Is there a way I can evaluate a JavaScript function with default parameters using PyV8?

Question

I am trying to call a function from JavaScript in python. I am using PyV8 and I can successfully call a function and print out the result. However, if the function contains a default parameter then I get a Syntax Error.

This works fine.

import PyV8

ctxt = PyV8.JSContext()
ctxt.enter()
ctxt.eval("function example(a){return a;}")
render = ctxt.eval("example('hello');")
print render

However, when I make example contain a default parameter like this:

import PyV8

ctxt = PyV8.JSContext()
ctxt.enter()
ctxt.eval("function example(a = 'hello'){return a;}")
render = ctxt.eval("example();")
print render

I get SyntaxError: SyntaxError: Unexpected token = ( @ 1 : 19 ) -> function example(a = 'hello'){return a;}

Any help is appreciated.

score 2 · Accepted Answer · answered Sep 17 '19 at 20:50

Default arguments are a feature of ES6.
PyV8 doesn't support ES6 syntax. You need to use a shim/polyfil:

import PyV8

jsFunc = """
function test(msg, name) {
  (msg === undefined) && (msg = 'hello');
  (name === undefined) && (name = 'world');
  return msg + ' ' + name
}
"""


ctxt = PyV8.JSContext()
ctxt.enter()
ctxt.eval(jsFunc)
render1 = ctxt.eval("test();")
render2 = ctxt.eval("test('hi');")
print render1
print render2

Prints:

hello world
hi world

Is there a way I can evaluate a JavaScript function with default parameters using PyV8?

1 Answers1