revision:
The Document.createAttributeNS() method creates a new attribute node with the specified namespace URI and qualified name, and returns it. The object created is a node implementing the Attr interface. The DOM does not enforce what sort of attributes can be added to a particular element in this manner.
createAttributeNS(namespaceURI, qualifiedName)
Parameters:
namespaceURI : a string that specifies the namespace URI to associate with the attribute. The namespaceURI property of the created attribute is initialized with the value of namespaceURI.
qualifiedName : a string that specifies the name of attribute to be created. The name property of the created attribute is initialized with the value of qualifiedName.
const node = document.getElementById("svg");
const a = document.createAttributeNS("http://www.w3.org/2000/svg", "viewBox");
a.value = "0 0 100 100";
node.setAttributeNode(a);
console.log(node.getAttribute("viewBox")); // "0 0 100 100"