Аналог `element.style.left = variable + 'px'`, только без 'px'

var $ = (function () { 
  var allowNumber = ['lineHeight']; 
 
  function $(el) { 
    if (this instanceof $) { 
      this.el = el; 
    } else { 
      return new $(el); 
    } 
  } 
 
  $.prototype.css = function (key, value) { 
    key = key.replace(/-(\w)/g, function (m, ch) { return ch.toUpperCase() }); 
   
    if (+value == value && allowNumber.indexOf(key) === -1) { 
      value += 'px'; 
    } 
     
    this.el.style[key] = value; 
     
    return this; 
  } 
   
  return $; 
})(); 
 
$(document.querySelector("div")).css('left', 100).css('line-height', 3);

div { 
  position: absolute; 
  left: 0; 
  top: 0; 
  background: silver; 
}

<div>123</div>