simple parser stack: tokenizer, ast builder, and evaluator for basic arithmetic expressions

expr_parser.js

function tokenize(s) {  //  OPS     TERMS
	return s.match(/([+*\-\/])|([\d\.]+)/g).map(function Token(t, i) {
		return {
			token: t,
			type: isFinite(t) ? "num" : "op",
			index: i,
			value: isFinite(t) ? +t : "-+*\/".indexOf(t), // data value/op priority
		};
	});
}//end tokenize()

function AST(tokens){
 var sortedOps = tokens.filter(x=>x.type=='op').sort((a,b)=>b.value-a.value);
 return sortedOps.map(function group(op){
   var buff = [op, tokens[op.index-1], tokens[op.index+1]];
    tokens[op.index-1] = null;
    tokens[op.index+1] = null;
   return buff;
 });
}//end AST()

function solve(ast){
  return ast.map(function expr2value(eq, i, whole){
   let a = eq[1] ? eq[1].value : whole[i-1],
       b = eq[2] ? eq[2].value : whole[i-1];
   switch(eq[0].token){ // clobber eq with a mock result token:
     case "*": return ast[i]= a * b;
     case "/": return ast[i]= a / b;
     case "+": return ast[i]= a + b;
     case "-": return ast[i]= a - b;
  }
 }).pop(); // return final result
} // end solve()


solve(AST(tokenize("5 + 12 * 34 - 12"))); // == 401