Parsing Typescript in Go using QuickJS
Posted on 26 April 2022
This post is in series where I talk on how to parse Typescript code in Java and Go using v8go. Instead of using v8go, this time we will use QuickJS-Go to parse and obtain the abstract syntax tree.
As before, there are many use-cases on why this may be required:
- you want to build a new documentation tool for
Javascript/Typescript - you are building a new version of Hacker Rank
- you want to enable scripting in your own code
Back to getting the job done, here are the broader steps:
- Initialize and load QuickJS
- Load Typescript JS library
- Load the Typescript code that you would like to parse
- And, finally call the TS parser code to obtain the AST
Let’s dig into the details. The first step is to initialize a new
QuickJS instance. You would notice the use of runtime.LockOSThread() -
this is to ensure that QuickJS always operates in the exact same thread.
// this is a must-step as per QuickJS documentation
stdruntime.LockOSThread()
// create new runtime
runtime := quickjs.NewRuntime()
defer runtime.Free()
// obtain a new context that we will work with
context := runtime.NewContext()
defer context.Free()
Once V8 is up, we need to load the Typescript JS code. We make use of a locally
saved typescript.js file for the same. Read it in memory and then use the EvalFile
method to load the script.
tsSource, err := ioutil.ReadFile("/path/to/ts/on/disk/typescript.js")
if err != nil {
panic(err) // panic if load failed
}
// load TS source code
result, err := context.EvalFile(string(typeScript), 0, "typescript.js")
check(err)
defer result.Free()
Notice the use of check(err) function call above. It is a convenience function
borrowed from the documentation that allows us to visit the stack/cause in case
something fails inside the QuickJS runtime. The function is as under.
func check(err error) {
if err != nil {
var evalErr *quickjs.Error
if errors.As(err, &evalErr) {
fmt.Println(evalErr.Cause)
fmt.Println(evalErr.Stack)
}
panic(err)
}
}
Once Typescript is loaded, we need to build the compiler options to let TS know that we would like to use the latest syntax version for parsing.
// never free this - throws cgo error at app termination
globals := context.Globals()
ts := globals.Get("ts")
defer ts.Free()
scriptTarget := ts.Get("ScriptTarget")
defer scriptTarget.Free()
system := scriptTarget.Get("Latest")
defer system.Free()
args := make([]quickjs.Value, 4)
args[0] = context.String("index.ts")
args[1] = context.String(string(sourceCode))
args[2] = context.String("")
args[3] = context.Bool(true)
Next, obtain the function createSourceFile from the loaded ts object. This allows
us to invoke the function directly from Go.
parseCode := ts.Get("createSourceFile")
defer parseCode.Free()
Load the typescript code that you would like to parse, and then simply use
context.Call to execute the parser.
sourceCode, err := ioutil.ReadFile("/path/on/disk/typescript/code/index.ts")
if err != nil {
panic(err)
}
result, err = context.Call(globals, parseCode, args)
check(err)
defer result.Free()
If there was no error, result contains the AST as an object. However, you will need
to iterate over it to convert to a pure Go object or a strongly-typed object. It is left as
an exercise for the reader.
if result.IsObject() {
// print the property names available
names, err := result.PropertyNames()
check(err)
fmt.Println("Object:")
for _, name := range names {
val := result.GetByAtom(name.Atom)
defer val.Free()
fmt.Printf("'%s': %s\n", name, val)
}
} else {
fmt.Println(result.String())
}
Complete code is available in this gist
This concludes the series on different ways to parse Typescript code in Java using J2V8, Go with v8go and [Go with QuickJS][post3].
Happy Hacking.