Problem with arrow function with Bangle.js

Now test with this code :

function Test() {
  this.p1 = {x:0, y:80};
  this.p2 = {x:240, y:80};
}

Test.prototype.draw = _=> {
  g.clear();
  g.drawLine(0,80,240,80);
  return true;
};

var t= new Test();
if (t.draw()) {
  g.drawLine(0,160,240,160);
}

It works in the emulator but not on Bangle, only the first line is drawn and an error message appear on console ( see screen copy )

I have tested on Pico with this similar code :

function Test() {
  this.p1 = {x:0, y:80};
  this.p2 = {x:240, y:80};
}

Test.prototype.draw = _=> {
//  g.clear();
//  g.drawLine(0,80,240,80);
  console.log('line1');
  return true;
};

var t= new Test();
if (t.draw()) {
//  g.drawLine(0,160,240,160);
  console.log('line2');
}

same problem , with this message error :

---

| |_ ___ ___ _ ||___ ___
| |_ -| . | _| | | | | . |
||_| || |_|||_|_|

     |_| espruino.com

2v04 (c) 2019 G.Williams
line1
Uncaught SyntaxError: BREAK statement outside of SWITCH, FOR or WHILE loop
at line 4 col 22
console.log('line1');return true;
^
in function "draw" called from line 7 col 10
if(t.draw()){console.log('line2');

(test are made without minification)
Strange !