how to alert after all images loaded?

how to alert after all images loaded?

im beginner in javascript i`m building javascript game and i want to alert
after all images loaded i tried this code but its not working
function loadEveryThing(){
var imgNumber = 0;
img1 = new Image();
img1.src= "1.png"
img1.onload = function(){
imgNumber =imgNumber+1;
}
img2 = new Image();
img2.src= "2.png"
img2.onload = function(){
imgNumber =imgNumber+1;
}
img3 = new Image();
img3.src= "3.png"
img3.onload = function(){
imgNumber =imgNumber+1;
}
if(imgNumber==3)alert("done")
}