gtagjs

gtag.js, short for Global Site Tag, is Google's unified tagging framework for websites. It provides a single tag to implement and configure Google Analytics, Google Ads, and other Google marketing products. The goal is to simplify tagging by standardizing data collection across services and reducing the number of separate tags a site must manage.

The library loads asynchronously from Google's servers and uses a dataLayer array to store commands and data. When a page loads, gtag.js creates a global gtag function that pushes instructions into the dataLayer. Google services read those instructions to initialize properties, configure tracking, and log events, enabling consistent event measurement across linked products.

Typical usage involves including the gtag.js script in the page and issuing configuration calls, for example gtag('js', new Date()); gtag('config', 'GA_MEASUREMENT_ID'); Additional options can customize behavior, such as sending page views or setting user properties. GA4 property IDs begin with G- (for example, G-XXXXXXX), while Universal Analytics properties use IDs that begin with UA-.

gtag.js is the recommended tagging method for new implementations and it supersedes older tagging approaches like analytics.js. It supports multiple Google products from a single tag and can manage configurations for both GA4 and UA properties, though Google encourages migration toward GA4-focused configurations. Throughout its use, developers should consider privacy, consent, and compliance requirements, and debugging tools such as browser consoles or Google’s Tag Assistant to verify proper data collection.