[findTransformECC] (https://docs.opencv.org/master/dc/d6b/group__video__track.html#ga1aa357007eaec11e9ed03500ecbcbe47) has 2 optional parameter (inputMask and gaussFiltSize) but if you don't include them you get an error.
so what should be used for inputMask? "null" does not work.
function Align_img(){
let image_baseline = cv.imread(imgElement_Baseline);
let image = cv.imread('imageChangeup');
let im1_gray = new cv.Mat();
let im2_gray = new cv.Mat();
let im2_aligned = new cv.Mat();
//get size of baseline image
var width1 = image_baseline.cols;
var height1 = image_baseline.rows;
//resize image to baseline image
let dim1 = new cv.Size(width1, height1);
cv.resize(image, image, dim1, cv.INTER_AREA);
// Convert images to grayscale
cv.cvtColor(image_baseline, im1_gray, cv.COLOR_BGR2GRAY);
cv.cvtColor(image, im2_gray, cv.COLOR_BGR2GRAY);
// Find size of image1
let dsize = new cv.Size(image_baseline.rows, image_baseline.cols);
// Define the motion model
const warp_mode = cv.MOTION_HOMOGRAPHY;
// Define 3x3 matrix and initialize the matrix to identity
let warp_matrix = cv.Mat.eye(3, 3, cv.CV_8U);
// Specify the number of iterations.
const number_of_iterations = 5000;
// Specify the threshold of the increment in the correlation coefficient between two iterations
const termination_eps = 0.0000000001; //1e-10;
// Define termination criteria
//const criteria = (cv.TERM_CRITERIA_EP | cv.TERM_CRITERIA_COUNT, number_of_iterations, termination_eps);
let criteria = new cv.TermCriteria(cv.TERM_CRITERIA_EP | cv.TERM_CRITERIA_COUNT, number_of_iterations, termination_eps);
//Run the ECC algorithm. The results are stored in warp_matrix.
//let inputMask = new cv.Mat.zeros(im1_gray.size(), cv.CV_8UC3); //uint8
cv.findTransformECC(im1_gray, im2_gray, warp_matrix, warp_mode, criteria, null, 5);
// Use warpPerspective for Homography
cv.warpPerspective (image, im2_aligned, warp_matrix, dsize, cv.INTER_LINEAR + cv.WARP_INVERSE_MAP);
cv.imshow('imageChangeup', im2_aligned);
im1_gray.delete();
im2_gray.delete();
im2_aligned.delete();
};