Fixed a bug when a containment feature is defined with a upper bound set to 1. In this case, it is possible to set the feature by using the set function of EObject, but the model cannot be serialized. The problem is located here (https://github.com/ghillairet/ecore.js/blob/master/src/xmi.js#L214):

if (root.eContainer.isKindOf('Resource')) {

Indeed, eContainer of the root object (in this case, the EObject set in the containment feature) is undefined (the set function of EObject does not check whether the reference feature is a containment feature or not).

You can test with the following model defined using Eclipse:

<?xml version="1.0" encoding="UTF-8"?>
<ecore:EPackage xmi:version="2.0" xmlns:xmi="http://www.omg.org/XMI" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:ecore="http://www.eclipse.org/emf/2002/Ecore" name="sample" nsURI="http://www.example.org/sample" nsPrefix="sample">
  <eClassifiers xsi:type="ecore:EClass" name="A">
    <eStructuralFeatures xsi:type="ecore:EReference" name="b" eType="#//B" containment="true"/>
  </eClassifiers>
  <eClassifiers xsi:type="ecore:EClass" name="B"/>
</ecore:EPackage>

This (meta)model defines two concepts A and B, with a containment feature A::b (lowerBound = 0, upperBound = 1). If you try to create a model conformed to this metamodel and then serialize it, it will fail.

Commit ac68cd2 fixes this bug.
Commit 6c62f2e adds a test for this bug.

Besides, commit 45fef67 ensures that for a containment feature whose upper bound is equal to 1, if an object is set and then a second object is set, the first one is unset, and its eContainer and eContainingFeature properties are cleared. Commit 1c89e74 adds a test for it.

By the way, https://github.com/ghillairet/ecore.js/blob/master/src/ecore.js#L191 checks if the reference feature is a containment feature, by checking its value (true or false) and type (boolean). In case of the above metamodel defined with Eclipse, the containment property is a string so isContainment is always false (the variable seems to be never used though).