shift.js

const shift = (offset, points) => points.map(pt => {
    const point = typeof pt == 'object' ? pt : JSON.parse(pt);    
    return {x: point.x + offset.x, y: point.y + offset.y};
});

const polyline = [
    {x: 0, y: 0},
    {x: 10, y: 10},
    '{"x": 20, "y": 20}',
    {x: 30, y: 30},
];
const shifted = shift({x: 10, y: 10}, polyline);

console.log(shifted);
console.log(polyline);

• Don't use ==
• I'd prefer to avoid ternary here
  • Use collection for branching (strategy pattern)
• Respect separation of concerns principle
  • Decompose shift to parsing, iteration, and moving functions
  • Use class or closure to hold intermediate state
  • Avoid long expressions