compressing object hierarchies in JavaScript

Question

Is there a generic approach to "compressing" nested objects to a single level:

var myObj = {
    a: "hello",
    b: {
        c: "world"
    }
}

compress(myObj) == {
    a: "hello",
    b_c: "world"
}

I guess there would be some recursion involved, but I figured I don't need to reinvent the wheel here... !?

Answer 1

Here's a quick one, but watch out, b/c it will not work w/ arrays and null values (b/c their typeof returns "object").

var flatten = function(obj, prefix) {
  if(typeof prefix === "undefined") {
    prefix = "";
  }
  var copy = {};
  for (var p in obj) {
    if(obj.hasOwnProperty(p)) {
      if(typeof obj[p] === "object") {
        var tmp = flatten(obj[p], p + "_");
        for(var q in tmp) {
          if(tmp.hasOwnProperty(q)) {
            copy[prefix + q] = tmp[q];
          }
        }
      }
      else {
        copy[prefix + p] = obj[p];
      }
    }
  }
  return copy;
}

var myObj = {
  a: "level 1",
  b: {
    a: "level 2",
    b: {
      a: "level 3",
      b: "level 3"
    }
  }
}

var flattened = flatten(myObj);

Answer 2

Here's a quick CoffeeScript version based off Matthew Crumley's answer (I didn't use includePrototype as I had no need for it):

flatten = (obj, into = {}, prefix = '', sep = '_') ->
  for own key, prop of obj
    if typeof prop is 'object' and prop not instanceof Date and prop not instanceof RegExp
      flatten prop, into, prefix + key + sep, sep
    else
      into[prefix + key] = prop
  into

And a basic unflatten version, which would undoubtedly fail with repeated separators and other such trickiness:

unflatten = (obj, into = {}, sep = '_') ->
  for own key, prop of obj
    subKeys = key.split sep
    sub = into
    sub = (sub[subKey] or= {}) for subKey in subKeys[...-1]
    sub[subKeys.pop()] = prop
  into

FWIW, I use these functions to push object graphs into Redis hashes, which only support a single depth of key/value pairs.

Answer 3

function flatten(obj, includePrototype, into, prefix) {
    into = into || {};
    prefix = prefix || "";

    for (var k in obj) {
        if (includePrototype || obj.hasOwnProperty(k)) {
            var prop = obj[k];
            if (prop && typeof prop === "object" &&
                !(prop instanceof Date || prop instanceof RegExp)) {
                flatten(prop, includePrototype, into, prefix + k + "_");
            }
            else {
                into[prefix + k] = prop;
            }
        }
    }

    return into;
}

You can include members inherited members by passing true into the second parameter.

A few caveats:

• recursive objects will not work. For example:

  var o = { a: "foo" };
  o.b = o;
  flatten(o);
  

  will recurse until it throws an exception.

• Like ruquay's answer, this pulls out array elements just like normal object properties. If you want to keep arrays intact, add "|| prop instanceof Array" to the exceptions.

• If you call this on objects from a different window or frame, dates and regular expressions will not be included, since instanceof will not work properly. You can fix that by replacing it with the default toString method like this:

  Object.prototype.toString.call(prop) === "[object Date]"
  Object.prototype.toString.call(prop) === "[object RegExp]"
  Object.prototype.toString.call(prop) === "[object Array]"