sebmck/6to5

import { declare } from "@babel/helper-plugin-utils";
import syntaxFunctionBind from "@babel/plugin-syntax-function-bind";
import { types as t } from "@babel/core";

export default declare(api => {
  api.assertVersion(7);

  function getTempId(scope) {
    let id = scope.path.getData("functionBind");
    if (id) return id;

    id = scope.generateDeclaredUidIdentifier("context");
    return scope.path.setData("functionBind", id);
  }

  function getStaticContext(bind, scope) {
    const object = bind.object || bind.callee.object;
    return scope.isStatic(object) && object;
  }

  function inferBindContext(bind, scope) {
    const staticContext = getStaticContext(bind, scope);
    if (staticContext) return t.cloneNode(staticContext);

    const tempId = getTempId(scope);
    if (bind.object) {
      bind.callee = t.sequenceExpression([
        t.assignmentExpression("=", tempId, bind.object),
        bind.callee,
      ]);
    } else {
      bind.callee.object = t.assignmentExpression(
        "=",
        tempId,
        bind.callee.object,
      );
    }
    return tempId;
  }

  return {
    name: "proposal-function-bind",
    inherits: syntaxFunctionBind,

    visitor: {
      CallExpression({ node, scope }) {
        const bind = node.callee;
        if (!t.isBindExpression(bind)) return;

        const context = inferBindContext(bind, scope);
        node.callee = t.memberExpression(bind.callee, t.identifier("call"));
        node.arguments.unshift(context);
      },

      BindExpression(path) {
        const { node, scope } = path;
        const context = inferBindContext(node, scope);
        path.replaceWith(
          t.callExpression(
            t.memberExpression(node.callee, t.identifier("bind")),
            [context],
          ),
        );
      },
    },
  };
});