Type Annotations Can Only Be Used in TypeScript Files

Type annotations are one of TypeScript's superpowers. They provide a way to declare the type of a variable, function parameter, function return type, and other constructs. This can drastically improve code clarity, prevent type-related bugs, and aid in refactoring.

But it's essential to understand where and when you can use these annotations. As the title suggests, type annotations are exclusive to TypeScript and are not valid in plain JavaScript files.

What is a Type Annotation?

In TypeScript, a type annotation is a way to explicitly specify the type of a variable, function, or property. It follows the pattern:

let variableName: type;

For example:

let age: number = 30;
let name: string = "Alice";

JavaScript vs. TypeScript Files

Before we delve deeper, let's clarify the difference between .js and .ts files:

• .js stands for JavaScript files. These files contain plain JavaScript code.
• .ts stands for TypeScript files. These files contain TypeScript code, which can be a superset of JavaScript, including type annotations.

Only in .ts files can you use TypeScript-specific syntax, such as type annotations.

Why You Can't Use Type Annotations in JavaScript Files

If you attempt to use type annotations in a .js file, you will encounter a syntax error. Here's why:

• JavaScript Doesn't Understand Types: JavaScript is a dynamically typed language. Variables do not have a fixed type; they can change during runtime. Hence, native JavaScript doesn't recognize or utilize type annotations.
• Browsers Can't Interpret TypeScript Directly: Browsers understand JavaScript but not TypeScript. This is why TypeScript needs to be transpiled into JavaScript before being run in a browser. Using TypeScript syntax in a plain JavaScript file can lead to unexpected behavior or errors since the browser won't recognize that syntax.

The Solution: Convert .js to .ts

If you want to use type annotations (or any other TypeScript features) in a file, you'll need to rename the file from .js to .ts. Once it's a TypeScript file, you can freely add type annotations.

Handling Type Annotations with JSDoc in JavaScript

If you're not ready to transition to TypeScript but still want some type-checking in JavaScript, you can use JSDoc annotations. Many modern IDEs understand JSDoc comments and can use them to provide intelligent autocompletions and some level of type checking.

Here's an example:

/**
 * @param {number} a
 * @param {number} b
 * @returns {number}
 */
function add(a, b) {
    return a + b;
}

While not as powerful or strict as TypeScript's type annotations, JSDoc can still provide significant utility in a plain JavaScript context.

Conclusion

Type annotations are an excellent tool for enhancing code clarity and reliability. However, they are exclusive to TypeScript and can't be used directly in JavaScript files. If you find yourself wanting the benefits of type annotations in a JavaScript project, consider converting to TypeScript or using JSDoc for a similar (though less powerful) effect.