I'm not familiar with React, but why would that not be the case without this lib? {} !=== {}

EDIT: Maybe I understand. You're saying a parent component might do something like: foo.bar = 'blah'; and then doing a simple equality check foo === foo is not enough, you have to check that the properties have not changed either. Is that correct?

Yes, that's how for I've got, too. But then you have a problem if your React component has multiple attributes. Suddenly your check beckomes much larger:

  return (nextProps.attr1 !== this.props.attr1 ||
          nextProps.attr2 !== this.props.attr2 ||
          nextProps.attr3 !== this.props.attr3 ||
          nextProps.attr4 !== this.props.attr4);

Which would result in a lot of boilerplate code if you had to do this for every single attribute in every component in your application. It would be much simpler to just do this check:

  return nextProps !== this.props;

But that's impossible if you're using JSX, because the JSX compiler automatically inserts a normal JavaScript object into your code, instead of an immutable object from this library.

So from my point of view this immutable-js library and JSX seem incompatible, which is kind of weird since Facebook apparently uses JSX internally for their React code.