babel-core/lib/babel/transformation/helpers/build-comprehension.js

Turn ES6 code into readable vanilla ES5 with source maps

"use strict";

var t = require("../../types");

module.exports = function build(node, buildBody) {
  var self = node.blocks.shift();
  if (!self) return;

  var child = build(node, buildBody);
  if (!child) {
    // last item
    child = buildBody();

    // add a filter as this is our final stop
    if (node.filter) {
      child = t.ifStatement(node.filter, t.blockStatement([child]));
    }
  }

  return t.forOfStatement(
    t.variableDeclaration("let", [t.variableDeclarator(self.left)]),
    self.right,
    t.blockStatement([child])
  );
};