如何摆脱未定义？

Question

When I console.log the array below, undefined is a part of each array item, I would like to know why is that and how to get rid of it?

new_colors.addEventListener("click", function() {
    var rgb_guesses = new Array();
    for(var i = 0; i < color_squares.length; i++) {
        var rgb_value = "rgb" + "(" + random() + "," + " " + random() + "," + " " + random() + ")";
        color_squares[i].style.background = rgb_value;
        rgb_guesses[i] += rgb_value;
    }
    guess_rgb.textContent = color_squares[3].style.background;
    console.log(rgb_guesses);
});

Answer 1

You've created an empty array

So, the statement

rgb_guesses[i] += rgb_value; 

is shorthand for

rgb_guesses[i] = rgb_guesses[i] + rgb_value;

Now as the array is empty, rgb_guesses[i] will be undefined until you assign a value to it ... undefined when coerced to a string, is "undefined"

So, your code is doing the equivalent of

rgb_guesses[i] = "undefined" + rgb_value;

Since you are only ever assigning a single value to each element in the array, you can change your code to simply

rgb_guesses[i] = rgb_value;

Or

rgb_guesses.push(rgb_value);