index.js

/* @noflow */

import type { NodePath } from "babel-traverse";
import nameFunction from "babel-helper-function-name";
import template from "babel-template";
import * as t from "babel-types";
import rewriteForAwait from "./for-await";

let buildWrapper = template(`
  (() => {
    var REF = FUNCTION;
    return function NAME(PARAMS) {
      return REF.apply(this, arguments);
    };
  })
`);

let namedBuildWrapper = template(`
  (() => {
    var REF = FUNCTION;
    function NAME(PARAMS) {
      return REF.apply(this, arguments);
    }
    return NAME;
  })
`);

let awaitVisitor = {
  Function(path) {
    if (path.isArrowFunctionExpression() && !path.node.async) {
      path.arrowFunctionToShadowed();
      return;
    }
    path.skip();
  },

  AwaitExpression({ node }, { wrapAwait }) {
    node.type = "YieldExpression";
    if (wrapAwait) {
      node.argument = t.callExpression(wrapAwait, [node.argument]);
    }
  },

  ForAwaitStatement(path, { file, wrapAwait }) {
    let { node } = path;

    let build = rewriteForAwait(path, {
      getAsyncIterator: file.addHelper("asyncIterator"),
      wrapAwait
    });

    let { declar, loop } = build;
    let block = loop.body;

    // ensure that it's a block so we can take all its statements
    path.ensureBlock();

    // add the value declaration to the new loop body
    if (declar) {
      block.body.push(declar);
    }

    // push the rest of the original loop body onto our new body
    block.body = block.body.concat(node.body.body);

    t.inherits(loop, node);
    t.inherits(loop.body, node.body);

    if (build.replaceParent) {
      path.parentPath.replaceWithMultiple(build.node);
      path.remove();
    } else {
      path.replaceWithMultiple(build.node);
    }
  }

};

function classOrObjectMethod(path: NodePath, callId: Object) {
  let node = path.node;
  let body = node.body;

  node.async = false;

  let container = t.functionExpression(null, [], t.blockStatement(body.body), true);
  container.shadow = true;
  body.body = [
    t.returnStatement(t.callExpression(
      t.callExpression(callId, [container]),
      []
    ))
  ];
}

function plainFunction(path: NodePath, callId: Object) {
  let node = path.node;
  let isDeclaration = path.isFunctionDeclaration();
  let asyncFnId = node.id;
  let wrapper = buildWrapper;

  if (path.isArrowFunctionExpression()) {
    path.arrowFunctionToShadowed();
  } else if (!isDeclaration && asyncFnId) {
    wrapper = namedBuildWrapper;
  }

  node.async = false;
  node.generator = true;

  node.id = null;

  if (isDeclaration) {
    node.type = "FunctionExpression";
  }

  let built = t.callExpression(callId, [node]);
  let container = wrapper({
    NAME: asyncFnId,
    REF: path.scope.generateUidIdentifier("ref"),
    FUNCTION: built,
    PARAMS: node.params.map(() => path.scope.generateUidIdentifier("x"))
  }).expression;

  if (isDeclaration) {
    let declar = t.variableDeclaration("let", [
      t.variableDeclarator(
        t.identifier(asyncFnId.name),
        t.callExpression(container, [])
      )
    ]);
    declar._blockHoist = true;

    path.replaceWith(declar);
  } else {
    let retFunction = container.body.body[1].argument;
    if (!asyncFnId) {
      nameFunction({
        node: retFunction,
        parent: path.parent,
        scope: path.scope
      });
    }

    if (!retFunction || retFunction.id || node.params.length) {
      // we have an inferred function id or params so we need this wrapper
      path.replaceWith(t.callExpression(container, []));
    } else {
      // we can omit this wrapper as the conditions it protects for do not apply
      path.replaceWith(built);
    }
  }
}

export default function (path: NodePath, file: Object, helpers: Object) {
  path.get("body").traverse(awaitVisitor, {
    file,
    wrapAwait: helpers.wrapAwait
  });

  if (path.isClassMethod() || path.isObjectMethod()) {
    classOrObjectMethod(path, helpers.wrapAsync);
  } else {
    plainFunction(path, helpers.wrapAsync);
  }
}