What is TypeScript?

TypeScript is an open-source language created and managed by Microsoft. It serves as a superset of JavaScript, meaning any valid JavaScript is also valid TypeScript. TypeScript introduces optional static typing, classes, interfaces, and other features, making it easier to develop and maintain large applications.

A notable characteristic of TypeScript is its static typing system, which enables developers to identify errors during compilation rather than at runtime, simplifying debugging and testing. It also supports advanced features like modules, decorators, and async/await, which may not yet be fully implemented across all browsers.

How Does TypeScript Transpile to JavaScript?

As a compiled language, TypeScript must be converted to JavaScript before execution. The TypeScript compiler, a command-line utility, takes TypeScript code and outputs JavaScript code ready for execution.

When TypeScript code is written, it is saved in a .ts file. This file is processed by the TypeScript compiler, which generates a corresponding .js file containing the transpiled code. Additionally, the compiler produces a source map file (.js.map), linking the generated JavaScript back to the original TypeScript code, thus aiding in debugging.

The following steps outline the process the TypeScript compiler follows to transpile TypeScript to JavaScript:

1. Parsing

   The compiler begins by parsing the TypeScript code, forming an Abstract Syntax Tree (AST). This tree-like structure represents the code's organization, enabling the compiler to analyze and optimize it.

2. Type Checking

   Next, the compiler conducts type checking on the AST using the type annotations present in the code to deduce the variable types, function parameters, and return values. Any type errors identified will be flagged at this stage.

3. Emitting JavaScript

   After successful type checking, the compiler produces optimized JavaScript code that corresponds to the original TypeScript code. This generated code is minified, and any unused portions are discarded.

4. Generating Source Maps

   Finally, the compiler creates a source map file that connects the generated JavaScript code back to the original TypeScript code, allowing for easier debugging in compatible browsers or editors.

TypeScript Features Requiring Transpilation

Several TypeScript features necessitate transpilation to JavaScript, including:

1. Classes

   TypeScript allows the use of classes, which are not natively supported in JavaScript. When a class is defined in TypeScript, the compiler generates JavaScript that creates the class as a constructor function, with methods as properties on the prototype.

2. Interfaces

   Interfaces, which define a contract that a class or object must fulfill, are also supported by TypeScript but not by JavaScript. The TypeScript compiler produces JavaScript that defines an object conforming to the interface's specified properties and methods.

3. Enums

   TypeScript introduces enums, allowing the definition of sets of named constants. The compiler generates JavaScript that represents these constants as properties of an object.