How to get CSS transform rotation value in degrees with JavaScript

Question

I'm using the code found at CSS-Tricks to get the current rotation transform (in CSS) with JavaScript.

JavaScript function:

function getCurrentRotation( elid ) {
  var el = document.getElementById(elid);
  var st = window.getComputedStyle(el, null);
  var tr = st.getPropertyValue("-webkit-transform") ||
       st.getPropertyValue("-moz-transform") ||
       st.getPropertyValue("-ms-transform") ||
       st.getPropertyValue("-o-transform") ||
       st.getPropertyValue("transform") ||
       "fail...";

  if( tr !== "none") {
    console.log('Matrix: ' + tr);

    var values = tr.split('(')[1];
      values = values.split(')')[0];
      values = values.split(',');
    var a = values[0];
    var b = values[1];
    var c = values[2];
    var d = values[3];

    var scale = Math.sqrt(a*a + b*b);

    // arc sin, convert from radians to degrees, round
    /** /
    var sin = b/scale;
    var angle = Math.round(Math.asin(sin) * (180/Math.PI));
    /*/
    var angle = Math.round(Math.atan2(b, a) * (180/Math.PI));
    /**/

  } else {
    var angle = 0;
  }

  // works!
  console.log('Rotate: ' + angle + 'deg');
  $('#results').append('<p>Rotate: ' + angle + 'deg</p>');
}

According to the post, this works, however, for values over 180 degrees, I get negative numbers, and 360deg returns zero. I need to be able to correctly return the degree value from 180-360 degrees.

What am I doing wrong with this code that won't let it return the correct degree turn over 180 degrees?

It will make a lot more sense if you view the demo: See the pen for a demo of this in action.

Can you not simply add 360 to angle if it is smaller than or = to 0? — Brian Hannay, Oct 24 '13 at 18:56
Yes, just added an answer with my updated code. Figured it had to do with my lack of geometry knowledge. Found help here: http://stackoverflow.com/questions/10343448/javascript-atan2-function-not-giving-expected-results — jjeaton, Oct 24 '13 at 19:05

danicotra · Answer 1 · 2019-02-03T10:30:05.213

I came in need of something like this too and decided to start from the initial code, doing a little clean up and some little improvement; then I modified as for the OP needing, so I wanted to share it here now:

function getCurrentRotation(el){
  var st = window.getComputedStyle(el, null);
  var tm = st.getPropertyValue("-webkit-transform") ||
           st.getPropertyValue("-moz-transform") ||
           st.getPropertyValue("-ms-transform") ||
           st.getPropertyValue("-o-transform") ||
           st.getPropertyValue("transform") ||
           "none";
  if (tm != "none") {
    var values = tm.split('(')[1].split(')')[0].split(',');
    /*
    a = values[0];
    b = values[1];
    angle = Math.round(Math.atan2(b,a) * (180/Math.PI));
    */
    //return Math.round(Math.atan2(values[1],values[0]) * (180/Math.PI)); //this would return negative values the OP doesn't wants so it got commented and the next lines of code added
    var angle = Math.round(Math.atan2(values[1],values[0]) * (180/Math.PI));
    return (angle < 0 ? angle + 360 : angle); //adding 360 degrees here when angle < 0 is equivalent to adding (2 * Math.PI) radians before
  }
  return 0;
}

Use it like this:

getCurrentRotation(document.getElementById("el_id"));

Thanks for sharing your efforts! I really appreciate it! – Ricalsin May 29 '19 at 18:17 — Ricalsin, May 29 '19 at 18:17

score 11 · Accepted Answer · answered Oct 24 '13 at 19:04

Found the answer in another SO question, you have to add (2 * PI) if the result in radians is less than zero.

This line:

var angle = Math.round(Math.atan2(b, a) * (180/Math.PI));

Needs to be replaced with this:

var radians = Math.atan2(b, a);
if ( radians < 0 ) {
  radians += (2 * Math.PI);
}
var angle = Math.round( radians * (180/Math.PI));

How to get CSS transform rotation value in degrees with JavaScript

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