babel/packages/babel-plugin-transform-es2015-classes/src/loose.js

import nameFunction from "babel-helper-function-name";
import VanillaTransformer from "./vanilla";
import * as t from "babel-types";

export default class LooseClassTransformer extends VanillaTransformer {
  constructor() {
    super(...arguments);
    this.isLoose = true;
  }

  _processMethod(node, scope) {
    if (!node.decorators) {
      // use assignments instead of define properties for loose classes

      let classRef = this.classRef;
      if (!node.static) classRef = t.memberExpression(classRef, t.identifier("prototype"));
      let methodName = t.memberExpression(classRef, node.key, node.computed || t.isLiteral(node.key));

      let func = t.functionExpression(null, node.params, node.body);
      let key = t.toComputedKey(node, node.key);
      if (t.isStringLiteral(key)) {
        func = nameFunction({
          node: func,
          id: key,
          scope
        });
      }

      let expr = t.expressionStatement(t.assignmentExpression("=", methodName, func));
      t.inheritsComments(expr, node);
      this.body.push(expr);
      return true;
    }
  }
}