TypeScript Compiler API Advanced Usage and Examples

The TypeScript Compiler API provides powerful tools for programmatically interacting with TypeScript code. It allows developers to analyze, transform, and generate TypeScript code in sophisticated ways. This article delves into advanced usage scenarios and examples to illustrate the capabilities of the TypeScript Compiler API.

Getting Started with TypeScript Compiler API

Before diving into advanced usage, it's essential to set up the TypeScript Compiler API. This involves installing TypeScript and writing a basic script to interact with the API.

import * as ts from 'typescript';

const sourceCode = `let x: number = 1;`;
const sourceFile = ts.createSourceFile('example.ts', sourceCode, ts.ScriptTarget.ES2015);

console.log(sourceFile.text);

Parsing TypeScript Code

The Compiler API allows for parsing TypeScript code into an abstract syntax tree (AST). This can be useful for code analysis and transformation tasks.

const sourceFile = ts.createSourceFile('example.ts', sourceCode, ts.ScriptTarget.ES2015);

function visit(node: ts.Node) {
  if (ts.isVariableDeclaration(node)) {
    console.log(`Variable name: ${node.name.getText()}`);
  }
  ts.forEachChild(node, visit);
}

visit(sourceFile);

Transforming TypeScript Code

The API provides tools for transforming TypeScript code. This example shows how to use a transformer to modify code.

function transformer<T extends ts.Node>(context: ts.TransformationContext) {
  function visit(node: T): T {
    if (ts.isVariableDeclaration(node)) {
      return ts.updateVariableDeclaration(node, node.name, node.type, ts.createLiteral(42)) as T;
    }
    return ts.visitEachChild(node, visit, context);
  }
  return (rootNode: T) => ts.visitNode(rootNode, visit);
}

const result = ts.transform(sourceFile, [transformer]);
const printer = ts.createPrinter();
const transformedCode = printer.printFile(result.transformed[0] as ts.SourceFile);

console.log(transformedCode);

Generating TypeScript Code

Generating TypeScript code programmatically is another powerful feature of the API. Here's an example of how to create a new TypeScript file from scratch.

const newSourceFile = ts.createSourceFile(
  'newFile.ts',
  `const greeting: string = 'Hello, TypeScript!';`,
  ts.ScriptTarget.ES2015
);

const printer = ts.createPrinter();
const newCode = printer.printFile(newSourceFile);

console.log(newCode);

Handling Diagnostics and Errors

The Compiler API provides mechanisms for handling diagnostics and errors. This example demonstrates how to use diagnostics to report issues in TypeScript code.

const program = ts.createProgram(['example.ts'], {});
const diagnostics = ts.getPreEmitDiagnostics(program);

diagnostics.forEach(diagnostic => {
  const message = ts.flattenDiagnosticMessageText(diagnostic.messageText, '\n');
  console.log(`Error: ${message}`);
});

Conclusion

The TypeScript Compiler API offers a rich set of features for working with TypeScript code programmatically. By mastering these advanced capabilities, developers can create powerful tools for analyzing, transforming, and generating TypeScript code, leading to more efficient and flexible development workflows.