These are the 2 interfaces I currently have
export interface Contact {
first_name: string;
last_name: string;
emails?: (EmailsEntity)[] | null;
company: string;
job_title: string;
}
export interface EmailsEntity {
email: string;
label: string;
}
Can you explain what
emails?: (EmailsEntity)[] | null;EmailsEntity in parentheses?
How is it different from this notation: emails?: EmailsEntity[] | null;?
This piece of code doesn't actually serve any purpose. It's structured similarly to:
emails?: EmailsEntity[] | null;
The use of parentheses here is redundant. They are only needed when changing the order of operations. You can learn more about operator precedence to get a better understanding.
When it comes to (EmailsEntity)[] and EmailsEntity[], there is actually no distinction between the two in Typescript language. The use of parentheses () in this context serves to adjust the precedence of operators when working with certain type operations. Unless these parentheses are specifically defining a function, they hold no additional significance within the type definitions.
Check out this demonstration that shows the importance of parentheses in coding:
type MyUnion1 = { a: string } | EmailsEntity & { c: boolean }
// {a: string; } | { email: string; label: string; c: boolean; }
type MyUnion2 = ({ a: string } | EmailsEntity) & { c: boolean }
// ^ ^
// { a: string; c: boolean; } | { email: string; label: string; c: boolean; }
The & (intersection) operator takes precedence over the | (union) operator. Using parentheses can modify the order of operations.
Looking at the provided example, there seems to be no distinction. However, let's delve into another scenario:
const b: (string | number)[] = [1, 1];
const a: (string | number)[] = ['Hello!', 'Hello!'];
const c: (string | number)[] = [1, 'Hello!'];
In the above snippet, notice how the type annotation for the arrays is enclosed in parentheses. Such an array with this specific type can contain purely numbers, purely strings, or even a mixture of both.
Now, consider the following instance:
const d: string[] | number[] = [1, 1];
const e: string[] | number[] = ['Hello!', 'Hello!'];
// const f: string[] | number[] = [1, 'Hello!']; // ERROR
In this alternate case, the type declaration for the arrays does not involve parentheses. An array with such a type can consist of solely numbers or only strings, but it cannot accommodate a combination of both.
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