Is TypeScript bending the rules?
var x: number = 5
var y: Object = x
It seems strange that a number is being assigned to an Object. Maybe x is being automatically converted to an object, but a simple check reveals otherwise:
if (!(y instanceof Object)) {
console.log(typeof y)
}
This code snippet will print:
number
Just to clarify:
$ tsc --version
Version 1.8.10
When it comes to type compatibility in TypeScript, it follows a structural subtyping approach rather than nominal typing. To illustrate this point, consider the following interface definitions:
interface IFoo { X: number }
interface IBar { X: number; Y: number }
Does IBar extend IFoo? Not quite.
However, is IFoo compatible with IBar? Indeed it is.
The members of IFoo form a subset of IBar members, allowing for the assignment of any IBar to IFoo. The reverse operation is not possible:
var x: IFoo;
var y: IBar;
x = y // perfectly valid
y = x // results in a type error due to missing Y member in x
In TypeScript, all types can be considered compatible with Object, akin to treating it as an empty interface. This feature enables smooth interaction with JavaScript libraries by passing any valid TypeScript value to functions accepting Object.
I recommend delving into the details of Type Compatibility explained in the documentation, particularly focusing on the distinction between Subtype vs Assignment.
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