JavaScript How do I make a div go left and then right in javascript? (no jQuery) make a left and then a right

This should be simple but I guess no jQuery makes it a bit difficult.
I want to repeat a process where a div goes 100px to the right (with animation) and then 100px to the left (so i want a continuous movement).
There seems to be plenty of jQuery answers to this question yet no pure javascript solution. I'm probably missing something obvious here yet I can't find it.
Here is the code:

var left = 0;
var id = setInterval(function(){goRight()}, 10);
var ed = setInterval(function(){goLeft()}, 10);

function goRight(){
    var redpixel = document.getElementById("redpixel");
    left++;
    redpixel.style.left = left + "px";
    if (left>100) {
        clearInterval(id)
        goLeft();
    }
}

function goLeft(){
    var redpixel = document.getElementById("redpixel");
    left-=1;
    redpixel.style.left = left + "px";
    if (left<100) {
        clearInterval(ed);
        goRight()
    }
}

HTML: 

    <button onclick="goRight()">Go Right</button>
    <div id="redpixel"></div>

CSS:

.container {
    width: 480px;
    height: 800px;
    border: 1px solid black;
}

#redpixel {
    position: absolute;
    top: 200px;
    left: 0;
    background: red;
    width: 25px;
    height: 25px;
}

Last comments:

  1. The animation starts without me calling any function (without using the button), how is that possible?
  2. The animation works but stops when it hits the first 100px.
  3. (Additional question) - if i put the var redpixel out of the function it doesn't work at all, why?

All help appreciated, thanks!

Answer:1

The problem with your code is that you set left and right animations at the same time, and the left one is cleared immediately because left<100. Fixed code:

var left = 0,
    id = setInterval(goRight, 10); 
    ed;

function goRight() {
    var redpixel = document.getElementById("redpixel");
    left++;
    redpixel.style.left = left + "px";
    if (left > 100) {
        clearInterval(id);
        ed = setInterval(goLeft, 10);
    }
}

function goLeft() {
    var redpixel = document.getElementById("redpixel");
    left -= 1;
    redpixel.style.left = left + "px";
    if (left < 1) {
        clearInterval(ed);
        id = setInterval(goRight, 10);
    }
}
#redpixel {
    position: absolute;
    top: 50px;
    left: 0;
    background: red;
    width: 25px;
    height: 25px;
}
<div id="redpixel"></div>
Answer:2

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