The dataLayer, explained: what actually happens when you push

If you have worked with Google Tag Manager for more than a week, you have typed dataLayer.push(). But what actually happens after you hit enter? In this post we open the hood and look at the message queue, the internal data model and the moment triggers get evaluated.

What the dataLayer really is

The dataLayer is a plain JavaScript array that GTM turns into a message bus. Every object you push becomes a message; GTM processes messages one by one, first in, first out.

// always initialise before the GTM snippet
window.dataLayer = window.dataLayer || [];

dataLayer.push({
  event: 'add_to_cart',
  ecommerce: {
    currency: 'USD',
    value: 95.00,
    items: [{ item_id: 'gtm-beginners', quantity: 1 }]
  }
});

What happens on push()

GTM overrides the array’s native push method. When a message arrives, three things happen in strict order:

1. The message is merged into GTM’s internal data model.
2. If the message has an event key, all triggers are evaluated.
3. Tags attached to matching triggers fire, respecting priority and sequencing.

Why order matters

Because messages are processed sequentially, a variable only has the value you expect if the push that sets it happened before the event that reads it. This is the root cause of most “my purchase tag has no value” tickets.

Three silent mistakes

1. Re-declaring the array

Writing dataLayer = [] after GTM has loaded replaces the instrumented array with a dumb one. Nothing breaks visibly — events just stop arriving.

2. Pushing strings instead of objects

GTM ignores anything that is not an object or a function. No error, no warning.

3. Forgetting to clear ecommerce

Nested objects are merged recursively, so stale items can leak into the next event.

Download the test container

Summary

The dataLayer is a queue, not a bag of variables. Push objects, include an event when you want something to fire, and mind the order.