JavaScript AST annotation helpers
Last updated 7 years ago by sqs .
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astannotate is a node.js package that supports querying a JavaScript AST annotated with comments. It is useful for testing libraries that perform JavaScript source introspection because it lets you define test expectations inline in source code instead of having to put them in separate test files.

For example, suppose we want to test a function findDecl that determines the definition location of any JavaScript identifier in source code. You can use astannotate.js to help you test this function by first writing JavaScript code interspersed with comments as follows:

var /*DECL*/a = 8/*DECL:a*/;
var x = a/*REF:a*/ + 8;

And then define AST annotation visitors:

var astannotate = require('astannotate');
var decls = {};
var declVisitor = astannotate.rangeVisitor('DECL', null, function(range, name) {
  decls[name] = range;
var refVisitor = astannotate.nodeVisitor('REF', 'Identifier', function(identNode, declName) {
  // call our findDecl function on the Identifier node
  var decl = findDecl(identNode);
  if ( !== declName) {
    throw new Error('Expected Identifier at ' + identNode.start + '-' + identNode.end +
                    ' to refer to decl ' + declName + ', but findDecl gave ' +;

Then we can run this simple test case as follows, and it will throw an error if the output of findDecl doesn't match the expectations we've defined in the source code annotations.

astannotate.multi([declVisitor, refVisitor])(
  'var /*DECL*/a = 8/*DECL:a*/;' +
  'var x = a/*REF:a*/ + 8;'


Install the library by running npm install astannotate or by adding it to your package.json file.

var astannotate = require('astannotate');

The two main functions are astannotate.nodeVisitor and astannotate.rangeVisitor, which visit annotated AST nodes and ranges, respectively. Both functions let you define a comment directive and specify a test (constraint) function on the matched AST nodes.


var astannotate = require('astannotate');

// Simple directive.
astannotate.nodeVisitor('#', null, function(node, x) {
  console.log('Found node', node.type, 'with data:', x);
})('var a/*#:A*/;');
// outputs:
// Found node VariableDeclaration with data: A

// Use a different directive and a node test ('Identifier') to constrain the matched nodes.
astannotate.nodeVisitor('VAR', 'Identifier', function(node, x) {
  console.log('Found node', node.type,, 'with data:', x);
})('var a, b/*VAR:foo*/, c/*VAR:bar*/;');
// outputs:
// Found node Identifier b with data: foo
// Found node Identifier c with data: bar

// Visit annotated ranges.
astannotate.rangeVisitor('DECL', 'VariableDeclarator', function(range, x) {
  console.log('Found node', range.node.type, 'with data:', x);
})('var /*DECL*/a = 7/*DECL:foo*/;');
// outputs:
// Found node VariableDeclarator with data: foo

These examples are contained in example.js. See the test file for more examples.

Known issues

  • When using rangeVisitor, the range between the start/end comments must exactly contain a node in order for it to be defined on the callback range's node property. If there is extra whitespace or nested comments, such as /*##*/foo/*hello*//*##:*/, then the node will not be detected.

Running tests

Run npm test.


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