Get a VSCode extension's version using the (undocumented) marketplace API.

ext-version.ts

// To use this in a `.js` file, just remove the type annotations.

/**
 * (Makes reverse engineered internal API calls from https://marketplace.visualstudio.com)
 * @param {string} extFullName Org name + "." + extension name. e.g: `"DeepSourceCorp.deepsource-vscode"`
 * @returns The latest version of the DeepSource extension from the marketplace, as a semver string.
 */
async function fetchLatestExtensionVersion(extFullName: string): Promise<string | null> {
  const resp = await fetch(
    "https://marketplace.visualstudio.com/_apis/public/gallery/extensionquery",
    {
      method: "POST",
      headers: {
        Accept: "application/json;api-version=7.2-preview.1;excludeUrls=true",
        "Content-Type": "application/json",
      },
      body: JSON.stringify({
        // this request body was taken from the browser's network tab when observing the requests
        // made by the VSCode marketplace web page.
        assetTypes: null,
        filters: [
          {
            criteria: [
              {
                filterType: 7,
                value: extFullName,
              },
            ],
            direction: 2,
            pageSize: 100,
            pageNumber: 1,
            sortBy: 0,
            sortOrder: 0,
            pagingToken: null,
          },
        ],
        flags: 55,
      }),
    },
  );

  const json: unknown = await resp.json();

  // The shape of the response was once again taken from the network tab.
  if (json && typeof json === "object" && "results" in json && Array.isArray(json.results)) {
    const result: unknown = json.results[0];
    if (
      !(
        result &&
        typeof result === "object" &&
        "extensions" in result &&
        Array.isArray(result.extensions)
      )
    )
      return null;

    const extension: unknown = result.extensions[0];
    if (
      !(
        extension &&
        typeof extension === "object" &&
        "versions" in extension &&
        Array.isArray(extension.versions)
      )
    )
      return null;

    const versions = extension.versions.filter((v) => !isPrelease(v));
    const version = versions[0];

    if (!(version && typeof version === "object" && "version" in version)) return null;
    return version.version;
  }

  return null;
}

/**
 * @param version A version object from the marketplace API.
 * @returns  `true` if the version is a pre-release, `false` otherwise.
 */
function isPrelease(version: unknown) {
  if (
    !(
      typeof version === "object" &&
      version !== null &&
      "properties" in version &&
      Array.isArray(version.properties)
    )
  ) {
    return false;
  }

  const properties: unknown[] = version.properties;

  return properties.some((p) => {
    return (
      typeof p === "object" &&
      p &&
      "key" in p &&
      "value" in p &&
      p.key === "Microsoft.VisualStudio.Code.PreRelease" &&
      p.value === "true"
    );
  });
}