JSON schema validator

I wrote a simple JSON schema validator.
The full code is over here on gist.github.com
However, the code without comment is...

validate = function(schema, instance) {
    var i;
    var errors = 0;
    
    var getType = function(attr) {
        return Object.prototype.toString.call(attr);
    }
 
    var addError = function(msg, attrs) {
        console.error(msg, attrs);
        errors += 1;
    }
 
    if(getType(schema) !== getType(instance)) {
        addError("Type Mismatch", [schema, instance]);
        return errors;
    }
 
    for(i in schema) {
        if(schema.hasOwnProperty(i)) {
 
            if(instance[i] == undefined) {
                addError("Property Not found", i);
                
            }
            //Special Handling for arrays
            else if( getType(schema[i]) === getType([]) ) {
                var zeroSchema = schema[i][0];
                var zeroInstance = instance[i][0];
                if(zeroInstance === undefined) {
                    continue;
                }
                for(var j=0;j<instance[i].length;j++) {
                    errors += validate(zeroSchema, instance[i][j]);    
                }
                
            }
            //Special Handling for nested objects
            else if( getType(schema[i]) === getType({}) ) {
                errors += validate(schema[i], instance[i]);
            }
 
        }
    }
    return errors;
 
}

The code doesn't need the schema object to explicitly specify the type of object properties
unlike the official one

I'm fairly new to javascript. How do I improve the code?

I'd definitely change:

if(instance[i] == undefined) {
    addError("Property Not found", i);
}

to something more robust, e.g.

if (!instance.hasOwnProperty(i)) {
    addError("Property Not found", i);
}

Stylistically I'd change a few things as well:

Add a space between function names and (): function() to function ()

Add a space between if, for etc and (: for(i in schema) to for (i in schema)

Add in semi-colons after defining your functions:

var getType = function(attr) {
    return Object.prototype.toString.call(attr);
}; // <-- here.

Also, because JavaScript doesn't have block scope I like to define all my variables at the top of the function. e.g. you have:

validate = function (schema, instance) {

    // omitted code.

    for(var j=0; j<instance[i].length; j++) {
        errors += validate(zeroSchema, instance[i][j]); 
    }

    // more code.
}

Whereas I would prefer j defined at the top of the function (as well as the extra spaces I've added).

Having said all of that, I wonder whether your solution is flexible enough - how would you specify an optional property? A property with a minimum and/or maximum value?

StackExchange Code Review Q#23950, answer score: 4