There are 28 free clock instances per day for frontend instances only when you use one of the standard environments or runtimes (python, java, php and go), and not when you use virtual machine-driven applications. All VM managed environments are based on the Compute Engine and are all available for payment.
For the nodejs application, you are definitely using a managed virtual machine.
To enjoy hours of free instances, you need to use one of the standard runtimes.
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