In TypeScript, assertions are expressions used to inform the TypeScript compiler about the type of a variable or expression when the type is known to you but cannot be automatically inferred by the compiler. Assertions are a way to provide explicit type information, essentially overriding TypeScript's inherent type inference mechanisms.
The as [type] assertion specifies the type of an expression where the type is known but TypeScript cannot infer it.
let someValue:any="This is a string"; let strLength:number=(someValue asstring).length;
another example
interfacePerson{ name:string; age:number; } // Let's say we have a variable `unknownPerson` of type `any`: let unknownPerson:any={ name:"Alice", age:30}; // We know more about the type, so we assert it to the `Person` type: let person:Person= unknownPerson asPerson; console.log(person.name);// Output: Alice console.log(person.age);// Output: 30
Non-null assertions are a way to tell the TypeScript compiler that you are certain a value will not be null or undefined at the time it is used, even when TypeScript's type checking might consider such a case possible. This assertion can be particularly useful when you are confident about the presence of a value due to the logic of your application, but TypeScript's static analysis cannot infer this certainty.
// Assume we have an optional interface property which may be undefined interfaceUser{ id:number; name?:string; } let user:User={ id:123}; // Using the non-null assertion to access 'name' which is optionally undefined let userName:string= user.name!;// We assert that name is definitely not null or undefined here // Using 'as' type assertion let userName2:string= user.nameasstring;// Similarly asserting non-nullability console.log(userName);// Note: This will throw a runtime error if name is actually undefined
The satisfies keyword in TypeScript is a relatively new feature introduced in TypeScript 4.9.
It is used for type assertion, allowing you to check if a value satisfies a particular type without changing the resulting type of the expression.
This keyword is particularly useful in situations where you want to ensure that a value conforms to a specific interface or type, but you don't want to change its original type.
The satisfies keyword acts like a type assertion but does not perform type casting. Instead, it checks that the value can be assigned to the type specified after satisfies and if not, TypeScript will raise a type error at compile time. However, the expression itself still retains its original type.
interfaceUser{ id:number; name:string; email:string; } const user ={ id:1, name:"Alice", email:"alice@example.com", isAdmin:true, }; // Using 'satisfies' to ensure 'user' matches the 'User' interface user satisfies User; // 'user' still retains its original type with 'isAdmin' property console.log(user.isAdmin);// Outputs: true // If we mistakenly add or omit a property, TypeScript will throw an error const newUser ={ id:2, name:"Bob", // email is missing here isAdmin:false, } satisfies User;// TypeScript Error: Property 'email' is missing in type ...
Validating Shape without Narrowing: Use it when you need to validate that an object meets a specific type interface without wanting to narrow the type of the object to that interface.
Advanced Configuration Objects: Particularly useful for configuration objects where you expect certain properties to meet specific criteria.
Complex Conditions: Where multiple types or interfaces may apply to a single object, and you need to ensure it meets one of them without affecting its usability elsewhere in your code.
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