Create a basic arithmetic syntax tree using javascript

arithsyntaxtree.js

function AstNode(operator, left, right) {
	this.operator = operator;
	this.left = left;
	this.right = right;
}

AstNode.prototype.evaluate = function() {
	return this.operator(this.left.evaluate(), this.right.evaluate());
};

function ValueNode(val) {
	this.value = val;
}

ValueNode.prototype.evaluate = function() {
	return this.value;
};

function VariableNode(context, keyName) {
	this.context = context;
	this.keyName = keyName;
}

VariableNode.prototype.evaluate = function() {
	return this.context.get(this.keyName);
};

var operators = {
	'*': function(left, right) {
		return left * right;
	},
	'/': function(left, right) {
		return left / right;
	},
	'+': function(left, right) {
		return left + right;
	},
	'-': function(left, right) {
		return left - right;
	},
	'^': function(left, right) {
		return Math.pow(left, right);
	},
	'%': function(left, right) {
		return left % right;
	}
};

var operatorDict = {
	'+': [operators['+'], 2, 2],
	'-': [operators['-'], 2, 2],
	'*': [operators['*'], 3, 3],
	'/': [operators['/'], 3, 3],
	'%': [operators['%'], 3, 3],
	'^': [operators['^'], 4, 5],
	'(': [function() {}, 0, 8]
};

function Parser() {
	this.operatorStack = [];
	this.nodeStack = [];
};

var funcDict = {
	valueFunc: function(value) {
		this.nodeStack.push(value);
		this.__proto__ = protoState2;
	},
	operatorFunc: function(operator) {
		var operatorPrecedence = operatorDict[operator][2];
		this.reduce(operatorPrecedence);
		this.operatorStack.push(operatorDict[operator]);
		this.__proto__ = protoState1;
	},
	closeParenFunc: function() {
		this.reduce(1);
		if (this.operatorStack.length) {
			this.operatorStack.pop();
		} else {
			this.syntaxErr('Error - no open parenthesis matches close parenthesis');
		}
		this.__proto__ = protoState2;
	},
	syntaxErr: function(err) {
		throw new Error(err);
	},
	end: function() {
		this.reduce(0);
		return this.nodeStack.pop();
	},
	reduce: function(precedence) {
		while (this.operatorStack.length) {
			var tailOperator = this.operatorStack[this.operatorStack.length - 1];
			if (tailOperator[1] < precedence) break;
			tailOperator = this.operatorStack.pop();
			var right = this.nodeStack.pop();
			var left = this.nodeStack.pop();
			this.nodeStack.push(new AstNode(tailOperator[0], left, right));
		}
	}
};

var protoState1 = {
	valueFunc: funcDict.valueFunc,
	operatorFunc: funcDict.syntaxErr,
	openParenFunc: funcDict.operatorFunc,
	closeParenFunc: funcDict.syntaxErr,
	syntaxErr: funcDict.syntaxErr,
	end: funcDict.end,
	reduce: funcDict.reduce
};

var protoState2 = {
	valueFunc: funcDict.syntaxErr,
	operatorFunc: funcDict.operatorFunc,
	openParenFunc: funcDict.syntaxErr,
	closeParenFunc: funcDict.closeParenFunc,
	syntaxErr: funcDict.syntaxErr,
	end: funcDict.end,
	reduce: funcDict.reduce
};

Parser.prototype.__proto__ = protoState1;

function Lexer(expression, parseObj, context) {
	if (!context) {
		context = {
			get: function(name) {
				return null;
			}
		};
	}
	for (var i = 0, len = expression.length; i < len; ++i) {
		var character = expression.charAt(i);
		switch (character) {
			case ' ':
			case '\n':
			case '\t':
				continue;
			case '(':
				parseObj.openParenFunc('(');
				break;
			case ')':
				parseObj.closeParenFunc();
				break;
			case '{':
				var name = '';
				while (i < len) {
					var ch = expression.charAt(++i);
					if (ch === '}') {
						if (/^[\w\.]+$/.test(name)) {
							parseObj.valueFunc(new VariableNode(context, name));
							break;
						} else {
							parseObj.syntaxErr('Invalid character in variable name at ' + (i));
						}
					} else {
						name += ch;
						if (i === len) {
							parseObj.syntaxErr('Unexpected end of output');
						}
					}
				}
				break;
			case '}':
				parseObj.syntaxErr('Unexpected } found at character' + i);
				break;
			default:
				if (typeof operatorDict[character] !== 'undefined') {
					parseObj.operatorFunc(character);
				} else {
					var numDecimals = 0, numBuffer = '';
					while (i < len) {
						if (numDecimals > 1) parseObj.syntaxErr('Unexpected . found at character ' + i);
						if (character === '.') numDecimals++;
						numBuffer += '' + character;
						var nextChar = expression.charAt(i + 1);
						if (nextChar === '.' || !isNaN(parseFloat(nextChar))) {
							character = expression.charAt(++i);
						} else {
							break;
						}
					}
					parseObj.valueFunc(new ValueNode(parseFloat(numBuffer)));
					numBuffer = '';
				}
		}
	}
	return parseObj.end();
}

var expr = '4 + (5 + 18.1) * 10 / {Stuff}';
expr = '5 / 10 % 3 + {Merf}';

var context = {
	data: {
		Stuff: 5
	},
	get: function(name) {
		if (typeof this.data[name] !== 'undefined') return this.data[name];
		return null;
	}
}

try {
	var tree = new Lexer(expr, new Parser(), context);
	console.log(tree, tree.evaluate());
} catch (e) {
	console.log(e.stack);
}