meteor/packages/spacebars-compiler/react.js

// A visitor to ensure that React components included via the `{{>
// React}}` template defined in the react-template-helper package are
// the only child in their parent component. Otherwise `React.render`
// would eliminate all of their sibling nodes.
//
// It's a little strange that this logic is in spacebars-compiler if
// it's only relevant to a specific package but there's no way to have
// a package hook into a build plugin.
ReactComponentSiblingForbidder = HTML.Visitor.extend();
ReactComponentSiblingForbidder.def({
  visitArray: function (array, parentTag) {
    for (var i = 0; i < array.length; i++) {
      this.visit(array[i], parentTag);
    }
  },
  visitObject: function (obj, parentTag) {
    if (obj.type === "INCLUSION" && obj.path.length === 1 && obj.path[0] === "React") {
      if (!parentTag) {
        throw new Error(
          "{{> React}} must be used in a container element"
            + (this.sourceName ? (" in " + this.sourceName) : "")
               + ". Learn more at https://github.com/meteor/meteor/wiki/React-components-must-be-the-only-thing-in-their-wrapper-element");
      }

      var numSiblings = 0;
      for (var i = 0; i < parentTag.children.length; i++) {
        var child = parentTag.children[i];
        if (child !== obj && !(typeof child === "string" && child.match(/^\s*$/))) {
          numSiblings++;
        }
      }

      if (numSiblings > 0) {
        throw new Error(
          "{{> React}} must be used as the only child in a container element"
            + (this.sourceName ? (" in " + this.sourceName) : "")
               + ". Learn more at https://github.com/meteor/meteor/wiki/React-components-must-be-the-only-thing-in-their-wrapper-element");
      }
    }
  },
  visitTag: function (tag) {
    this.visitArray(tag.children, tag /*parentTag*/);
  }
});