You can learn the basic in a few days or months. You can learn basic in a week. But not be job ready. It will take few months or years.
Learning javaScript is not hard. You need to spend four hours a day on learning javaScript.
1 hour in watching javascript lecture
1 hour in reading the javascript book like Javascript definitive guide.
1 hour in javascript coding.
1 hour researching on javascript function, DOM , variable etc.
Total = 120 hours in a month.
Original source