right association peg.js

grammar.pegjs

{
  const node = (name, children) => ({
    name,
    children: [children].flat(),
  });

  const rightAssociate = (...nodes) => {
  	const [last, penultimate, ...flat] = nodes.flat().reverse();
    return flat.reduce((current, previous) => node(
		current.name,
        [...(previous.children || [previous]), current]
    ), node(last.name, [...penultimate.children, ...last.children]));
   }
}

start = right_assoc_exprs

num = $[0-9]+

right_assoc_exprs = head:num tail:(
    '||' val:num { return node('or', val) }
    )* {
	return tail.length ?
    	rightAssociate(head, tail) :
        head;
}