Check if a javascript object(instance) has the same properties of the same type with respect to a known object(schema).

validate_properties.js

/*******************************************************************************************************
Checks if an object(instance) has the same properties of the same type with respect to a known
object(schema). 

validate is especially useful to test whether the server side code sends JSON as expected by the client


Usage Example: 
1. Create a sample expected object (sampleSchema).
2. When data(sampleInstance) is received, call validate(sampleSchema, sampleInstance)
3. validate returns 0 if no errors are found, else logs errors (Property not found/Type Mismatch) and 
returns error count.


sampleSchema = {
	"name": "nelly furtado",
	"age": 34,
	"albums": ["folklore", "loose"],
	"details": {
		"genres": ["pop", "latin", "hip-hop"]
	}
}

sampleInstance = {
	"name": "shakira",
	"age": 32,
	"albums": ["one", 2],
	"details": {
		"genres": ["pop", "latin"]
	}
}

> validate(sampleSchema, sampleInstance)
Type Mismatch ["folklore", 2] 
> 1

> validate(sampleSchema, sampleSchema)
> 0


*******************************************************************************************************/



validate = function(schema, instance) {

	var errors = 0;

	var i, j, arrLength, zeroSchema, zeroInstance;
	
	var getType = function(attr) {
		return Object.prototype.toString.call(attr);
	};

	var addError = function(msg, attrs) {
		console.error(msg, attrs);
		errors += 1;
	};

	var validateArray = function(schemaArr, instanceArr) {
		
		//Each element of instance array is compared with zeroth element of schema array
		//All elements of instance array MUST be of same type for no errors to occur

		var zeroSchema = schemaArr[0];
		var zeroInstance = instanceArr[0];
		if(zeroInstance === undefined) {
			return 0;
		}

		var j, arrLength = instanceArr.length;
		for(j=0; j<arrLength; j += 1) {
			errors += validate(zeroSchema, instanceArr[j]);	
		}

	};


	if( getType(schema) !== getType(instance) ) {
		addError("Type Mismatch", [schema, instance]);
	}
	
	else if( getType(schema) == getType("") ) {
		//Everything ok if both schema and instance types are strings
	}

	else if( getType(schema) === getType([]) ) {
		validateArray(schema, instance);
	}

	else {
		for(i in schema) {
			if(schema.hasOwnProperty(i)) {

				if(!instance.hasOwnProperty(i)) {
					addError("Property Not found", i);
					
				}
				//Special Handling for arrays
				else if( getType(schema[i]) === getType([]) ) {
					validateArray(schema[i], instance[i])
				}
				//Special Handling for nested objects
				else if( getType(schema[i]) === getType({}) ) {
					errors += validate(schema[i], instance[i]);
				}

			}
		}
	}

	return errors;

}

For the code review, see http://codereview.stackexchange.com/questions/23950/json-schema-validator