Understanding the TypeScript Error: "Spread Types May Only Be Created from Object Types" (TS2698)
The TypeScript error "Spread types may only be created from object types" (TS2698) is a common issue encountered when working with spread syntax (...) in TypeScript. This error arises when you attempt to use the spread operator on a type that isn't an object type. Let's delve into the specifics and explore how to effectively resolve this error.
What are Spread Types in TypeScript?
Spread types, denoted by the ellipsis (...), allow you to create a new type by combining existing types. They are particularly useful for extending interfaces and types, adding new properties, or merging existing properties. However, this powerful tool has limitations: it only works with object types.
Why This Error Occurs: A Deep Dive
The TS2698 error signifies that you're trying to use the spread syntax on a type that isn't considered an object type by TypeScript. This often happens in these scenarios:
-
Using spread on primitive types: Attempting to spread primitive types like
string,number,boolean,symbol,null, orundefinedwill result in this error. These types are not object types. -
Using spread on union or intersection types containing non-object types: If a union or intersection type includes primitive types, spreading it will likely cause this error. The spread operator expects a consistent object structure.
-
Incorrect type inference: TypeScript's type inference might sometimes fail to correctly identify a type as an object type, leading to this error. Explicit type annotations can rectify this.
-
Spread on
anytype: While you can spread ananytype, doing so loses type safety which is one of TypeScript's key benefits. It's best to avoid this and resolve the underlying type issues.
How to Fix the "Spread Types May Only Be Created from Object Types" Error
The solution depends on the specific context of the error. Here are some common solutions:
1. Ensure you're spreading object types: Double-check that the type you're spreading is indeed an object type. This might involve creating a new type or refining your existing types.
Example:
// Incorrect: Spreading a number type
type IncorrectSpread = { a: number } & { ...10 }; // TS2698 error
// Correct: Spreading an object type
type CorrectSpread = { a: number } & { ...{ b: string } };
2. Handle union and intersection types carefully: If you have a union or intersection type, you might need to use conditional types or type guards to ensure you're only spreading object types.
Example:
type UnionType = string | { a: number };
// Incorrect direct spread of UnionType
type IncorrectSpreadUnion = { b: string } & { ...UnionType }
// Correct using type guards and conditional types (advanced scenario)
type SpreadablePartOfUnion<T> = T extends { a: number } ? T : never;
type CorrectSpreadUnion = { b: string } & { ...SpreadablePartOfUnion<UnionType> };
3. Use explicit type annotations: If TypeScript is failing to infer the correct type, explicitly annotating the type can help resolve the issue.
4. Refactor your code to avoid spreading primitive types: Instead of spreading primitives, consider alternative approaches like creating new objects with the desired properties.
5. Avoid unnecessary any types: If you're using any to circumvent the error, try to identify and fix the underlying type issue. It sacrifices TypeScript's type safety benefits.
Example Scenarios and Solutions
Scenario 1: Spreading a primitive in an interface extension
// Incorrect
interface User {
id: number;
}
interface Admin extends User, { ...10 } {} // TS2698 error
// Correct
interface Admin extends User, { adminLevel: number } {}
Scenario 2: Spread in a type alias with a primitive union member
// Incorrect
type MyType = string | { name: string };
type ExpandedType = { age: number } & { ...MyType }; // TS2698 error
//Correct: Use conditional types to handle the string case
type ExpandedTypeCorrect = {age: number} & ({name: string} extends MyType ? MyType : never)
By carefully examining your code and applying the appropriate solution, you can effectively resolve the "Spread types may only be created from object types" (TS2698) error and maintain the type safety and clarity of your TypeScript code. Remember to prioritize explicit type annotations for enhanced maintainability and readability.
