// Credit goes to where it is due
function throwException(cause, index) {
    // Execute when there is an error in Brainf*** code
    $('p#output').text("Interpreting Terminated because " + cause + " at char index " + index);
}

function runBrainfuck(data) {
    console.time("Running Brainf*** code");
    var memory = new Array(); // memory

    // Initialize memory
    console.time("Initialize memory");
    for(var i = 0; i < 30000; i++) {
        memory[i] = 0;
    }
    console.timeEnd("Initialize memory");

    var p = 0; // pointer to memory
    $('p#output').text("");

    // Loop through the Brainf*** code
    var data = $('#code').val();

    for(var j = 0; j < data.length; j++) {
        switch(true) {
            case data[j] === ">": // go right one cell
               if(p < 30000) {
                   p++;
               } else {
                   throwException("There are only 30000 usable cells of memory.", j); // are you using too many cells?
               }
               break;
            case data[j] === "<": // go left one cell
               if(p > 0) {
                   p--;
               } else {
                   throwException("Cannot decrement pointer when it is at first cell.", j); // are you going below cell 0?
               }
               break;
            case data[j] === "+": // Increment cell value
               memory[p]++;
               if(memory[p] > 255) {
                   memory[p] = 0; // Whoops, overflown the memory!
               }
               break;
            case data[j] === "-": // decrement cell value
               if(memory[p] > 0) {
                   memory[p]--;
               } else {
                   memory[p] = 255; // Overflow back to 255
               }
               break;
            case data[j] === ".": // put character to screen
               var memoryChar = String.fromCharCode(memory[p]);
               if(memoryChar == '\n') {
                   memoryChar = ""; // Turn newlines into linebreaks
               }
               $('p#output').append(String.fromCharCode(memory[p]));
               break;
            case data[j] === ",":
               memory[p] = window.prompt("Please enter one character").charCodeAt(0); // Set memory to the character code
               break;
            case data[j] === "[": // start loop
               if(memory[p] != 0) {
                   continue;
               } else {
                   var openLoopCnt = 0; // number of open loops
                   for(var k = j; k < data.length; k++) {
                       if(data[k] === '[') {
                           openLoopCnt++; // Another open loop is in existence
                       } else if(data[k] === ']') {
                           openLoopCnt++; // Decrement open count

                           if(openLoopCnt === 0) {
                               j = k;
                               break;
                           }
                       }
                   }
                   if(openLoopCnt != 0) {
                       throwException("Open loop.", j);
                   }
               }
               break;
           case data[j] === "]": // end loop
               if(memory[p] != 0) {
                   var closedLoopCnt = 0; // Due to the fact that we are at closing loops, we use closedLoopCnt
                   for(var l = j; l >= 0; l--) {
                        if(data[l] === "]") {
                            closedLoopCnt++;
                        } else if(data[l] === "[") {
                            closedLoopCnt--;
                        }

                        if(closedLoopCnt === 0) {
                            j = l;
                            break;
                        }
                   }
                   if(closedLoopCnt != 0) {
                        throwException("Too many loop closings.", i);
                   }
               }
               break;
        }
    }
    console.timeEnd("Running Brainf*** code");

    console.time("Saving output in localStorage");
    localStorage.setItem("brainfuck_output", $('#output').text());
    console.timeEnd("Saving output in localStorage");
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<h1>Interpret your Brainf*** code!</h1>
<form action="#" method="get" id="brainf***" onsubmit="runBrainfuck($('#code').val())">
    <label for="brainf***">Brainf*** code:</label> <br />
    <textarea name="code" rows="25" cols="70" id="code"></textarea> <br />
    <input type="submit" value="Interpret Brainf*** code!" />
</form>
<h2>Brainf*** output:</h2>
<p id="output"></p>

// Credit goes to where it is due
function throwException(cause, index) {
    // Execute when there is an error in Brainf*** code
    $('p#output').text("Interpreting Terminated because " + cause + " at char index " + index);
}

function runBrainfuck(data) {
    console.time("Running Brainf*** code");
    var memory = new Array(); // memory

    // Initialize memory
    console.time("Initialize memory");
    for(var i = 0; i < 30000; i++) {
        memory[i] = 0;
    }
    console.timeEnd("Initialize memory");

    var p = 0; // pointer to memory
    $('p#output').text("");

    // Loop through the Brainf*** code
    var data = $('#code').val();

    for(var j = 0; j < data.length; j++) {
        switch(true) {
            case data[j] === ">": // go right one cell
               if(p < 30000) {
                   p++;
               } else {
                   throwException("There are only 30000 usable cells of memory.", j); // are you using too many cells?
               }
               break;
            case data[j] === "<": // go left one cell
               if(p > 0) {
                   p--;
               } else {
                   throwException("Cannot decrement pointer when it is at first cell.", j); // are you going below cell 0?
               }
               break;
            case data[j] === "+": // Increment cell value
               memory[p]++;
               if(memory[p] > 255) {
                   memory[p] = 0; // Whoops, overflown the memory!
               }
               break;
            case data[j] === "-": // decrement cell value
               if(memory[p] > 0) {
                   memory[p]--;
               } else {
                   memory[p] = 255; // Overflow back to 255
               }
               break;
            case data[j] === ".": // put character to screen
               var memoryChar = String.fromCharCode(memory[p]);
               if(memoryChar == '\n') {
                   memoryChar = ""; // Turn newlines into linebreaks
               }
               $('p#output').append(String.fromCharCode(memory[p]));
               break;
            case data[j] === ",":
               memory[p] = window.prompt("Please enter one character").charCodeAt(0); // Set memory to the character code
               break;
            case data[j] === "[": // start loop
               if(memory[p] != 0) {
                   continue;
               } else {
                   var openLoopCnt = 0; // number of open loops
                   for(var k = j; k < data.length; k++) {
                       if(data[k] === '[') {
                           openLoopCnt++; // Another open loop is in existence
                       } else if(data[k] === ']') {
                           openLoopCnt--; // Decrement open count

                           if(openLoopCnt === 0) {
                               j = k;
                               break;
                           }
                       }
                   }
                   if(openLoopCnt != 0) {
                       throwException("Open loop.", j);
                   }
               }
               break;
           case data[j] === "]": // end loop
               if(memory[p] != 0) {
                   var closedLoopCnt = 0; // Due to the fact that we are at closing loops, we use closedLoopCnt
                   for(var l = j; l >= 0; l--) {
                        if(data[l] === "]") {
                            closedLoopCnt++;
                        } else if(data[l] === "[") {
                            closedLoopCnt--;
                        }

                        if(closedLoopCnt === 0) {
                            j = l;
                            break;
                        }
                   }
                   if(closedLoopCnt != 0) {
                        throwException("Too many loop closings.", i);
                   }
               }
               break;
        }
    }
    console.timeEnd("Running Brainf*** code");

    console.time("Saving output in localStorage");
    localStorage.setItem("brainfuck_output", $('#output').text());
    console.timeEnd("Saving output in localStorage");
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<h1>Interpret your Brainf*** code!</h1>
<form action="#" method="get" id="brainf***" onsubmit="runBrainfuck($('#code').val())">
    <label for="brainf***">Brainf*** code:</label> <br />
    <textarea name="code" rows="25" cols="70" id="code"></textarea> <br />
    <input type="submit" value="Interpret Brainf*** code!" />
</form>
<h2>Brainf*** output:</h2>
<p id="output"></p>

// Credit goes to where it is due
function throwException(cause, index) {
    // Execute when there is an error in Brainf*** code
    $('p#output').text("Interpreting Terminated because " + cause + " at char index " + index);
}

function runBrainfuck(data) {
    console.time("Running Brainf*** code");
    var memory = new Array(); // memory

    // Initialize memory
    console.time("Initialize memory");
    for(var i = 0; i < 30000; i++) {
        memory[i] = 0;
    }
    console.timeEnd("Initialize memory");

    var p = 0; // pointer to memory
    $('p#output').text("");

    // Loop through the Brainf*** code
    var data = $('#code').val();

    for(var j = 0; j < data.length; j++) {
        switch(true) {
            case data[j] === ">": // go right one cell
               if(p < 30000) {
                   p++;
               } else {
                   throwException("There are only 30000 usable cells of memory.", j); // are you using too many cells?
               }
               break;
            case data[j] === "<": // go left one cell
               if(p > 0) {
                   p--;
               } else {
                   throwException("Cannot decrement pointer when it is at first cell.", j); // are you going below cell 0?
               }
               break;
            case data[j] === "+": // Increment cell value
               memory[p]++;
               if(memory[p] > 255) {
                   memory[p] = 0; // Whoops, overflown the memory!
               }
               break;
            case data[j] === "-": // decrement cell value
               if(memory[p] > 0) {
                   memory[p]--;
               } else {
                   memory[p] = 255; // Overflow back to 255
               }
               break;
            case data[j] === ".": // put character to screen
               var memoryChar = String.fromCharCode(memory[p]);
               if(memoryChar == '\n') {
                   memoryChar = ""; // Turn newlines into linebreaks
               }
               $('p#output').append(String.fromCharCode(memory[p]));
               break;
            case data[j] === ",":
               memory[p] = window.prompt("Please enter one character").charCodeAt(0); // Set memory to the character code
               break;
            case data[j] === "[": // start loop
               if(memory[p] != 0) {
                   continue;
               } else {
                   var openLoopCnt = 0; // number of open loops
                   for(var k = j; k < data.length; k++) {
                       if(data[k] === '[') {
                           openLoopCnt++; // Another open loop is in existence
                       } else if(data[k] === ']') {
                           openLoopCnt++; // Decrement open count

                           if(openLoopCnt === 0) {
                               j = k;
                               break;
                           }
                       }
                   }
                   if(openLoopCnt != 0) {
                       throwException("Open loop.", j);
                   }
               }
               break;
           case data[j] === "]": // end loop
               if(memory[p] != 0) {
                   var closedLoopCnt = 0; // Due to the fact that we are at closing loops, we use closedLoopCnt
                   for(var l = j; l >= 0; l--) {
                        if(data[l] === "]") {
                            closedLoopCnt++;
                        } else if(data[l] === "[") {
                            closedLoopCnt--;
                        }

                        if(closedLoopCnt === 0) {
                            j = l;
                            break;
                        }
                   }
                   if(closedLoopCnt != 0) {
                        throwException("Too many loop closings.", i);
                   }
               }
               break;
        }
    }
    console.timeEnd("Running Brainf*** code");

    console.time("Saving output in localStorage");
    localStorage.setItem("brainfuck_output", $('#output').text());
    console.timeEnd("Saving output in localStorage");
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<h1>Interpret your Brainf*** code!</h1>
<form action="#" method="get" id="brainf***" onsubmit="runBrainfuck($('#code').val())">
    <label for="brainf***">Brainf*** code:</label> <br />
    <textarea name="code" rows="25" cols="70" id="code"></textarea> <br />
    <input type="submit" value="Interpret Brainf*** code!" />
</form>
<h2>Brainf*** output:</h2>
<p id="output"></p>