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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
The first thing boa will do is generate tokens from source code. If the token generation is wrong the rest of the operation will be wrong, this is usually a good starting place.
To print the tokens to stdout, you can use the boa_cli command-line flag
--dump-tokens or -t, which can optionally take a format type. Supports
these formats: Debug, Json, JsonPretty. By default it is the Debug
format.
cargo run -- test.js --dump-tokens # token dump format is Debug by default.
or with interactive mode (REPL):
cargo run -- --dump-tokens # token dump format is Debug by default.
Seeing the order of tokens can be a big help to understanding what the parser is working with.
Note: flags --dump-tokens and --dump-ast are mutually exclusive. When
using the flag --dump-tokens, the code will not be executed.
AST nodes
Assuming the tokens looks fine, the next step is to see the AST. You can use
the boa_cli command-line flag --dump-ast, which can optionally take a
format type. 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.
These methods will print out the entire parse tree.
Note: flags --dump-tokens and --dump-ast are mutually exclusive. When
using the flag --dump-ast, the code will not be executed.
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.
Execution
Once the tree has been generated exec will begin to
run through each node. If the tokens and tree looks fine, you can start looking
here. We usually just add dbg!() in the relevent places to see what the
output is at the time.
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]
VM
For debugging the new VM see here