {
eggplant: 'purple',
tomato: 'red',
carrot: 'orange'
};
function recode(arr: any[], dict: Record<string, string> ) {
return arr.map(el => dict[el])
}
// calling recode()
recode(myArr, myDictionary)
// returns
// ["purple", "red", "orange", undefined]
But I want the output to be但我希望 output 成为
// ["purple", "red", "orange", null]
Is there a simple enough way to achieve this, considering that I'm using typescript (not sure it makes a difference)?考虑到我使用的是typescript (不确定它是否有所作为),是否有足够简单的方法来实现这一点?
You can use the nullish coalescing operator ( ??
) to resolve null
in cases of undefined
(and use a generic type parameter to infer the type of values from the dict
parameter):在undefined
的情况下,您可以使用无效合并运算符 ( ??
)解析null
(并使用泛型类型参数从dict
参数推断值的类型):
const myArr: string[] = ['eggplant', 'tomato', 'carrot', 'cabbage'];
const myDictionary: Record<string, string> = {
eggplant: 'purple',
tomato: 'red',
carrot: 'orange'
};
function recode <T extends Record<string, any>>(
arr: readonly string[],
dict: T,
): (T[keyof T] | null)[] {
return arr.map(el => dict[el] ?? null);
}
const result = recode(myArr, myDictionary); // (string | null)[]
console.log(result);
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