class Superhero {
name: string = ''
}
const superheroesList: Superhero[] = [];
const superheroesList2 = [] as Superhero[];
As I was exploring TypeScript, I stumbled upon these two distinct methods of declaring an array. This got me thinking whether it is simply a matter of syntax or if there is some deeper rationale that I am overlooking.
When you provide an example, you are essentially informing the compiler about the type of a variable at the time of its creation. The methods you illustrated are both valid and can be used interchangeably. However, the as operator is typically utilized within a code block to convert a variable from one type to another, as opposed to specifying the type during the variable declaration using :type.
Consider this pseudo code based on your Hero example:
function Y() : any {
return <something> // here, <something> represents an object known to be of type Hero, but the return type cannot be explicitly defined in the function signature due to external constraints
}
function performActions() {
if ((Y() as Hero).name == 'Frodo') {
print('The hero from LOTR has been located')
}
}
y
If you want to spot the distinction, take a look at this code snippet:
class Hero {
name: string = ''
}
// `hero` is specified with type `Hero`, causing an error due to `extra` not being part of `Hero`.
const hero: Hero = {name: '', extra: ''};
// The next declaration does not result in an error.
// Here, we are converting the object into type Hero.
// The object meets the requirements of the `Hero` type as it contains the necessary properties.
const hero2 = {name: '', extra: ''} as Hero;
Run example on TypeScript Playground
For further information, visit
Expressing the same concept in two different ways. When utilizing the as keyword, a type assertion is performed, like demonstrated here:
const heroes2 = [] as Hero[];
This instructs the compiler to create the heroes2 array and interpret it as a Hero type array. Alternatively,
const heroes: Hero[] = [];
is an explicit declaration, considered the standard practice. Ultimately, both methods are essentially just syntactic sugar, so use whichever one you find more readable.
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