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Debugging
There are multiple ways to debug what Boa is doing. Or maybe you just want to know how it works under the hood. Or even test some JavaScript.
One way to do so is to create a file in the root of the repository. For example
test.js. Then execute cargo run -- test.js to run the file with boa. You can
compile a list of JavaScript files by running cargo run -- file1.js file2.js
and so on.
You can also run boa interactively by simply calling cargo run without any
arguments to start a shell to execute JS.
These are added in order of how the code is read:
Tokens and AST nodes
The first thing boa will do is to generate tokens from the source code. These tokens are then parsed into an abstract syntax tree (AST). Any syntax errors should be thrown while the AST is generated.
You can use the boa_cli command-line flag --dump-ast to print the AST.
The flag supports these formats: Debug, Json, JsonPretty. By default
it is the Debug format.
Dumping the AST of a file:
cargo run -- test.js --dump-ast # AST dump format is Debug by default.
or with interactive mode (REPL):
cargo run -- --dump-ast # AST dump format is Debug by default.
Bytecode generation and Execution
Once the AST has been generated, boa will compile it into bytecode, which is then executed by the VM.
You can print the bytecode and the executed instructions with the command-line flag --trace.
For more detailed information about the VM and the trace output look here.
Instruction flowgraph
We can also get the VM instructions flowgraph, which is a visual representation of the instruction flow.
The Start (in green) and End (in red) node in the graph represents the start and end point of execution.
They are not instructions, just markers.
The conditional instructions are diamond shaped, with the "YES" branch in green and the "NO" branch in red.
The push and pop evironment pairs match colors and are connected by a dotted line.
You can use the --flowgraph (or --flowgraph=mermaid for mermaid format) flag which outputs
graphviz format by default, and pipe it to dot (from the graphviz package which is installed
on most linux distros by default) or use an online editor like: https://dreampuf.github.io/GraphvizOnline to
view the graph.
cargo run -- test.js --flowgraph | dot -Tpng > test.png
You can specify the -Tsvg to generate a svg instead of a png file.
Mermaid graphs can be displayed on github natively without third-party programs.
By using a mermaid block as seen below.
```mermaid
// graph contents here...
```
Additionaly you can specify the direction of "flow" by using the --flowgraph-direction cli option,
for example --flowgraph-direction=left-to-right, the default is top-to-bottom.
Debugging through the debug object $boa
Certain debugging actions in JavaScript land are difficult to impossible, like triggering a GC collect.
For such puroposes we have the $boa object that contains useful utilities that can be used to debug JavaScript in JavaScript.
The debug object becomes available with the --debug-object cli flag, It injects the $boa debug object in the context as global variable,
the object is separated into modules gc, function, object, etc.
We can now do $boa.gc.collect(), which force triggers a GC collect.
If you want to trace only a particular function (without being flodded by the --trace flag, that traces everything),
for that we have the $boa.function.trace(func, this, ...args).
The full documentation of the $boa object's modules and functionalities can be found here.
Compiler panics
In the case of a compiler panic, to get a full backtrace you will need to set
the environment variable RUST_BACKTRACE=1.
Debugger
VS Code Debugger
The quickest way to get debugging is to use the CodeLLDB plugin and add breakpoints. You can get more information here.
LLDB Manual debugging
You can also use rust-lldb. The Dockerfile already has this enabled, you
should be able to use that environment to run your code.
rust-lldb ./target/debug/boa [arguments]