The JavaScript style class return empty string [duplicate]

When i use style property on JavaScript for an existence element it return to me an empty string

alert(element.style.width);

JavaScript return an empty alert box i tried this with many predeclared css property but the same output

var images =    document.querySelectorAll(".slider_image");

document.getElementById("next").onclick = function() {
   for (var i = 0; i < images.length; i++) {
   alert(images[i].style.width);
   }
};

And here is the html

  <div id="first-part">
  <div id="slider">
    <img src="img/1.jpg" class="slider_image" id="1"/>
    <img src="img/2.jpg" class="slider_image" id="2"/>
    <img src="img/3.jpg" class="slider_image" id="3"/>
    <img src="img/next.png" id="next-img"/>
  </div>
  <span id="next">Next</span>  <span id="before">Before</span>
</div>

And here is my css :

.slider_image{
  width:100%;
  position:relative;
  right:0px;
}

What might cause this ?