JS problem declaring array in class

Question

var dynamic_particle=function(){
        this.init();
    }
dynamic_particle.prototype={
       init:function(){
           this.a=new Array();
           this.a.push(1);
           alert(a[0]);
        }
    }

a error is undefined

Answer 1

var dynamic_particle=function(){
        this.init();
    }
dynamic_particle.prototype={
       init:function(){
           this.a=new Array();
           this.a.push(1);
           alert(this.a[0]);
        }
    }

You all know how to use this.a before. Why not add this. when using alert... an error will definitely be reported

Answer 2

Encapsulation method is generally used like this:

var defOpt = {
    a: [1]
}

var dynamic_particle=function(options){
    if(!(this instanceof dynamic_particle)){
        return new dynamic_particle(options);
    }
    this.opt = $.extend({}, defOpt, options);
    this.init();
}
dynamic_particle.prototype={

    init:function(){
       console.log('初始化数据：', this.opt);
    },
    other: function() {
        console.log('调用option中的新数据', this.opt.newData);
    }
}
var dp = dynamic_particle({
    newData: [2] 
})
dp.other()