Superfast JavaScript forEach function

gistfile1.js

function forEach(arr, fn) {
    var impl = fn.toString();
    var m = impl.match(/function \((\w+),?\s*(\w+)?\) { (.*) }/);
    var arg0 = m[1];
    var arg1 = m[2];
    var body = m[3];
    var program = [];
    program.push('var ' + arg0 + ';');
    if (arg1)
        program.push('var ' + arg1 + ';');

    var val;
    for (var i = 0; i < arr.length; i++) {
        val = arr[i];
        program.push(arg0 + ' = ' + val.toString() + ';');
        if (arg1) 
            program.push(arg1 + ' = ' + i + ';');
        program.push(body);
    }
    program = program.join('');
    console.log(program);
    eval(program);
}

var arr = [1, 2, 3];
forEach(arr, function(val, i) { arr[i] += 1; });
console.log(arr);