OAuth2 in Google Apps Script: Authenticate External APIs Step by Step

Third-party SaaS APIs often require OAuth2 authorization code or client credentials flows. Apps Script can host the callback in a web app doGet that writes tokens to Script Properties.

Use Google's OAuth2 Apps Script library for refresh tokens, or implement token exchange manually with UrlFetchApp.post to the provider token URL.

Contrast with service accounts in /blog/apps-script-service-account when no human login exists.

Example code

function exchangeCodeForToken(code) {
  const props = PropertiesService.getScriptProperties();
  const res = UrlFetchApp.fetch('https://provider.example/oauth/token', {
    method: 'post',
    payload: {
      grant_type: 'authorization_code',
      code: code,
      client_id: props.getProperty('CLIENT_ID'),
      client_secret: props.getProperty('CLIENT_SECRET'),
      redirect_uri: props.getProperty('REDIRECT_URI'),
    },
  });
  const tokens = JSON.parse(res.getContentText());
  props.setProperty('access_token', tokens.access_token);
  props.setProperty('refresh_token', tokens.refresh_token);
}

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