Visitor

Every new operation touches every class

Circle, Square, Triangle rarely change — but you keep needing new ops: area, export to SVG, describe. Each one means editing every element (or a big type-switch).

function area(shape) {
  if (shape.type === 'circle')   return Math.PI * shape.r ** 2;
  if (shape.type === 'square')   return shape.side ** 2;
  if (shape.type === 'triangle') return 0.5 * shape.base * shape.h;
}
function toSVG(shape)   { /* …same type-switch, again */ }
function describe(shape) { /* …and again */ }
// new op? write a NEW switch over every type.

Elements accept a visitor (double dispatch)

Each element just knows how to accept a visitor. Each visitor implements one method per element type. New operation = new visitor class.

class Circle { accept(v){ return v.visitCircle(this); } }

class AreaVisitor {
  visitCircle(c){ return Math.PI * c.r * c.r; }
  visitSquare(s){ return s.side * s.side; }
}

Pick a visitor — run it over every shape

Same three shapes, same shape classes. Pick a visitor and it runs across all of them — proving you added an operation WITHOUT changing a single shape class.

Tip: switch visitors — the shape classes never change.

New operations, unchanged elements

• Add an operation = add a visitor class. Element classes stay untouched.
• Great fit: ASTs, compilers, document trees — structure is stable, operations grow.
• Caution: adding a new ELEMENT type forces updating every visitor — the trade-off is inverted from most patterns.