preload.js

// 1つの場合
function preloader() {
    heavyImage = new Image(); 
    heavyImage.src = "sample.jpg";
}

// 複数ある場合
function preloader() {
    // counter
    var i = 0;

    // create object
    imageObj = new Image();

    // set image list
    images = [];
    images[0] = "sample1.jpg"
    images[1] = "sample2.jpg"
    images[2] = "sample3.jpg"
    images[3] = "sample4.jpg"
    
    // start preloading
    for(i=0; i<images.length; i++) {
        imageObj.src = images[i];
    }
}