var babyEyeCount = this.babyEyeCount;为什么要设置临时变量呢

var babyEyeCount = this.babyEyeCount;

ctx1.drawImage(babyEye[babyEyeCount],-babyEye[babyEyeCount].width*0.5,-babyEye[babyEyeCount].height*0.5);

这里的babyEyeCount为什么要用临时变量去赋值？直接用this.babyEyeCount不是一样吗？