Question

I’m experimenting with JavaScript, and just for learning purposes, I was writing a forEach iterator that can iterate through nested arrays or any other iterable object that include a length property.

This is what I wrote:

var forEach = function(obj, callback, options) {
  var options = options || {};
  var context = options.context || this;    

  if(!isEmpty(obj)) { // isEmpty function just evaluates `return !(!!obj.length);`
    for(var x = 0; x < obj.length; x++) {
      if(!isEmpty(obj[x]) && options.deep === true) {
        forEach.call(context, obj[x], callback, options);
        continue;
      }
      callback.call(context, obj[x]);
    }
  }
};

If I pass a nested array I get RangeError: Maximum call stack size exceeded:

forEach(['a', 'b', ['c', 'd']], function(x) {
  console.log(x);
}, { deep: true });

But that only seems to happen if I check for length property in obj[x]

If I replace:

if(!isEmpty(obj[x]) && options.deep === true) {

For:

if((obj[x] instanceof Array) && options.deep === true) {

I will magically work. Hovewer, not only Arrays have a length property. String have it to, so It’s not a wide approach.

How can I prevent the RangeError but still check for length property?

EDIT: I’m running the example on NodeJS v0.8.12

Answer 1

Consider that 'a'[0][0][0][0][0][0][0]... is valid ad infinitum, and every value is type string. If the type is string, then you should not recursively iterate it. Note also that function objects have a length property, and a function argument could be a self-reference to the function itself. This would cause another infinite recursion. I think it would probably make more sense to handle different types differently instead of trying to develop a catch-all function.

You could also have a maxLevel property that limits the depth of the recursion, with a default value of, say, 10. This way infinite recursion should not be easily possible.

forEach(['a', 'b', ['c', 'd']], function(x) {
  console.log(x);
}, { deep: true, maxLevel: 10 });

var forEach = function(obj, callback, options, level) {
    var options = options || {};
    var context = options.context || this;

    if (!level) level = 1;
    if (!options.maxLevel) options.maxLevel = 10;

    if (level > options.maxLevel) return;
    ...
        forEach.call(context, obj[x], callback, options, level + 1);
    ...
}

Demo: http://jsfiddle.net/bjpx5/