function BefungeBoard(source, constraints) {
    constraints = constraints || {
        width: 80,
        height: 25
    };

    this.constraints = constraints;
    this.grid = source.split(/\r\n|[\n\v\f\r\x85\u2028\u2029]/).map(function (line) {
        return (line + String.repeat(' ', constraints.width - line.length)).split('');
    });
    for (var i = this.grid.length; i < constraints.height; i++) {
        this.grid[i] = String.repeat(' ', constraints.width).split('');
    }

    this.pointer = {
        x: 0,
        y: 0
    };

    this.direction = Direction.RIGHT;
}

BefungeBoard.prototype.nextPosition = function () {
    var vector = this.direction.toVector(),
        nextPosition = {
            x: this.pointer.x + vector[0],
            y: this.pointer.y + vector[1]
        };

    nextPosition.x = nextPosition.x < 0 ? this.constraints.width - 1 : nextPosition.x;
    nextPosition.y = nextPosition.y < 0 ? this.constraints.height - 1 : nextPosition.y;

    nextPosition.x = nextPosition.x >= this.constraints.width ? 0 : nextPosition.x;
    nextPosition.y = nextPosition.y >= this.constraints.height ? 0 : nextPosition.y;

    return nextPosition;
};

BefungeBoard.prototype.advance = function () {
    this.pointer = this.nextPosition();
    if (this.onAdvance) {
        this.onAdvance.call(null, this.pointer);
    }
};

BefungeBoard.prototype.currentToken = function () {
    return this.grid[this.pointer.y][this.pointer.x];
};

BefungeBoard.prototype.nextToken = function () {
    var nextPosition = this.nextPosition();
    return this.grid[nextPosition.y][nextPosition.x];
};

var Direction = (function () {
    var vectors = [
        [1, 0],
        [-1, 0],
        [0, -1],
        [0, 1]
    ];

    function Direction(value) {
        this.value = value;
    }

    Direction.prototype.toVector = function () {
        return vectors[this.value];
    };

    return {
        UP: new Direction(2),
        DOWN: new Direction(3),
        RIGHT: new Direction(0),
        LEFT: new Direction(1)
    };
})();

function BefungeStack() {
    this.stack = [];
}

BefungeStack.prototype.pushAscii = function (item) {
    this.pushNumber(item.charCodeAt());
};

BefungeStack.prototype.pushNumber = function (item) {
    if (isNaN(+item)) {
        throw new Error(typeof item + " | " + item + " is not a number");
    }

    this.stack.push(+item);
};

BefungeStack.prototype.popAscii = function () {
    return String.fromCharCode(this.popNumber());
};

BefungeStack.prototype.popNumber = function () {
    return this.stack.length === 0 ? 0 : this.stack.pop();
};

function Befunge(source, constraints) {
    this.board = new BefungeBoard(source, constraints);
    this.stack = new BefungeStack();
    this.stringMode = false;
    this.terminated = false;

    this.digits = "0123456789".split('');
}

Befunge.prototype.run = function () {
    for (var i = 1; i <= (this.stepsPerTick || 10); i++) {
        this.step();
        if (this.terminated) {
            return;
        }
    }

    requestAnimationFrame(this.run.bind(this));
};

Befunge.prototype.step = function () {
    this.processCurrentToken();
    this.board.advance();
};

Befunge.prototype.processCurrentToken = function () {
    var token = this.board.currentToken();
    if (this.stringMode && token !== '"') {
        return this.stack.pushAscii(token);
    }

    if (this.digits.indexOf(token) !== -1) {
        return this.stack.pushNumber(token);
    }

    switch (token) {
        case ' ':
            while ((token = this.board.nextToken()) == ' ') {
                this.board.advance();
            }
            return;
        case '+':
            return this.stack.pushNumber(this.stack.popNumber() + this.stack.popNumber());
        case '-':
            return this.stack.pushNumber(-this.stack.popNumber() + this.stack.popNumber());
        case '*':
            return this.stack.pushNumber(this.stack.popNumber() * this.stack.popNumber());
        case '/':
            var denominator = this.stack.popNumber(),
                numerator = this.stack.popNumber(),
                result;
            if (denominator === 0) {
                result = +prompt("Illegal division by zero. Please enter the result to use:");
            } else {
                result = Math.floor(numerator / denominator);
            }

            return this.stack.pushNumber(result);
        case '%':
            var modulus = this.stack.popNumber(),
                numerator = this.stack.popNumber(),
                result;
            if (modulus === 0) {
                result = +prompt("Illegal division by zero. Please enter the result to use:");
            } else {
                result = Math.floor(numerator / modulus);
            }

            return this.stack.pushNumber(result);
        case '!':
            return this.stack.pushNumber(this.stack.popNumber() === 0 ? 1 : 0);
        case '`':
            return this.stack.pushNumber(this.stack.popNumber() < this.stack.popNumber() ? 1 : 0);
        case '>':
            this.board.direction = Direction.RIGHT;
            return;
        case '<':
            this.board.direction = Direction.LEFT;
            return;
        case '^':
            this.board.direction = Direction.UP;
            return;
        case 'v':
            this.board.direction = Direction.DOWN;
            return;
        case '?':
            this.board.direction = [Direction.RIGHT, Direction.UP, Direction.LEFT, Direction.DOWN][Math.floor(4 * Math.random())];
            return;
        case '_':
            this.board.direction = this.stack.popNumber() === 0 ? Direction.RIGHT : Direction.LEFT;
            return;
        case '|':
            this.board.direction = this.stack.popNumber() === 0 ? Direction.DOWN : Direction.UP;
            return;
        case '"':
            this.stringMode = !this.stringMode;
            return;
        case ':':
            var top = this.stack.popNumber();
            this.stack.pushNumber(top);
            return this.stack.pushNumber(top);
        case '\\':
            var first = this.stack.popNumber(),
                second = this.stack.popNumber();
            this.stack.pushNumber(first);
            return this.stack.pushNumber(second);
        case '$':
            return this.stack.popNumber();
        case '#':
            return this.board.advance();
        case 'p':
            return this.board.grid[this.stack.popNumber()][this.stack.popNumber()] = this.stack.popAscii();
        case 'g':
            return this.stack.pushAscii(this.board.grid[this.stack.popNumber()][this.stack.popNumber()]);
        case '&':
            return this.stack.pushNumber(+prompt("Please enter a number:"));
        case '~':
            return this.stack.pushAscii(prompt("Please enter a character:")[0]);
        case '.':
            return this.print(this.stack.popNumber());
        case ',':
            return this.print(this.stack.popAscii());
        case '@':
            this.terminated = true;
            return;
    }
};

Befunge.prototype.withStdout = function (printer) {
    this.print = printer;
    return this;
};

Befunge.prototype.withOnAdvance = function (onAdvance) {
    this.board.onAdvance = onAdvance;
    return this;
};

String.repeat = function (str, count) {
    var repeated = "";
    for (var i = 1; i <= count; i++) {
        repeated += str;
    }

    return repeated;
};

window['requestAnimationFrame'] = window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function (callback) {
    window.setTimeout(callback, 1000 / 60);
};

(function () {
    var currentInstance = null;

    function resetInstance() {
        currentInstance = null;
    }

    function getOrCreateInstance() {
        if (currentInstance !== null && currentInstance.terminated) {
            resetInstance();
        }

        if (currentInstance === null) {
            var boardSize = Editor.getBoardSize();
            currentInstance = new Befunge(Editor.getSource(), {
                width: boardSize.width,
                height: boardSize.height
            });
            currentInstance.stepsPerTick = Editor.getStepsPerTick();

            currentInstance.withStdout(Editor.append);
            currentInstance.withOnAdvance(function (position) {
                Editor.highlight(currentInstance.board.grid, position.x, position.y);
            });
        }

        return currentInstance;
    }

    var Editor = (function (onExecute, onStep, onReset) {
        var source = document.getElementById('source'),
            sourceDisplay = document.getElementById('source-display'),
            sourceDisplayWrapper = document.getElementById('source-display-wrapper'),
            stdout = document.getElementById('stdout');
        var execute = document.getElementById('execute'),
            step = document.getElementById('step'),
            reset = document.getElementById('reset');
        var boardWidth = document.getElementById('board-width'),
            boardHeight = document.getElementById('board-height'),
            stepsPerTick = document.getElementById('steps-per-tick');

        function showEditor() {
            source.style.display = "block";
            sourceDisplayWrapper.style.display = "none";
            source.focus();
        }

        function hideEditor() {
            source.style.display = "none";
            sourceDisplayWrapper.style.display = "block";

            var computedHeight = getComputedStyle(source).height;
            sourceDisplayWrapper.style.minHeight = computedHeight;
            sourceDisplayWrapper.style.maxHeight = computedHeight;

            sourceDisplay.textContent = source.value;
        }

        function resetOutput() {
            stdout.value = null;
        }

        function escapeEntities(input) {
            return input.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
        }

        sourceDisplayWrapper.onclick = function () {
            resetOutput();
            showEditor();
            onReset && onReset.call(null);
        };
        execute.onclick = function () {
            resetOutput();
            hideEditor();
            onExecute && onExecute.call(null);
        };
        step.onclick = function () {
            hideEditor();
            onStep && onStep.call(null);
        };
        reset.onclick = function () {
            resetOutput();
            showEditor();
            onReset && onReset.call(null);
        };

        return {
            getSource: function () {
                return source.value;
            },

            append: function (content) {
                stdout.value = stdout.value + content;
            },

            highlight: function (grid, x, y) {
                var highlighted = [];
                for (var row = 0; row < grid.length; row++) {
                    highlighted[row] = [];
                    for (var column = 0; column < grid[row].length; column++) {
                        highlighted[row][column] = escapeEntities(grid[row][column]);
                    }
                }

                highlighted[y][x] = '<span class="activeToken">' + highlighted[y][x] + '</span>';
                sourceDisplay.innerHTML = highlighted.map(function (lineTokens) {
                    return lineTokens.join('');
                }).join('\n');
            },

            getBoardSize: function () {
                return {
                    width: +boardWidth.innerHTML,
                    height: +boardHeight.innerHTML
                };
            },

            getStepsPerTick: function () {
                return +stepsPerTick.innerHTML;
            }
        };
    })(function () {
        getOrCreateInstance().run();
    }, function () {
        getOrCreateInstance().step();
    }, resetInstance);
})();

.container {
    width: 100%;
}
.so-box {
    font-family:'Helvetica Neue', Arial, sans-serif;
    font-weight: bold;
    color: #fff;
    text-align: center;
    padding: .3em .7em;
    font-size: 1em;
    line-height: 1.1;
    border: 1px solid #c47b07;
    -webkit-box-shadow: 0 2px 2px rgba(0, 0, 0, 0.3), 0 2px 0 rgba(255, 255, 255, 0.15) inset;
    text-shadow: 0 0 2px rgba(0, 0, 0, 0.5);
    background: #f88912;
    box-shadow: 0 2px 2px rgba(0, 0, 0, 0.3), 0 2px 0 rgba(255, 255, 255, 0.15) inset;
}
.control {
    display: inline-block;
    border-radius: 6px;
    float: left;
    margin-right: 25px;
    cursor: pointer;
}
.option {
    padding: 10px 20px;
    margin-right: 25px;
    float: left;
}
input, textarea {
    box-sizing: border-box;
}
textarea {
    display: block;
    white-space: pre;
    overflow: auto;
    height: 75px;
    width: 100%;
    max-width: 100%;
    min-height: 25px;
}
span[contenteditable] {
    padding: 2px 6px;
    background: #cc7801;
    color: #fff;
}
#controls-container, #options-container {
    height: auto;
    padding: 6px 0;
}
#stdout {
    height: 50px;
}
#reset {
    float: right;
}
#source-display-wrapper {
    display: none;
    width: 100%;
    height: 100%;
    overflow: auto;
    border: 1px solid black;
    box-sizing: border-box;
}
#source-display {
    font-family: monospace;
    white-space: pre;
    padding: 2px;
}
.activeToken {
    background: #f88912;
}
.clearfix:after {
    content:".";
    display: block;
    height: 0;
    clear: both;
    visibility: hidden;
}
.clearfix {
    display: inline-block;
}
* html .clearfix {
    height: 1%;
}
.clearfix {
    display: block;
}

<div class="container">
    <textarea id="source" placeholder="Enter your Befunge-93 program here" wrap="off">&> :#v_,>:#,_@
 ^-1:<</textarea>
    <div id="source-display-wrapper">
        <div id="source-display"></div>
    </div>
</div>
<div id="controls-container" class="container clearfix">
    <input type="button" id="execute" class="control so-box" value="► Execute" />
    <input type="button" id="step" class="control so-box" value="Step" />
    <input type="button" id="reset" class="control so-box" value="Reset" />
</div>
<div id="stdout-container" class="container">
    <textarea id="stdout" placeholder="Output" wrap="off" readonly></textarea>
</div>
<div id="options-container" class="container">
    <div class="option so-box">Steps per Tick: <span id="steps-per-tick" contenteditable>500</span>

    </div>
    <div class="option so-box">Board Size: <span id="board-width" contenteditable>80</span> x <span id="board-height" contenteditable>25</span>

    </div>
</div>