AST visitors for TypeScript

npm install @brad-jones/tspoon@1.0.460




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AST visitors for Typescript.

API Reference

What Tspoon does

Typescript transpilation is usually source -> AST -> target. Tspoon uses Typescript's compiler API to allow pluggable pieces of logic (called visitor) to modify the AST before invoking the Typescript transpiler. The process will look like this source -> AST -visitors-> AST -> target. This technique enables early optimizations and error detection for custom language features.

In addition, Tspoon's validation api supports pre-validation code changes, allowing the developer to bypass otherwise unavoidable TypeScript diagnostics.

Users Documentation

Simple examples can be found here and here.

Getting started

Install tspoon using npm.

npm install tspoon

Currently, Tspoon exposes only a programmatic API. Meaning, it is used by other javacript code invoking it's transpile and validate methods.

tspoon.transpile(content, config)

content is a string containing the code to transpile, and config defines the visitors and transpilation parameters. The result is an instance of the TranspilerOutput interface, containing the transpiled code, a source map describing all changes done to the code, the diagnostics generated by the visitors and Typescript, and whether the operation failed or not.

// from src/transpile.ts
interface TranspilerOutput {
	code: string,
	sourceMap: RawSourceMap,
	diags: ts.Diagnostic[],
	halted: boolean
var tspoon = require('tspoon');
// from examples/poc/build.js
var config = {
    sourceFileName: 'src.ts',
    visitors: ... // insert visitors here
var sourceCode = fs.readFileSync(...);
var transpilerOut = tspoon.transpile(sourceCode, config);
fs.writeFileSync(path.join(__dirname, 'src.js'), transpilerOut.code, {encoding:'utf8'});


Documentation pending writing

How to write a visitor

A visitor is an instance of the visitor interface:

// from src/visitor.ts
interface Visitor {
	filter(node: ts.Node) : boolean;
	visit(node: ts.Node, context: VisitorContext, traverse: (...visitors: Visitor[]) => void): void;

Consider for example the following visitor:

// from examples/poc/deletePrivate.js
		return node.kind === ts.SyntaxKind.PropertyDeclaration
			&& node.modifiers
			&& node.modifiers.some(function(m){
				return m.kind === ts.SyntaxKind.PrivateKeyword;
	visit(node, context, traverse) {
		context.replace(node.getStart(), node.getEnd(), '');
		context.reportDiag(node, ts.DiagnosticCategory.Message, 'deleted field "' + node.getText()+'"', false);

This visitor only operates on nodes representing property declarations which have the private modifier. When such a node is encountered, it is deleted from the source code, and a diagnostic message notifying the delete action is emitted.

Visitor Composition

The code transformations often happens on multiple levels of the AST. For example, one needs to traverse all class declarations, but the actual transformation is performed on the respective syntax subtree (e.g. method bodies). In order to achieve this, TSpoon lets you to execute a visitor from another visitor:

var visitor = {
    filter(node) {
        return node.kind === ts.SyntaxKind.ClassDeclaration;

    visit(node, context, traverse) {
            filter(node) {
                return node.kind === ts.SyntaxKind.Block;

            visit(node, context, travers) {
                context.replace(node.getStart(), node.getEnd(), '');

(This example will remove execution blocks within classes.)

Developer Documentation

how to build and test locally from source

Clone this project locally. Then, at the root folder of the project, run:

npm install
npm test

how to run local continous test feedback

At the root folder of the project, run:

npm start

Then, open your browser at http://localhost:8080/webtest.bundle and see any changes you make in tests or code reflected in the browser


Currently Tspoon is in alpha mode. As such, it does not respect semver.


We use a custom license, see