Creating a namespace resolver for document.evaluate

ns-resolver.ts

// from the doc or node being parsed

const namespaceResolver = doc.createNSResolver(doc)

// from XML you control

const namespaceResolverDoc = new DOMParser().parseFromString(
  `<TEI xmlns="http://www.tei-c.org/ns/1.0"/>`,
  'application/xml'
)

const namespaceResolver = namespaceResolverDoc.createNSResolver(
  namespaceResolverDoc
)

// from a Document created from scratch

/*
const namespaceResolverDoc = new Document()

namespaceResolverDoc.appendChild(
  namespaceResolverDoc.createElementNS('http://www.tei-c.org/ns/1.0', 'TEI')
)

const namespaceResolver = namespaceResolverDoc.createNSResolver(
  namespaceResolverDoc
)
*/

// from a map
const namespaces = new Map<string | null, string>([
  [null, 'http://www.tei-c.org/ns/1.0'],
])

const namespaceResolver = (prefix: string | null) =>
  namespaces.get(prefix) || null

// or 

const namespaceResolver = {
  lookupNamespaceURI: (prefix: string | null) => namespaces.get(prefix) || null

}