javascript equivalent to python's dictionary.get

Question

I'm trying to validate a JSON object with node.js. Basically, if condition A is present then I want to make sure that a particular value is in an array which may not be present. I do this in python using dictionary.get because that will return a default value if I look up something that isn't present. This is what it looks like in python

if output.get('conditionA') and not 'conditionB' in output.get('deeply', {}).get('nested', {}).get('array', []):
    print "There is an error somewhere you need to be fixing."

I'd like to find a similar technique for javascript. I tried using defaults in underscore to create the keys if they aren't there but I don't think I did it right or I'm not using it the way it was intended.

var temp = _.defaults(output, {'deeply': {'nested': {'array': []}}});
if (temp.hasOwnProperty('conditionA') && temp.deeply.nested.array.indexOf('conditionB') == -1) {
    console.log("There is an error somewhere you need to be fixing.");
}

It seems like if it runs into an output where one of the nested objects is missing it doesn't replace it with a default value and instead blows with a TypeError: Cannot read property 'variety' of undefined where 'variety' is the name of the array I'm looking at.

Answer 1

Or better yet, here's a quick wrapper that imitates the functionality of the python dictionary.

http://jsfiddle.net/xg6xb87m/4/

function pydict (item) {
    if(!(this instanceof pydict)) {
       return new pydict(item);
    }
    var self = this;
    self._item = item;
    self.get = function(name, def) {
        var val = self._item[name];
        return new pydict(val === undefined || val === null ? def : val);
    };
    self.value = function() {
       return self._item;
    };
    return self;
};
// now use it by wrapping your js object
var output = {deeply: { nested: { array: [] } } };
var array = pydict(output).get('deeply', {}).get('nested', {}).get('array', []).value();

Edit

Also, here's a quick and dirty way to do the nested / multiple conditionals:

var output = {deeply: {nested: {array: ['conditionB']}}};
var val = output["deeply"]
if(val && (val = val["nested"]) && (val = val["array"]) && (val.indexOf("conditionB") >= 0)) {
...
}

Edit 2 updated the code based on Bergi's observations.