About the Go-to-JS bridge

All k6 and xk6 binaries have an embedded JavaScript engine, sobek, which your test scripts run on.

You will deepen your conceptual knowledge of how your k6 extension works if you understand the bridge between Go internals and the JavaScript runtime.

Go-to-JavaScript bridge features

The bridge has a few features we should highlight:

Go method names are converted from Pascal to Camel case when accessed in JS. For example, IsGreater becomes isGreater.
Go field names convert from Pascal to Snake case. For example, the struct field ComparisonResult string becomes comparison_result in JS.
Field names may be explicit using js struct tags. For example, declaring the field as ComparisonResult string `js:“result”` or hiding from JS using `js:"-"`.

Type conversion and native constructors

The JavaScript runtime transparently converts Go types like int64 to their JS equivalent. For complex types where this is impossible, your script might fail with a TypeError, requiring you to explicitly convert your object to a sobek.Object or sobek.Value.

func (*Compare) XComparator(call sobek.ConstructorCall, rt *sobek.Runtime) *sobek.Object {
	return rt.ToValue(&Compare{}).ToObject(rt)
}

The preceding snippet also demonstrates the native constructors feature from sobek, where methods can become JS constructors. Methods with this signature can create Comparator instances in JS with new compare.Comparator(). While this is more idiomatic to JS, it still has the benefit of receiving the sobek.Runtime.