When working with TypeScript, I encountered a strange issue. Can you help me understand why this code fails?
export class MyClass {
x:string = null //shows IDE warning (type 'null' is not assignable to 'String')
}
However, when I change it to this, it works fine:
export class MyClass {
x:string = null!
}
I'm puzzled by the use of null!. Is it some kind of guaranteed-to-be-not-null null?
! guarantees non-null values, eliminating null types and resulting in an empty union or never. The never type is unique in that it serves as a subtype of all types and can be assigned to any type (although it should never occur). This explains why no error is thrown.
For your reference, you can test this out yourself:
const x = null!; // hover over x to see its type as "never"
const y: string = 42 as never; // this won't result in an error because never can be assigned to anything
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