How to apply !important using .css()?

You can set the width directly using .width() like this:

$("#elem").width(100);

Updated for comments: You have this option as well, but it'll replace all css on the element, so not sure it's any more viable:

$('#elem').css('cssText', 'width: 100px !important');

const elem = $("#elem");
elem[0].style.removeAttribute('width');
elem[0].style.setProperty('width', '100px', 'important');

Note: Using Chrome may return an error such as:

elem[0].style.removeAttribute is not a function

Changing the line to use the .removeProperty function such as to elem[0].style.removeProperty('width'); fixed the issue.

I think I've found a solution. I've made it into a new function:

jQuery.style(name, value, priority);

You can use it to get values with .style('name') just like .css('name'), get the CSSStyleDeclaration with .style(), and also set values, with the ability to specify the priority as 'important'. See this.

Example

var div = $('someDiv');
console.log(div.style('color'));
div.style('color', 'red');
console.log(div.style('color'));
div.style('color', 'blue', 'important');
console.log(div.style('color'));
console.log(div.style().getPropertyPriority('color'));

Example output:

null
red
blue
important

---

The Function

(function($) {    
  if ($.fn.style) {
    return;
  }

  // Escape regex chars with \
  var escape = function(text) {
    return text.replace(/[-[\]{}()*+?.,\\^$|#\s]/g, "\\$&");
  };

  // For those who need them (< IE 9), add support for CSS functions
  var isStyleFuncSupported = !!CSSStyleDeclaration.prototype.getPropertyValue;
  if (!isStyleFuncSupported) {
    CSSStyleDeclaration.prototype.getPropertyValue = function(a) {
      return this.getAttribute(a);
    };
    CSSStyleDeclaration.prototype.setProperty = function(styleName, value, priority) {
      this.setAttribute(styleName, value);
      var priority = typeof priority != 'undefined' ? priority : '';
      if (priority != '') {
        // Add priority manually
        var rule = new RegExp(escape(styleName) + '\\s*:\\s*' + escape(value) +
            '(\\s*;)?', 'gmi');
        this.cssText =
            this.cssText.replace(rule, styleName + ': ' + value + ' !' + priority + ';');
      }
    };
    CSSStyleDeclaration.prototype.removeProperty = function(a) {
      return this.removeAttribute(a);
    };
    CSSStyleDeclaration.prototype.getPropertyPriority = function(styleName) {
      var rule = new RegExp(escape(styleName) + '\\s*:\\s*[^\\s]*\\s*!important(\\s*;)?',
          'gmi');
      return rule.test(this.cssText) ? 'important' : '';
    }
  }

  // The style function
  $.fn.style = function(styleName, value, priority) {
    // DOM node
    var node = this.get(0);
    // Ensure we have a DOM node
    if (typeof node == 'undefined') {
      return this;
    }
    // CSSStyleDeclaration
    var style = this.get(0).style;
    // Getter/Setter
    if (typeof styleName != 'undefined') {
      if (typeof value != 'undefined') {
        // Set style property
        priority = typeof priority != 'undefined' ? priority : '';
        style.setProperty(styleName, value, priority);
        return this;
      } else {
        // Get style property
        return style.getPropertyValue(styleName);
      }
    } else {
      // Get CSSStyleDeclaration
      return style;
    }
  };
})(jQuery);

See this for examples of how to read and set the CSS values. My issue was that I had already set !important for the width in my CSS to avoid conflicts with other theme CSS, but any changes I made to the width in jQuery would be unaffected since they would be added to the style attribute.

Compatibility

For setting with the priority using the setProperty function, This Article says there is support for IE 9+ and all other browsers. I have tried with IE 8 and it has failed, which is why I built support for it in my functions (see above). It will work on all other browsers using setProperty, but it will need my custom code to work in < IE 9.

The problem is caused by jQuery not understanding the !important attribute, and as such fails to apply the rule.

You might be able to work around that problem, and apply the rule by referring to it, via addClass():

.importantRule { width: 100px !important; }

$('#elem').addClass('importantRule');

Or by using attr():

$('#elem').attr('style', 'width: 100px !important');

The latter approach would unset any previously set in-line style rules, though. So use with care.

Of course, there's a good argument that @Nick Craver's method is easier/wiser.

The above, attr() approach modified slightly to preserve the original style string/properties, and modified as suggested by falko in a comment:

$('#elem').attr('style', function(i,s) { return (s || '') + 'width: 100px !important;' });