Polymorphic 'this' Type in TypeScript

TypeScript introduces several advanced types that enhance the language's capability for static type checking. One such feature is the polymorphic 'this' type, which provides a powerful way to write more reusable and type-safe code, especially in object-oriented programming. In this article, we will learn more about Polymorphic 'this' Type in TypeScript.

What is the Polymorphic 'this' Type?

The polymorphic this type is a way to address the current kind of class or interface with safety, unlike JavaScript's this keyword, which points to the current object instance, in TypeScript we have a proper representation of its data type through this, the significance of such typing arises when working with inheritance since one can have methods returning this, with correct types for derived classes.

Example 1: Fluent Interfaces and Method Chaining

Fluent interfaces and method chaining are two of the main uses for this polymorphic type, if you have a fluent interface, that means you can write code that is easier to read and understand through method chaining, the polymorphic 'this' type guarantees that every method in the chain returns a correct type, even if it is from a subclass.

In this example, every method in the QueryBuilder class returns this, so that methods can chain them, this type of this ensures that the type returned is always the current instance type; thus, any subclass of QueryBuilder will result in a return type from the sub-class and not the base class.

class QueryBuilder {
    private query: string = "";

    select(fields: string): this {
        this.query += `SELECT ${fields} `;
        return this;
    }

    from(table: string): this {
        this.query += `FROM ${table} `;
        return this;
    }

    where(condition: string): this {
        this.query += `WHERE ${condition} `;
        return this;
    }

    build(): string {
        return this.query.trim();
    }
}

const query = new QueryBuilder()
    .select("*")
    .from("users")
    .where("age > 18")
    .build();

console.log(query);  

Output

SELECT * FROM users WHERE age > 18

Example 2: Polymorphic 'this' in Inheritance

This polymorphic this type works quite well for inheritance, the moment a class extends another class, these types of methods will be able to be used and they will correctly infer what kind of object it is and allow subsequent method calls on derived classes.

In the given below example an additional method withEmail has been included in UserQueryBuilder which extends QueryBuilder, such methods in UserQueryBuilder return UserQueryBuilder as their return types due to polymorphic this (i.e., making possible continuous sequence of actions).

class QueryBuilder {
    protected query: string = "";

    select(fields: string): this {
        this.query += `SELECT ${fields} `;
        return this;
    }

    from(table: string): this {
        this.query += `FROM ${table} `;
        return this;
    }

    where(condition: string): this {
        this.query += `WHERE ${condition} `;
        return this;
    }

    build(): string {
        return this.query.trim();
    }
}

class UserQueryBuilder extends QueryBuilder {
    withEmail(email: string): this {
        this.query += `AND email = '${email}' `;
        return this;
    }
}

const userQuery = new UserQueryBuilder()
    .select("name, email")
    .from("users")
    .where("age > 18")
    .withEmail("[email protected]")
    .build();

console.log(userQuery);   

Output

SELECT name, email FROM users WHERE age > 18 AND email = '[email protected]'

Benefits of the Polymorphic this Type

1. Type Safety: It will ensures that method chains return the appropriate types, reducing the risk of runtime errors by maintaining type correctness throughout the method chain.
2. Extensibility: It will makes it easier to expand classes and support method chaining, while maintaining the chain’s integrity.
3. Code Readability: By using it code can become more understandable as well as expressive in nature.

Limitations and Considerations

Although it is a powerful feature, polymorphic this type has its limitations-

1. Complexity in Type Inference: In complex inheritance hierarchies, TypeScript may struggle with type inference. In such cases, manual type annotations might be necessary to ensure type correctness.
2. Limited to Class Methods: Basically, polymorphic this type is most useful within class-based methods and less valuable outside these contexts.

pankajbind

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Article Tags :

• JavaScript
• Web Technologies
• TypeScript
• TypeScript-Questions