Convert string in dot notation to get the object reference

Question

Consider this object in javascript,

var obj = { a : { b: 1, c: 2 } };

given the string "obj.a.b" how can I get the object this refers to, so that I may alter its value? i.e. I want to be able to do something like

obj.a.b = 5;
obj.a.c = 10;

where "obj.a.b" & "obj.a.c" are strings (not obj references). I came across this post where I can get the value the dot notation string is referring to obj but what I need is a way I can get at the object itself?

The nesting of the object may be even deeper than this. i.e. maybe

var obj = { a: { b: 1, c : { d : 3, e : 4}, f: 5 } }

Answer 1

To obtain the value, consider:

function ref(obj, str) {
    str = str.split(".");
    for (var i = 0; i < str.length; i++)
        obj = obj[str[i]];
    return obj;
}

var obj = { a: { b: 1, c : { d : 3, e : 4}, f: 5 } }
str = 'a.c.d'
ref(obj, str) // 3

or in a more fancy way, using reduce:

function ref(obj, str) {
    return str.split(".").reduce(function(o, x) { return o[x] }, obj);
}

Returning an assignable reference to an object member is not possible in javascript, you'll have to use a function like the following:

function set(obj, str, val) {
    str = str.split(".");
    while (str.length > 1)
        obj = obj[str.shift()];
    return obj[str.shift()] = val;
}

var obj = { a: { b: 1, c : { d : 3, e : 4}, f: 5 } }
str = 'a.c.d'
set(obj, str, 99)
console.log(obj.a.c.d) // 99

or use ref given above to obtain the reference to the containing object and then apply the [] operator to it:

parts = str.split(/\.(?=[^.]+$)/)  // Split "foo.bar.baz" into ["foo.bar", "baz"]
ref(obj, parts[0])[parts[1]] = 99

Answer 2

Similar to thg435's answer, but with argument checks and supports nest levels where one of the ancestor levels isn't yet defined or isn't an object.

setObjByString = function(obj, str, val) {
    var keys, key;
    //make sure str is a string with length
    if (!str || !str.length || Object.prototype.toString.call(str) !== "[object String]") {
        return false;
    }
    if (obj !== Object(obj)) {
        //if it's not an object, make it one
        obj = {};
    }
    keys = str.split(".");
    while (keys.length > 1) {
        key = keys.shift();
        if (obj !== Object(obj)) {
            //if it's not an object, make it one
            obj = {};
        }
        if (!(key in obj)) {
            //if obj doesn't contain the key, add it and set it to an empty object
            obj[key] = {};
        }
        obj = obj[key];
    }
    return obj[keys[0]] = val;
};

Usage:

var obj;
setObjByString(obj, "a.b.c.d.e.f", "hello");

Answer 3

If this javascript runs in a browser then you can access the object like this:

window['obj']['a']['b'] = 5

So given the string "obj.a.b" you have to split the it by .:

var s = "obj.a.b"
var e = s.split(".")
window[e[0]][e[1]][e[2]] = 5

Answer 4

Returning an assignable reference to an object member is not possible in javascript. You can assign value to a deep object member by dot notation with a single line of code like this.

new Function('_', 'val', '_.' + path + ' = val')(obj, value);

In you case:

var obj = { a : { b: 1, c: 2 } };

new Function('_', 'val', '_.a.b' + ' = val')(obj, 5); // Now obj.a.b will be equal to 5

Answer 5

var obj = { a : { b: 1, c: 2 } };
walkObject(obj,"a.b"); // 1

function walkObject( obj, path ){
  var parts = path.split("."), i=0, part;
  while (obj && (part=parts[i++])) obj=obj[part];
  return obj;
}

Or if you like your code terse:

function walkObject( o, path ){
  for (var a,p=path.split('.'),i=0; o&&(a=p[i++]); o=o[a]);
  return o;
}

Answer 6

Below is a simple class wrapper around dict:

class Dots(dict):
    def __init__(self, *args, **kargs):
            super(Dots, self).__init__(*args, **kargs)

    def __getitem__(self, key):
            try:
                    item = super(Dots, self).__getitem__(key)
            except KeyError:
                    item = Dots()
                    self.__setitem__(key, item)

            return Dots(item) if type(item) == dict else item

    def __setitem__(self, key, value):
            if type(value) == dict: value = Dots(value)
            super(Dots, self).__setitem__(key, value)

    __getattr__ = __getitem__
    __setattr__ = __setitem__

Example:

>>> a = Dots()
>>> a.b.c = 123
>>> a.b.c
123
>>> a.b
{'c': 123}
>>> a
{'b': {'c': 123}}

Missing key are created on the fly as empty Dots():

>>> if a.Missing: print "Exists"
...
>>> a
{'Missing': {}, 'b': {'c': 123}}