Question

Starting with a Firebase DataSnapshot, I want something so that, in plain English, “if all the children failed the condition, execute something”.

Here is what I have now:

     appleappsRef.on('value', function (allApplesSnapshot){
            allApplesSnapshot.forEach(function (appleSnapshot) {
                if (condition) {
                   //execute code
                } 
            });             
      });   

Answer 1

Per the docs, your forEach callback can return true to cancel enumeration and forEach() will return true to signal that enumeration was canceled.

This means you can do:

appleappsRef.on('value', function (allApplesSnapshot){
  var foundOne = allApplesSnapshot.forEach(function (appleSnapshot) {
    if (condition) {
       return true; // found one, cancel enumeration
    }
  });
  if (!foundOne) {
    // all children failed the condition.
  }
});