TypeError: Cannot convert undefined or null to object in waitForAllAttributesAndContinue

[dengribar]

Summary

👋 After updating Blaze from 2.7.1 to 2.8.0 in our Meteor (v2.14) app, we got Cannot convert undefined or null to object error in waitForAllAttributesAndContinue function (ref):

In our case it happens when an HTML element has only one attribute and it's value is calculated by a Template helper function that returns undefined.

Steps to reproduce

1. Create a Blaze app with command: meteor create --blaze simple-todos-blaze --prototype
2. Add disabled helper to hello Template in main.js module:

Template.hello.helpers({
  disabled() {
    return undefined;
  },

3. Add disabled attribute to button element in hello Template in main.html module:

<template name="hello">
  <button {{disabled}}>Click Me</button>

4. Run the app

Actual behaviour

The app crashes in browser with the described error

Expected behaviour

The element should be rendered with no attributes

[jankapunkt]

Should be easily fixed by making attrs = {}, I can make a PR

[jankapunkt]

The issue actually comes from Blaze._expandAttributes which should return and empty Object, instead of null, when no atts were found to be expandable. Otherwise multiple functions will fail (including waitForAllAttributesAndContinue)

Edit: note the issue only occurs when the element has computed atts but no other atts being set. For example, <button title="foo" {{disabled}}>Click Me</button> would not cause the error.

Anyway, it's fixed: #458

[dengribar]

This issue is fixed in Blaze 2.9.0, thanks to everyone involved! 🥇