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HTML

              
                <div class="preview" id="preview-old">
  <p class="title">Normal textarea behavior</p>
  <textarea spellcheck='false'>
  ## Feature detection + fallback

When using ***feature detection***, a pretty smart way to work when you aren't sure which features you can/cannot use consists in **several feature detections and consequent *fallbacks* to more basic methods** (or even creation of these methods from scratch) in case the features you want to use are not supported.

A simple example of *feature detection with fallback* may be applied to the `window.requestAnimationFrame` feature, which is not supported by all the browsers, and has several different prefixes depending on the browser you're working on. In this case, you can easily detect and *fallback* like this:

    requestAnimationFrame = 
       window.requestAnimationFrame       // Standard name
    || window.webkitRequestAnimationFrame // Fallback to -webkit (Chrome or Safari)
    || window.mozRequestAnimationFrame    // Fallback to -moz (Mozilla Firefox)
    || false;                             // Feature not supported :(

    // Same goes for cancelAnimationFrame
    cancelAnimationFrame = window.cancelAnimationFrame || window.webkitCancelAnimationFrame || window.mozCancelAnimationFrame || window.msCancelAnimationFrame || window.oCancelAnimationFrame || false;

    if (!requestAnimationFrame) {
        // Not supported? Build it by yourself!
        requestAnimationFrame = function(callback) {
            return setTimeout(callback, 0);
        }
		
		// No requestAnim. means no cancelAnim. Built that too.
        cancelAnimationFrame = function(id) {
            clearTimeout(id);
        }
    }

    // Now you can use requestAnimationFrame 
    // No matter which browser you're running
    var animationID = requestAnimationFrame(myBeautifulFunction);  
  </textarea>
</div>

<div class="preview" id="preview-new">
  <p class="title">Using textarea-devtab.js</p>
  <textarea spellcheck='false'>
  ## Feature detection + fallback

When using ***feature detection***, a pretty smart way to work when you aren't sure which features you can/cannot use consists in **several feature detections and consequent *fallbacks* to more basic methods** (or even creation of these methods from scratch) in case the features you want to use are not supported.

A simple example of *feature detection with fallback* may be applied to the `window.requestAnimationFrame` feature, which is not supported by all the browsers, and has several different prefixes depending on the browser you're working on. In this case, you can easily detect and *fallback* like this:

    requestAnimationFrame = 
       window.requestAnimationFrame       // Standard name
    || window.webkitRequestAnimationFrame // Fallback to -webkit (Chrome or Safari)
    || window.mozRequestAnimationFrame    // Fallback to -moz (Mozilla Firefox)
    || false;                             // Feature not supported :(

    // Same goes for cancelAnimationFrame
    cancelAnimationFrame = window.cancelAnimationFrame || window.webkitCancelAnimationFrame || window.mozCancelAnimationFrame || window.msCancelAnimationFrame || window.oCancelAnimationFrame || false;

    if (!requestAnimationFrame) {
        // Not supported? Build it by yourself!
        requestAnimationFrame = function(callback) {
            return setTimeout(callback, 0);
        }
		
		// No requestAnim. means no cancelAnim. Built that too.
        cancelAnimationFrame = function(id) {
            clearTimeout(id);
        }
    }

    // Now you can use requestAnimationFrame 
    // No matter which browser you're running
    var animationID = requestAnimationFrame(myBeautifulFunction);  
  </textarea>
</div>
              
            
!

CSS

              
                body {
  width: 100%;
  margin: 0;
  padding: 0;
  font-family: monospace;
}

.preview {
  position: absolute;
  display: block;
  width: 50%;
  height: 100%;
  text-align: center;
}

.preview textarea {
  position: relative;
  display: inline-block;
  width: 90%;
  height: 80%;
  resize: none;
}

.preview textarea:focus {
  outline: none;
}

.title {
  padding: 0;
  margin: 10px 0 10px 0;
  font-size: 18px;
  font-weight: bold;
}

#preview-old { right: 50%; }
#preview-new { left: 50%; }
#preview-old textarea { border: solid 2px crimson; }
#preview-new textarea { border: solid 2px green; }
#preview-old textarea:focus {  background: rgba(255, 77, 0, 0.11); }
#preview-new textarea:focus {  background: rgba(0, 255, 71, 0.11) }
              
            
!

JS

              
                /*
 * Title: Textarea developer TAB key (beta)
 * Author: Marco Bonelli (http://stackoverflow.com/users/3889449)
 *
 * Description:
 *   Enables extended TAB functionality on textareas of the current page for indenting/dedenting single/multiple lines.
 *   The default indentation is 4 spaces ("    "); change the var TAB_CHAR if you want to use another TAB delimiter.
 *   Originally developed for Stack Overflow Markdown editor.
 *
 * Implementation:
 *   Use 'element.enableTextareaDev()' to enable the feature for all the textareas inside the element (including the element itself).
 *   Use 'element.disableTextareaDev()' to disable the previously enabled feature.
 *
 * Usage:
 *   Single line: place the cursor where you want and press [TAB] to add a tab character, or [SHIFT]+[TAB] to remove a previously inserted tab character (before the caret position).
 *   Multi line: select multiple lines of code and press [TAB] to add a tab character at the beginning of each line, or [SHIFT]+[TAB] to remove previously inserted tab characters.
 *
 */

(function() {
    function textareaDev(e) {
        var TAB_CHAR = '    ',
            t = e.target,
            ds = 0,
            s, ss, se, v;

        if (t.tagName == 'TEXTAREA' && e.keyCode == 9) {
            e.stopPropagation();
            e.preventDefault();

            ss = t.selectionStart;
            se = t.selectionEnd;
            s = t.value.substring(ss, se);
            v = t.value;

            if (~s.indexOf('\n')) {
                if (e.shiftKey) {
                    s = s.split('\n').map(function(el) {
                        if (el.substr(0,TAB_CHAR.length) == TAB_CHAR) return el.substr(4);
                        return el;
                    }).join('\n');

                    t.value = v.substr(0, ss) + s + v.substr(se);
                } else {
                    s = s.split('\n').map(function(el) { return TAB_CHAR + el }).join('\n');
                    t.value = v.substr(0, ss) + s + v.substr(se);
                }

                t.selectionStart = ss;
                t.selectionEnd = ss+s.length;
            } else {
                if (e.shiftKey) {
                    if (v.substr(ss-TAB_CHAR.length, TAB_CHAR.length) == TAB_CHAR) {
                        t.value = v.substr(0, ss-TAB_CHAR.length) + v.substr(se);
                        ds = -TAB_CHAR.length;
                    }
                } else {
                    t.value = v.substr(0, ss) + TAB_CHAR + v.substr(se);
                    ds = TAB_CHAR.length;
                }

                t.selectionStart = t.selectionEnd = ss + ds;
            }
        }
    }

    Object.defineProperties(Element.prototype, {
       enableTextareaDevTab: {
           value: function() {
               this.addEventListener('keydown', textareaDev, true);
           }
       },
       disableTextareaDevTab: {
           value: function() {
               this.removeEventListener('keydown', textareaDev);
           }
       }
    });

    document.getElementById('preview-new').enableTextareaDevTab();
})();
              
            
!
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