I found the source code like this. I am familiar with javascript anonymous function though,
what is :?
For example
When I call createOverlays function with argument, how can I make it ? 
I am googling around about : but unable to find good explanation.
I might not understand the basic javascript structure.
Please give some hint.
var World = {
    init: function initFn() {   
        this.createOverlays();
    },
    createOverlays: function createOverlaysFn() {
         // some function
    },
}
simply solved. I can pass the argument like this.
var World = {
    init: function initFn() {   
        this.createOverlays(1);
    },
    createOverlays: function createOverlaysFn(arg) {
         // some function
         console.log(arg) //show 1
    },
}
 
    