Comparing AST nodes in Vitest/Jest/etc

[wms]

In my unit tests, I would like to assert that given some AST, the correct Node(s) are returned using something like expect(aNode).toBe(bNode). When I get my implementation right, the nodes are identical and the tests pass. However, as is more often the case, the assertion will fail. In the process of trying to show me the inequality, my test runner (Vitest in my case, but can imagine Jest would behave in a similar fashion) will crash as it attempts to pretty-print a cyclical structure.

Has anybody here had a similar experience and found some way of tweaking the test environment (or Services Module) so that the test runner does a shallower comparison/print-out when comparing AST Nodes?

[msujew]

Hey @wms,

I would assume that you're testing that your scoping/linking logic is working correctly and is resolving a reference to a specific AST node correctly. Is that right?

For that we have built something like this in our projects in the past for vitest:

// TypeScript declaration
declare module 'vitest' {
    interface Assertion<T = any> {
        toBeAstNode(expected: AstNode): void;
    }
}

// Implementation
expect.extend({
  toBeAstNode(received: AstNode, expected: AstNode, _options: unknown) {
    const { isNot } = this;
    return {
      pass: (received !== expected) === Boolean(isNot),
      message: () => {
        const addressExpected = expected 
          ? AstUtils.getDocument(expected).uri.toString() + '#' + sharedServices.workspace.AstNodeLocator.getAstNodePath(expected) 
          : '<<undefined>>';
        const addressReceived = received
          ? AstUtils.getDocument(expected).uri.toString() + '#' + sharedServices.workspace.AstNodeLocator.getAstNodePath(received) 
          : '<<undefined>>';
        return isNot
           ? `Received ${addressExpected} but expected another AstNode`
           : `Expected ${addressExpected}, but got ${addressReceived}`;
      }
    }
  },
});

// Usage
expect(refTarget).toBeAstNode(expectedTarget);

That way, you circumvent the issue completely by generating a completely custom message that uses the AST node path (i.e. a unique identifier for each node of an AST) for the error message.

[wms]

This pretty good, thank you. I was hoping to find some way of hooking directly into vitest's diff logic so I could tell it to skip over node properties like $container, but it looks like those things aren't directly surfaced in the vitest API.

You're right that testing scoping & linking logic is one use-case, but I also have other functions that work directly over the AST, and want to be able to use the standard matchers -- expect([...someNodes]).toContain(aNode), etc.

However, using this dedicated toBeAstNode matcher that prints the document URI and node path is a pretty good compromise, thank you.