值不断返回未定义

Question

Not sure why i keep getting undefined for this results any help would be great. The result is suppose to the the array with the x value at the beginning. thanks

var tester = [1,2,4];
Array.prototype.cons = function(x){
    function reduce(results,y){
        if(y == 1){
            results.unshift(x);
            return results;
        }
        else{
            results.push(this[y-1]);
            y = y-1;
            reduce(results, y);
        }
    }
    return reduce([], this.length);
}
document.getElementById('test').innerHTML = tester.cons(0)

Answer 1

You designed your reduce function to return results, but in your recusive call of it

else{
    results.push(this[y-1]);
    y = y-1;
    reduce(results, y);  // <--- HERE
}

You aren't doing anything with the returned value (such as returning it up the stack). This means that evaluation continues down your function, at the bottom of which there is no return statement. In JavaScript, no return statement means that the return value of the function call will be undefined

Answer 2

If you're just trying to move an element in the array to the front, you can simply use this instead of recursively going through the array.

var tester = [1,2,4];
Array.prototype.cons = function(x){
    // Copy the array. This only works with simple vars, not objects
    var newArray = this.slice(0);        

    // Check to make sure the element you want to move is in the array
    if (x < this.length) {
        // Remove it from the array
        var element = newArray.splice(x, 1);
        // Add to the beginning of the array
        newArray.unshift(element);
    }
    return newArray;
}
document.getElementById('test').innerHTML = tester.cons(4)​;​

EDIT: Made a copy of the array