babel-plugin-transform-negative-indices/dist/index.js

Turn negative array indices into offsets from the end of the array

'use strict';

function index ({
  types: t
}) {
  return {
    visitor: {
      MemberExpression: {
        enter({
          node
        }) {
          const prop = node.property;

          if (node.computed && prop.type === 'UnaryExpression' && prop.operator === '-' && prop.prefix === true) {
            node.property = t.binaryExpression('-', t.memberExpression(node.object, t.identifier('length'), false), prop.argument);
          }
        }

      }
    }
  };
}

module.exports = index;