Element: setAttributeNS() method

setAttributeNS adds a new attribute or changes the value of an attribute with the given namespace and name.

Syntax

setAttributeNS(namespace, name, value)

namespace is a string specifying the namespace of the attribute.
name is a string identifying the attribute by its qualified name; that is, a namespace prefix followed by a colon followed by a local name.
value is the desired string value of the new attribute.

let d = document.getElementById("d1");
d.setAttributeNS(
  "http://www.mozilla.org/ns/specialspace",
  "spec:align",
  "center"
);

DOM methods dealing with element's attributes:

Not namespace-aware, most commonly used methods	Namespace-aware variants (DOM Level 2)	DOM Level 1 methods for dealing with `Attr` nodes directly (seldom used)	DOM Level 2 namespace-aware methods for dealing with `Attr` nodes directly (seldom used)
`setAttribute` (DOM 1)	`setAttributeNS`	`setAttributeNode`	`setAttributeNodeNS`
`getAttribute` (DOM 1)	`getAttributeNS`	`getAttributeNode`	`getAttributeNodeNS`
`hasAttribute` (DOM 2)	`hasAttributeNS`	-	-
`removeAttribute` (DOM 1)	`removeAttributeNS`	`removeAttributeNode`	-

setAttributeNS is the only method for namespaced attributes which expects the fully qualified name, i.e. "namespace:localname".

Specification
DOM Standard # ref-for-dom-element-setattributens①

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