JavaScript subset for Brainfuck interpreter: array, locals

Program source

// <![CDATA[
function skipToLoopEnd(program, startPc) {
  var nest = 1;
  var pc = startPc - 1;
  while (nest > 0) {
    pc = pc + 1;
    if (pc >= program.length) {
      nest = 0
    } else if (program[pc] == "[") {
      nest = nest + 1
    } else if (program[pc] == "]") {
      nest = nest - 1
    }
  };
  return pc
}

function exec(program, read) {
  var pc = 0;
  var rp = 0;
  var output = "";

var mem = [0];
  var mp = 0;
  var stack = [];
  var sp = 0;
  var c;
  var str;

while (pc < program.length) {
    c = program[pc];
    pc = pc + 1;
    if (c == "<") {
      mp = mp - 1
    } else if (c == ">") {
      mp = mp + 1;
      if (mem[mp] === undefined) {
        mem[mp] = 0
      }
    } else if (c == "+") {
      mem[mp] = mem[mp] + 1
    } else if (c == "-") {
      mem[mp] = mem[mp] - 1
    } else if (c == ",") {
      if (rp < read.length) {
        str = read[rp];
        rp = rp + 1
        mem[mp] = str.charCodeAt(0);
      } else {
        mem[mp] = 0
      }
    } else if (c == ".") {
      output = output + String.fromCharCode(mem[mp])
    } else if (c == "[") {
      stack[sp] = pc;
      sp = sp + 1;
      pc = skipToLoopEnd(program, pc)
    } else if (c == "]") {
      if (mem[mp] > 0) {
        pc = stack[sp - 1]
      } else {
        sp = sp - 1
      }
    }
  };
  return output
}

var output = exec(
  "++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.,[+.,]",
  "aXY"
);
document.write(output)
// ]]>