mungedb-aggregate/lib/pipeline/expressions/ConcatExpression.js

"use strict";
var ConcatExpression = module.exports = (function(){
	// CONSTRUCTOR
	/** 
	 * Creates an expression that concatenates a set of string operands.
	 *
	 * @class ConcatExpression
	 * @namespace mungedb.aggregate.pipeline.expressions
	 * @module mungedb-aggregate
	 * @constructor
	 **/
	var klass = module.exports = function ConcatExpression(){
		if(arguments.length !== 0) throw new Error("zero args expected");
		base.call(this);
	}, base = require("./NaryExpression"), proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

	// DEPENDENCIES
	var Value = require("../Value");

	// PROTOTYPE MEMBERS
	proto.getOpName = function getOpName(){
		return "$concat";
	};

	proto.getFactory = function getFactory(){
		return klass;	// using the ctor rather than a separate .create() method
	};

	/**
	 * Concats a string of values together.
	 * @method evaluate
	 **/
	proto.evaluate = function evaluate(doc) {
		var result = "";
		for (var i = 0, n = this.operands.length; i < n; ++i) {
			var val = this.operands[i].evaluate(doc);
			if (val === null)
				return null; // if any operand is null, return null for all

			if (typeof(val) != "string") throw new Error("$concat only supports strings, not " + typeof(val) + "; code 16702");

			result = result + Value.coerceToString(val);
		}

		return result;
	};

	return klass;
})();