Typescript: How to define only one type of javascript object

October 25, 2022

Suppose that i have an object like:

const obj = [
  { createdAt: "2022-10-25T08:06:29.392Z", updatedAt: "2022-10-25T08:06:29.392Z"},
  { createdAt: "2022-10-25T08:06:29.392Z", animal: "cat"}
]

I want to create a function to order by createdAt, which is the only field i’m sure it will be in the object.
The function will be something like:

export const sortArrayByCreatedAt = (arr: TypeArr) => {
    return arr.sort(function (a, b) {
        return new Date(b.createdAt).valueOf() - new Date(a.createdAt).valueOf();
    });
};

How can i define the type of arr?

Type TypeArr {
  createdAt: string
}

Is it good practice to define the type of the only known var?

I think that if someone else will see this function he will think that obj contains only createdAt, but i didn’t find a better solution.

>Solution :

I would define my TypeArr as an interface and the sort method as a generic method. So it wouldn’t change the return type.

export const sortArrayByCreatedAt = <T extends TypeArr>(arr: Array<T>) => {
    return arr.sort(function (a, b) {
        return new Date(b.createdAt).valueOf() - new Date(a.createdAt).valueOf();
    });
};

interface TypeArr{
    createdAt :string
}

const obj = [
  { createdAt: "2022-10-25T08:06:29.392Z", updatedAt: "2022-10-25T08:06:29.392Z"},
  { createdAt: "2022-10-25T08:06:29.392Z", animal: "cat"}
]

const sorted = sortArrayByCreatedAt(obj);

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