The callback for Array.prototype.forEach is given three arguments, the current value, the index, and the array itself. val is the second argument (which should really be given a more appropriate name), so it is the index, which is a number.

This may help you understand:

let obj = {a: '1', b: '2'};

Object.keys(obj).forEach(function(key, idx, arr){
    console.log("Key:", key);
    console.log("Index:", idx);
    console.log("Original Array:", arr);
    console.log("Value:", obj[key]);
});

Alternatively, you can use Object.entries to directly access values (with destructuring):

let obj = {a: '1', b: '2'};

Object.entries(obj).forEach(function([key, val], idx, arr){
    console.log("Key:", key);
    console.log("Index:", idx);
    console.log("Original Array:", arr);
    console.log("Value:", val);
});

for (var key in validation_messages) {
    // skip loop if the property is from prototype
    if (!validation_messages.hasOwnProperty(key)) continue;

    var obj = validation_messages[key];
    for (var prop in obj) {
        // skip loop if the property is from prototype
        if (!obj.hasOwnProperty(prop)) continue;

        // your code
        alert(prop + " = " + obj[prop]);
    }
}

What you want is lazy iteration over an object or array. This is not possible in ES5 (thus not possible in node.js). We will get this eventually.

The only solution is finding a node module that extends V8 to implement iterators (and probably generators). I couldn't find any implementation. You can look at the spidermonkey source code and try writing it in C++ as a V8 extension.

You could try the following, however it will also load all the keys into memory

Object.keys(o).forEach(function(key) {
  var val = o[key];
  logic();
});

However since Object.keys is a native method it may allow for better optimisation.

Benchmark

As you can see Object.keys is significantly faster. Whether the actual memory storage is more optimum is a different matter.

var async = {};
async.forEach = function(o, cb) {
  var counter = 0,
    keys = Object.keys(o),
    len = keys.length;
  var next = function() {
    if (counter < len) cb(o[keys[counter++]], next);
  };
  next();
};

async.forEach(obj, function(val, next) {
  // do things
  setTimeout(next, 100);
});