How to get CSS class property in Javascript?

For modern browsers you can use getComputedStyle:

var elem,
    style;
elem = document.querySelector('.test');
style = getComputedStyle(elem);
style.marginTop; //`20px`
style.marginRight; //`20px`
style.marginBottom; //`20px`
style.marginLeft; //`20px`

margin is a composite style, and not reliable cross-browser. Each of -top -right, -bottom, and -left should be accessed individually.

fiddle

The accepted answer is the best way to get the computed values. I personally needed the pre-computed value. Say for instance 'height' being set to a 'calc()' value. I wrote the following jQuery function to access the value from the style sheet. This script handles nested 'media' and 'supports' queries, CORS errors, and should provide the final cascaded precomputed value for accessible properties.

$.fn.cssStyle = function() {
  var sheets = document.styleSheets, ret = [];
  var el = this.get(0);
  var q = function(rules){
   for (var r in rules) {
    var rule = rules[r];
    if(rule instanceof CSSMediaRule && window.matchMedia(rule.conditionText).matches){
     ret.concat(q(rule.rules || rule.cssRules));
    } else if(rule instanceof CSSSupportsRule){
     try{
      if(CSS.supports(rule.conditionText)){
       ret.concat(q(rule.rules || rule.cssRules));
      }
     } catch (e) {
      console.error(e);
     }
    } else if(rule instanceof CSSStyleRule){
     try{
      if(el.matches(rule.selectorText)){
       ret.push(rule.style);
      }
     } catch(e){
      console.error(e);
     }
    }
   }
  };
  for (var i in sheets) {
   try{
    q(sheets[i].rules || sheets[i].cssRules);
   } catch(e){
    console.error(e);
   }
  }
  return ret.pop();
 };
  
  // Your element
  console.log($('body').cssStyle().height);

Another approach (still experimental) could be computedStyleMap:

const computedStyleMap = document.getElementById('my-id').computedStyleMap();
computedStyleMap.get('overflow-x'); // {value: 'scroll'}
computedStyleMap.has('padding-right'); // false
computedStyleMap.entries(); // Iterator {}