How can I declare a key in a JS object with an ! before the colon?
MikroORM syntax for class
@Entity()
export class Post {
// Using @PrimaryKey() decorator to designate primary key
@PrimaryKey()
id!: number;
@Property({ type: "date", default: "NOW()" })
createdAt = new Date();
@Property({ type: "date", default: "NOW()", onUpdate: () => new Date() })
updatedAt = new Date();
@Property({ type: "text" })
title!: string;
}
When working with JavaScript, the code snippet you provided would result in a syntax error; however, this is actually TypeScript code. TypeScript extends JavaScript by adding type information. In TypeScript, the 'text!: string' declaration serves as a type definition that denotes:
text (typically not an object property in this context).string.Upon reviewing the complete class structure provided, here are the key attributes specified within the Post class:
A number typed id property, guaranteed to be initialized despite lacking explicit initialization within the class.
The createdAt attribute initiated using new Date() (TypeScript infers its type as Date based on this).
Initialization of updatedAt with new Date().
The title property, declared as a string type, similar to id, is definitely assigned although such initiation isn't visibly present in the class structure.
Utilization of various ORM-related decorators like @PrimaryKey, linking these attributes to the model and specifying the ORM operations concerning them.
Such assertions regarding definite assignment or initialization are commonly found in class implementations utilizing an ORM.
The exclamation mark operator ! in Typescript is known as the definite assignment assertion operator.
It serves as a way to instruct the compiler to "disregard the fact that this variable may be null, even though it shouldn't be".
In cases where a variable should not be null when it's a class field and you have configured the --strictPropertyInitialization flag in Typescript, using the exclamation mark can resolve issues like the following:
class MyClass {
let x; // This triggers an error stating "Property 'x' has no initializer"
}
class MyClass {
let x!; // No error reported
}
This feature proves useful when you are confident that the variable will be initialized later in your code.
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