TS1173: ‘extends’ clause must precede ‘implements’ clause

TypeScript is a powerful superset of JavaScript that adds static types to the language. This means you can define types (which specify what kind of data a variable can hold) for your variables, functions, and objects. By using types, developers can catch errors during development instead of at runtime, leading to more robust code.

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In this article, we will delve into the TypeScript error TS1173: ‘extends’ clause must precede ‘implements’ clause. To clarify our understanding, let’s break down the concepts involved in this error.

Understanding Types in TypeScript

Types in TypeScript are used to specify the kind of data a variable can store. For example, you can have types like string, number, or even custom types defined using interfaces or classes.

For instance:

let name: string = "John Doe";
let age: number = 30;

In this example, name is of type string, and age is of type number.

What is TS1173: ‘extends’ clause must precede ‘implements’ clause?

The error TS1173: ‘extends’ clause must precede ‘implements’ clause occurs when you define a class that uses both extends and implements clauses in the wrong order.

Example of the Error

Consider the following code snippet that will trigger this error:

class ExampleClass implements ExampleInterface extends BaseClass {
    // class body
}

Here, the extends clause is placed after the implements clause. TypeScript will raise an error indicating that you must place the extends clause first.

Correcting the Error

To fix this error, simply place the extends clause before the implements clause like this:

class ExampleClass extends BaseClass implements ExampleInterface {
    // class body
}

Now the error TS1173: ‘extends’ clause must precede ‘implements’ clause will be resolved, and your class will compile without issues.

Important to Know!

• The order of the clauses is crucial in TypeScript class definitions. Always remember to position extends before implements.
• Familiarize yourself with both the class inheritance (extends) and interface implementation (implements) concepts to prevent such errors.

Frequently Asked Questions

Q: What happens if I forget the order of extends and implements?
A: You will encounter the TS1173: 'extends' clause must precede 'implements' clause error, indicating a syntax issue in your class definition.

Q: Can I have multiple interfaces in an implements clause?
A: Yes, you can implement multiple interfaces. For example:

class MultiInterfaceClass extends BaseClass implements FirstInterface, SecondInterface {
    // class body
}

Important Things to Know

1. Syntax Order: Always order extends before implements in class declarations.
2. Interface vs. Class: Understand the difference between interfaces (which can define a contract) and classes (which can provide implementation).
3. Type Checking: TypeScript performs type checking at compile time, which helps catch errors before the code runs.

Recap On TS1173: ‘extends’ clause must precede ‘implements’ clause

Whenever you encounter the error TS1173: ‘extends’ clause must precede ‘implements’ clause, remember to check the order of your class declarations. It’s key to ensuring your TypeScript code compiles correctly and behaves as intended.

Learning TypeScript can be challenging, but understanding the nuances like the TS1173: ‘extends’ clause must precede ‘implements’ clause error is part of the journey. Embrace the learning process, and don’t hesitate to reach for resources like gpteach to enhance your skills. Happy coding!