If a data layer push unexpectedly overwrites an existing key-value pair instead of creating a new one, what is the most likely cause?

The most likely cause of a data layer push overwriting an existing key-value pair, instead of creating a new one, is that the new push uses the same key as the existing variable. The data layer, in its simplest form, is a JavaScript array of objects. When you push an object to the data layer with a key that already exists at the top level, the existing value for that key is replaced with the new value. For instance, if the data layer initially contains { 'pageType': 'homepage' } and a later push includes { 'pageType': 'product' }, the 'pageType' variable will be updated to 'product', overwriting the original value. This behavior is standard JavaScript object assignment. It's crucial to ensure unique key names when pushing data to prevent unintended overwrites. This can happen when reusing code snippets, implementing multiple tags that rely on the same key, or simply making typographical errors when defining data layer variables. To avoid this, ensure each piece of data pushed to the data layer has a unique key. If you intend to add more details to an existing key, the initial data layer object must be structured as a nested object, allowing later pushes to add new properties without overwriting the existing top-level key. For example, instead of pushing {'productName':'widget'}, consider pushing {'product': {'name': 'widget'}}. This allows later additions via {'product': {'price': 19.99}} without overwriting the initial 'product' object, resulting in {'product': {'name': 'widget', 'price': 19.99}}.