Geth supports tracing via [custom Javascript tracers](/docs/evm-tracing/custom-tracer#custom-javascript-tracing). This document provides a tutorial with examples on how to achieve this.
### A simple filter
Filters are Javascript functions that select information from the trace to persist and discard based on some conditions. The following Javascript function returns only the sequence of opcodes executed by the transaction as a comma-separated list. The function could be written directly in the Javascript console, but it is cleaner to write it in a separate re-usable file and load it into the console.
1. Create a file, `filterTrace_1.js`, with this content:
More information about the `JSON.stringify` function is available [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify).
The commands above worked by calling the same `debug.traceTransaction` function that was previously explained in [basic traces](https://geth.ethereum.org/docs/dapp/tracing), but with a new parameter, `tracer`. This parameter takes the JavaScript object formated as a string. In the case of the trace above, it is:
In this case, `retVal` is used to store the list of strings to return in `result`.
The `step` function adds to `retVal` the program counter and the name of the opcode there. Then, in `result`, this list is returned to be sent to the caller.
### Filtering with conditions
For actual filtered tracing we need an `if` statement to only log relevant information. For example, to isolate the transaction's interaction with storage, the following tracer could be used:
The `step` function here looks at the opcode number of the op, and only pushes an entry if the opcode is `SLOAD` or `SSTORE` ([here is a list of EVM opcodes and their numbers](https://github.com/wolflo/evm-opcodes)). We could have used `log.op.toString()` instead, but it is faster to compare numbers rather than strings.
The output looks similar to this:
```js
[
"5921: SLOAD",
.
.
.
"2413: SSTORE",
"2420: SLOAD",
"2475: SSTORE",
"6094: SSTORE"
]
```
### Stack Information
The trace above reports the program counter (PC) and whether the program read from storage or wrote to it. That alone isn't particularly useful. To know more, the `log.stack.peek` function can be used to peek into the stack. `log.stack.peek(0)` is the stack top, `log.stack.peek(1)` the entry below it, etc.
The values returned by `log.stack.peek` are Go `big.Int` objects. By default they are converted to JavaScript floating point numbers, so you need `toString(16)` to get them as hexadecimals, which is how 256-bit values such as storage cells and their content are normally represented.
#### Storage Information
The function below provides a trace of all the storage operations and their parameters. This gives a more complete picture of the program's interaction with storage.
One piece of information missing from the function above is the result on an `SLOAD` operation. The state we get inside `log` is the state prior to the execution of the opcode, so that value is not known yet. For more operations we can figure it out for ourselves, but we don't have access to the
storage, so here we can't.
The solution is to have a flag, `afterSload`, which is only true in the opcode right after an `SLOAD`, when we can see the result at the top of the stack.
So the storage has been treated as if there are only 2<sup>256</sup> cells. However, that is not true. Contracts can call other contracts, and then the storage involved is the storage of the other contract. We can see the address of the current contract in `log.contract.getAddress()`. This value is the execution context - the contract whose storage we are using - even when code from another contract is executed (by using
[`CALLCODE` or `DELEGATECALL`](https://docs.soliditylang.org/en/v0.8.14/introduction-to-smart-contracts.html#delegatecall-callcode-and-libraries)).
However, `log.contract.getAddress()` returns an array of bytes. To convert this to the familiar hexadecimal representation of Ethereum addresses, `this.byteHex()` and `array2Hex()` can be used.
