"use strict";

/**
 * An $ifNull pipeline expression.
 * @see evaluate
 * @class IfNullExpression
 * @namespace mungedb-aggregate.pipeline.expressions
 * @module mungedb-aggregate
 * @constructor
 */
var IfNullExpression = module.exports = function IfNullExpression() {
	if (arguments.length !== 0) throw new Error(klass.name + ": expected args: NONE");
	base.call(this);
}, klass = IfNullExpression, base = require("./FixedArityExpressionT")(IfNullExpression, 2), proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

var Expression = require("./Expression");

proto.evaluateInternal = function evaluateInternal(vars) {
	var left = this.operands[0].evaluateInternal(vars);
	if (left !== undefined && left !== null)
		return left;
	var right = this.operands[1].evaluateInternal(vars);
	return right;
};

Expression.registerExpression("$ifNull", base.parse);

proto.getOpName = function getOpName() {
	return "$ifNull";
};