Converts Google service user OAuth2 credentials into an access token in Cloudflare-compatible JS

getGoogleAuthToken.js

/**
 * Get a Google auth token given service user credentials. This function
 * is a very slightly modified version of the one found at
 * https://community.cloudflare.com/t/example-google-oauth-2-0-for-service-accounts-using-cf-worker/258220
 * 
 * @param {string} user   the service user identity, typically of the 
 *   form [user]@[project].iam.gserviceaccount.com
 * @param {string} key    the private key corresponding to user
 * @param {string} scope  the scopes to request for this token, a 
 *   listing of available scopes is provided at
 *   https://developers.google.com/identity/protocols/oauth2/scopes
 * @returns a valid Google auth token for the provided service user and scope or undefined
 */
async function getGoogleAuthToken(user, key, scope) {
  function objectToBase64url(object) {
    return arrayBufferToBase64Url(
      new TextEncoder().encode(JSON.stringify(object)),
    )
  }
  function arrayBufferToBase64Url(buffer) {
    return btoa(String.fromCharCode(...new Uint8Array(buffer)))
      .replace(/=/g, "")
      .replace(/\+/g, "-")
      .replace(/\//g, "_")
  }
  function str2ab(str) {
    const buf = new ArrayBuffer(str.length);
    const bufView = new Uint8Array(buf);
    for (let i = 0, strLen = str.length; i < strLen; i++) {
      bufView[i] = str.charCodeAt(i);
    }
    return buf;
  };
  async function sign(content, signingKey) {
    const buf = str2ab(content);
    const plainKey = signingKey
      .replace("-----BEGIN PRIVATE KEY-----", "")
      .replace("-----END PRIVATE KEY-----", "")
      .replace(/(\r\n|\n|\r)/gm, "");
    const binaryKey = str2ab(atob(plainKey));
    const signer = await crypto.subtle.importKey(
      "pkcs8",
      binaryKey,
      {
        name: "RSASSA-PKCS1-V1_5",
        hash: { name: "SHA-256" }
      },
      false,
      ["sign"]
    );
    const binarySignature = await crypto.subtle.sign({ name: "RSASSA-PKCS1-V1_5" }, signer, buf);
    return arrayBufferToBase64Url(binarySignature);
  }

  const jwtHeader = objectToBase64url({ alg: "RS256", typ: "JWT" });
  try {
    const assertiontime = Math.round(Date.now() / 1000)
    const expirytime = assertiontime + 3600
    const claimset = objectToBase64url({
      "iss": user,
      "scope": scope,
      "aud": "https://oauth2.googleapis.com/token",
      "exp": expirytime,
      "iat": assertiontime
    })

    const jwtUnsigned = jwtHeader + "." + claimset
    const signedJwt = jwtUnsigned + "." + (await sign(jwtUnsigned, key))
    const body = "grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=" + signedJwt;
    const response = await fetch("https://oauth2.googleapis.com/token", {
      method: "POST",
      headers: {
        "Content-Type": "application/x-www-form-urlencoded",
        "Cache-Control": "no-cache",
        "Host": "oauth2.googleapis.com"
      },
      body: body
    });
    const oauth = await response.json();
    return oauth.access_token;
  } catch (err) {
    console.log(err)
  }
}