demo/typescript.js
function checkElementTypeOfIterable(iterable, errorNode) {
            var elementType = getElementTypeOfIterable(iterable, errorNode);
            // Now even though we have extracted the iteratedType, we will have to validate that the type
            // passed in is actually an Iterable.
            if (errorNode && elementType) {
                checkTypeAssignableTo(iterable, createIterableType(elementType), errorNode);
            }
            return elementType || anyType;
        }
function checkElementTypeOfIterable(iterable, errorNode) {
            var elementType = getElementTypeOfIterable(iterable, errorNode);
            // Now even though we have extracted the iteratedType, we will have to validate that the type
            // passed in is actually an Iterable.
            if (errorNode && elementType) {
                checkTypeAssignableTo(iterable, createIterableType(elementType), errorNode);
            }
            return elementType || anyType;
        }
When errorNode is undefined, it means we should not report any errors.