Summary - Transformer

Write a logging injector with a transformer.

Logging injector summary

A logging injector transformer combines three pieces: you produce a console.log(...) statement with ts.factory, apply the transformer to the source with ts.transform, and print the transformed source with ts.createPrinter. The injector can add a statement at the top of the file or at the start of each function body.

const result = ts.transform(sf, [transformer]);
const printer = ts.createPrinter();
process.stdout.write(printer.printFile(result.transformed[0]));

Exercise Logging injector

Create a SourceFile from const x = 1;. Write a transformer that adds a console.log("called") statement at the start of the source file. Produce the statement with ts.factory, apply with ts.transform, and print with ts.createPrinter.

Show example solution

import ts from "typescript";

const sourceText = "const x = 1;";
const sf = ts.createSourceFile("app.ts", sourceText, ts.ScriptTarget.Latest, true);

const transformer: ts.TransformerFactory<ts.SourceFile> = (context) => {
  return (node) => {
    const logStmt = ts.factory.createExpressionStatement(
      ts.factory.createCallExpression(
        ts.factory.createPropertyAccessExpression(
          ts.factory.createIdentifier("console"),
          ts.factory.createIdentifier("log")
        ),
        undefined,
        [ts.factory.createStringLiteral("called")]
      )
    );
    return ts.factory.updateSourceFile(node, [logStmt, ...node.statements]);
  };
};

const result = ts.transform(sf, [transformer]);
const printer = ts.createPrinter();
process.stdout.write(printer.printFile(result.transformed[0] as ts.SourceFile));
result.dispose();

Test yourself

Which trio comes together for a logging injector?
- getSourceFile + getText + console.log
- createSourceFile + JSON.stringify + writeFile
- getTypeChecker + writeFile + readFile
- ts.factory + ts.transform + ts.createPrinter
Show answer

ts.factory + ts.transform + ts.createPrinter — You produce the log statement with ts.factory, apply it with ts.transform, and print the result with ts.createPrinter. Together they give a full injector.

Create a printer

const result = ts.transform(sf, [transformer]);
const printer = ts.____;

Show answer

const result = ts.transform(sf, [transformer]);
const printer = ts.createPrinter();

Transformer summary

ts.factory

Produces AST nodes
ts.transform

Applies the transformer
ts.createPrinter

Writes a node to text
visitEachChild

Transforms child nodes

Next lesson: Type queries

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