a.js

for (let i = 0; i < 10; i++) {
    let read_img_path;

    if (i === 0) {
        read_img_path = "./blue_a.png"
    } else {
        read_img_path = "./blue_" + (i - 1) + ".png";
    }

    let img_write_path = "./blue_" + i + ".png";

    let img_i = cv.imread(read_img_path);
    result_img = convert_img(img_i);
    cv.imwrite(img_write_path, result_img);

}