Has anyone else used the JavaScript-based tracer that Geth provides, as part of its debug_traceTransaction method?

I want to look at segments of memory during a transaction's execution (specifically, I want to retrieve "input" data made by internal CALLs).

Using the JS tracer, one can access memory using log.memory, which has a method called log.memory.slice. The documentation says it,

"returns the specified segment of memory as a byte slice".

But I can't figure out what format this byte slice is in.

For example, when I request a segment 4 bytes long, I get this back: "FscnIQ==". How do I interpret that?

When I run debug_traceTransaction without the JS tracer, and print out the memory, I see that those 4 bytes should be "0x16c72721".

Similarly, when I ask for the first byte in memory when memory is all 0s, the JS tracer's log.memory.slice returns the string "AA==". The encoding is clearly not hex, and it doesn't seem to be ASCII either.

What kind of encoding maps "0x00" to "AA=="?

1 Answer 1


Ah, okay I figured it out. The output of log.memory.sliceis base64 encoded.

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