calculate.js

/** Because `eval(q)` is too easy... Based on the post
    https://www.linkedin.com/feed/update/urn:li:activity:6531594043288358912 */

/** Token types */
var TYPES = {
	 EOF     : -1
	,UNKNOWN : 0
	,NUM     : 1
	,OP      : 2
};

/** Returns the type of the character */
function getType(c) {
	if (c == '-' || c == '+')
		return TYPES.OP;
	else if (c >= '0' && c <= '9')
		return TYPES.NUM;

	return TYPES.UNKNOWN;
}

/** Returns the next token from the given position */
function nextToken(input, pos){
	if (pos >= input.length)
		return { type: TYPES.EOF };

	var start = pos;
	var char = input[start];
	var type = getType(char);

	for (pos=start + 1; pos < input.length; pos++){
		char = input[pos];
		if (getType(char) != type)
			break;
	}

	return {
		 type  : type
		,start : start
		,end   : pos - 1
		,text  : input.substring(start, pos)
	};
}

/** Returns an array of tokens from the input string */
function parse(input) {
	var tokens = [];
	var token = nextToken(input, 0);
	while (token.type != TYPES.UNKNOWN && token.type != TYPES.EOF) {
		tokens.push(token);
		token = nextToken(input, token.end + 1);
	};

	return tokens;
}

/** Parses the input string and calculates the total */
function calc(input) {
	 var tokens = parse(input);
	 var total = 0;
	 var op = "+";

	 for (var ix in tokens) {
	 	var token = tokens[ix];
	 	var type = token.type;

	 	switch (token.type) {
	 		case TYPES.OP:
	 			if (token.text == "-")
		 			op = "-";
		 		else if (token.text == "+")
		 			op = "+";
	 			break;
	 		case TYPES.NUM:
		 		var val = parseInt(token.text);
		 		if (op == "+")
		 			total += val;
		 		else if (op == "-")
		 			total -= val;
	 			break;
	 	} // switch
	 } // for ix

	 return total;
}

/** Prints to console the input and its result */
function calculate(q){
	console.log(`calculate("${q}") => ${calc(q)}`);
}

/** Runs the test examples */
function test(){
	calculate("6+9-12");
	calculate("1+2-3+4-5+6-7");
	calculate("100+200+300");
	calculate("1-2-0");
	calculate("255");
}