How to get an HTML element's style values in javascript?

I am looking for a way to retrieve the style from an element that has a style set upon it by the style tag.

<style> 
#box {width: 100px;}
</style>

In the body

<div id="box"></div>

I'm looking for straight javascript without the use of libraries.

I tried the following, but keep receiving blanks:

alert (document.getElementById("box").style.width);  
alert (document.getElementById("box").style.getPropertyValue("width"));

I noticed that I'm only able to use the above if I have set the style using javascript, but unable to with the style tags.

The element.style property lets you know only the CSS properties that were defined as inline in that element (programmatically, or defined in the style attribute of the element), you should get the computed style.

Is not so easy to do it in a cross-browser way, IE has its own way, through the element.currentStyle property, and the DOM Level 2 standard way, implemented by other browsers is through the document.defaultView.getComputedStyle method.

The two ways have differences, for example, the IE element.currentStyle property expect that you access the CCS property names composed of two or more words in camelCase (e.g. maxHeight, fontSize, backgroundColor, etc), the standard way expects the properties with the words separated with dashes (e.g. max-height, font-size, background-color, etc).

Also, the IE element.currentStyle will return all the sizes in the unit that they were specified, (e.g. 12pt, 50%, 5em), the standard way will compute the actual size in pixels always.

I made some time ago a cross-browser function that allows you to get the computed styles in a cross-browser way:

function getStyle(el, styleProp) {
  var value, defaultView = (el.ownerDocument || document).defaultView;
  // W3C standard way:
  if (defaultView && defaultView.getComputedStyle) {
    // sanitize property name to css notation
    // (hypen separated words eg. font-Size)
    styleProp = styleProp.replace(/([A-Z])/g, "-$1").toLowerCase();
    return defaultView.getComputedStyle(el, null).getPropertyValue(styleProp);
  } else if (el.currentStyle) { // IE
    // sanitize property name to camelCase
    styleProp = styleProp.replace(/\-(\w)/g, function(str, letter) {
      return letter.toUpperCase();
    });
    value = el.currentStyle[styleProp];
    // convert other units to pixels on IE
    if (/^\d+(em|pt|%|ex)?$/i.test(value)) { 
      return (function(value) {
        var oldLeft = el.style.left, oldRsLeft = el.runtimeStyle.left;
        el.runtimeStyle.left = el.currentStyle.left;
        el.style.left = value || 0;
        value = el.style.pixelLeft + "px";
        el.style.left = oldLeft;
        el.runtimeStyle.left = oldRsLeft;
        return value;
      })(value);
    }
    return value;
  }
}

The above function is not perfect for some cases, for example for colors, the standard method will return colors in the rgb(...) notation, on IE they will return them as they were defined.

I'm currently working on an article in the subject, you can follow the changes I make to this function here.

I believe you are now able to use Window.getComputedStyle()

Documentation MDN

var style = window.getComputedStyle(element[, pseudoElt]);

Example to get width of an element:

window.getComputedStyle(document.querySelector('#mainbar')).width

In jQuery, you can do alert($("#theid").css("width")).

-- if you haven't taken a look at jQuery, I highly recommend it; it makes many simple javascript tasks effortless.

Update

for the record, this post is 5 years old. The web has developed, moved on, etc. There are ways to do this with Plain Old Javascript, which is better.

Use getComputedStyle function, Computed style contains all the CSS properties set to an element. Even if do not set a property to an element. You will still find that property in the computed styles.

Example:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

    <style>
    #Body_element {
    color: green;
    }
    </style>

    <body id="Body_element">
    <script>
         alert(getComputedStyle(Body_element).color)
    </script>
    </body>

<!-- end snippet -->

This is a helper function if you want to get multiple style rules from the same element. You pass it the element and the styles you want as arguments, and it will return their values

const convertRestArgsIntoStylesArr = ([...args]) => {
	return args.slice(1);
}

const getStyles = function () {
	const args = [...arguments];
	const [element] = args;

	let stylesProps = [...args][1] instanceof Array ? args[1] : convertRestArgsIntoStylesArr(args);

	const styles = window.getComputedStyle(element);
	const stylesObj = stylesProps.reduce((acc, v) => {
		acc[v] = styles.getPropertyValue(v);
		return acc;
	}, {});

	return stylesObj;
};

Now, you can use this function like this:

const styles = getStyles(document.body, "height", "width");

OR

const styles = getStyles(document.body, ["height", "width"]);