Function jsval_to_primitive 

fn jsval_to_primitive(
    cx: JSContext,
    global: &GlobalScope,
    chunk: HandleValue<'_>,
    rval: MutableHandleValue<'_>,
    can_gc: CanGc,
) -> Fallible<()>

Converts a JS value to primitive type so that it can be used with ToString.

Set rval to chunk if chunk is a primitive JS value. Otherwise, convert chunk into a primitive JS value and then set rval to the converted primitive. This follows the ToString procedure with the exception that it does not convert the value to string.

See below for the ToString procedure in spec: https://tc39.es/ecma262/multipage/abstract-operations.html#sec-tostring