javascript变量发生了神奇的变化

Question

代码：

this.init = function(x, y, size, angle, speed, life, startColor, endColor, startAlpha, endAlpha, endSize)
{
    console.log("particle init called");
    this.x = x;
    this.y = y;
    this.size = size;
    this.motionX = speed * Math.sin(angle);
    this.motionY = speed * Math.cos(angle);
    this.life = life;
    this.timer = 0;
    this.startColor = startColor;
    this.endColor = endColor;
    this.startAlpha = startAlpha;
    this.endAlpha = endAlpha;
    this.endSize = endSize;
    console.log("here's what i just set stuff to: timer=" + this.timer);
    console.log(this);
};

当我实际致电init()时，日志显示：

here's what i just set stuff to: timer=0

随后是objct的转储，内容为：

timer: 54

那么，为什么timer的值读出为0，然后立即神奇地将其设置为某个随机数？