https://github.com/OfficeDev/office-js/issues/1643

Updating customXmlParts.WORD.yaml

name: Updating customXmlParts
description: Issue
host: WORD
api_set: {}
script:
  content: |
    $("#run").click(() => tryCatch(run));

    async function run() {
      await Word.run(async (context) => {
        console.log("run----------");
        const namespace = "https://schema.example.org/someschema";
        const exampleContent1 = '<?xml version="1.0" standalone="no"?><c:Collection xmlns:c="' + namespace + '" xmlns="' + namespace + '"><c:Item>Test 1</c:Item></c:Collection>';
        const exampleContent2 = '<?xml version="1.0" standalone="no"?><c:Collection xmlns:c="' + namespace + '" xmlns="' + namespace + '"><c:Item>Test 1</c:Item><c:Item>Test2</c:Item></c:Collection>';
        const part = await new Promise<Office.CustomXmlPart>((resolve, reject) =>
          Office.context.document.customXmlParts.getByNamespaceAsync(namespace, async x => {
            resolve(x.value[0]);
          }));
        const partDoesNotExists = part === undefined;
        if (partDoesNotExists) {
          console.log("create new part");
          const create = await new Promise<Office.CustomXmlPart>((resolve, reject) =>
            Office.context.document.customXmlParts.addAsync(exampleContent1, x => {
              console.log(x);
              resolve(x.value)
            }));
          await context.sync();
          console.log("created");
          const createdResult = await new Promise<string>((resolve, reject) => {
            create.getXmlAsync(e => resolve(e.value));
          });
          console.log(createdResult);
        } else {
          console.log("amend existing part");
          const rootNode = await new Promise<Office.CustomXmlNode>((resolve, reject) => {
            part.getNodesAsync("/", x => resolve(x.value[0]));
          });
          const existing = await new Promise<string>((resolve, reject) => {
            rootNode.getXmlAsync(e => resolve(e.value));
          });
          console.log(existing);
          
          const root = await new Promise<Office.CustomXmlNode>((resolve, reject) => {
            part.getNodesAsync("/", x => resolve(x.value[0]));
          });
          const updated = await new Promise<void>((resolve, reject) => {
            //rootNode.setNodeValueAsync(exampleContent2, e=> resolve()); // this line crashes word;
            //rootNode.setTextAsync(exampleContent2, e=> resolve()); // this line crashes Word;
            root.setXmlAsync(exampleContent2, e => { console.log (e); resolve(); });
          });
          const updatedResult = await new Promise<string>((resolve, reject) => {
            Office.context.document.customXmlParts.getByNamespaceAsync(namespace, async x => {
              x.value[0].getXmlAsync(x => resolve(x.value));
            });
          })
          console.log("Updated result: ");
          console.log(updatedResult); // is not updated
        }
      });
    }

    /** Default helper for invoking an action and handling errors. */
    async function tryCatch(callback) {
      try {
        await callback();
      } catch (error) {
        // Note: In a production add-in, you'd want to notify the user through your add-in's UI.
        console.error(error);
      }
    }
  language: typescript
template:
  content: |
    <button id="run" class="ms-Button">
        <span class="ms-Button-label">Run</span>
    </button>
  language: html
style:
  content: |-
    section.samples {
        margin-top: 20px;
    }

    section.samples .ms-Button, section.setup .ms-Button {
        display: block;
        margin-bottom: 5px;
        margin-left: 20px;
        min-width: 80px;
    }
  language: css
libraries: |
  https://appsforoffice.microsoft.com/lib/1/hosted/office.js
  @types/office-js

  office-ui-fabric-js@1.4.0/dist/css/fabric.min.css
  office-ui-fabric-js@1.4.0/dist/css/fabric.components.min.css

  core-js@2.4.1/client/core.min.js
  @types/core-js

  jquery@3.1.1
  @types/jquery@3.3.1