Element.prototype.classList polyfill

classList.js

if ( 'document' in self ) {

	// Full polyfill for browsers with no classList support
	// Including IE < Edge missing SVGElement.classList
	if ( !( 'classList' in document.createElement( '_' ) )
		|| document.createElementNS && !( 'classList' in document.createElementNS( 'http://www.w3.org/2000/svg', 'g' ) ) ) {

		( function( view ) {
			'use strict';
			var DOMEx;
			var checkTokenAndGetIndex;
			var ClassList;
			var classListGetter;
			var classListPropertyDescriptor;

			if ( !( 'Element' in view ) ) {
				return;
			}


			// Vendors: please allow content code to instantiate DOMExceptions
			DOMEx = function( type, message ) {
				this.name = type;
				this.code = DOMException[type];
				this.message = message;
			};

			// Most DOMException implementations don't allow calling DOMException's toString()
			// on non-DOMExceptions. Error's toString() is sufficient here.
			DOMEx.prototype = Error.prototype;

			checkTokenAndGetIndex = function( classList, token ) {
				if ( token === '' ) {
					throw new DOMEx( 'SYNTAX_ERR', 'An invalid or illegal string was specified' );
				}
				if ( /\s/.test( token ) ) {
					throw new DOMEx( 'INVALID_CHARACTER_ERR', 'String contains an invalid character' );
				}
				return [].indexOf.call( classList, token );
			};

			ClassList = function( elem ) {
				var trimmedClasses = String.prototype.trim.call( elem.getAttribute( 'class' ) || '' );
				var classes = trimmedClasses ? trimmedClasses.split( /\s+/ ) : [];
				var len = classes.length;
				var i = 0;

				for ( ; i < len; i++ ) {
					this.push( classes[i]);
				}
				this._updateClassName = function() {
					elem.setAttribute( 'class', this.toString() );
				};
			};

			ClassList.prototype = [];

			classListGetter = function() {
				return new ClassList( this );
			};

			ClassList.prototype.item = function( i ) {
				return this[i] || null;
			};

			ClassList.prototype.contains = function( token ) {
				token += '';
				return checkTokenAndGetIndex( this, token ) !== -1;
			};

			ClassList.prototype.add = function() {
				var tokens = arguments;
				var i = 0;
				var l = tokens.length;
				var token;
				var updated = false;

				do {
					token = tokens[i] + '';
					if ( checkTokenAndGetIndex( this, token ) === -1 ) {
						this.push( token );
						updated = true;
					}
				} while ( ++i < l );

				if ( updated ) {
					this._updateClassName();
				}
			};

			ClassList.prototype.remove = function() {
				var tokens = arguments;
				var i = 0;
				var l = tokens.length;
				var token;
				var updated = false;
				var index;

				do {
					token = tokens[i] + '';
					index = checkTokenAndGetIndex( this, token );
					while ( index !== -1 ) {
						this.splice( index, 1 );
						updated = true;
						index = checkTokenAndGetIndex( this, token );
					}
				} while ( ++i < l );

				if ( updated ) {
					this._updateClassName();
				}
			};

			ClassList.prototype.toggle = function( token, force ) {
				var result;
				var method;

				token += '';
				result = this.contains( token );
				method = result ? force !== true && 'remove' : force !== false && 'add';

				if ( method ) {
					this[method]( token );
				}

				if ( force === true || force === false ) {
					return force;
				}
				else {
					return !result;
				}
			};

			ClassList.prototype.toString = function() {
				return this.join( ' ' );
			};

			if ( Object.defineProperty ) {
				classListPropertyDescriptor = {
					'get': classListGetter,
					'enumerable': true,
					'configurable': true
				};
				try {
					Object.defineProperty( view.Element.prototype, 'classList', classListPropertyDescriptor );
				}
				catch ( exception ) { // IE 8 doesn't support enumerable:true
					if ( exception.number === -0x7FF5EC54 ) {
						classListPropertyDescriptor.enumerable = false;
						Object.defineProperty( view.Element.prototype, 'classList', classListPropertyDescriptor );
					}
				}
			}
			else if ( Object.prototype.__defineGetter__ ) {
				view.Element.prototype.__defineGetter__( 'classList', classListGetter );
			}

		}( self ) );

	}
	else {
		// There is full or partial native classList support, so just check if we need
		// to normalize the add/remove and toggle APIs.

		( function() {
			'use strict';
			var testElement = document.createElement( '_' );
			var createMethod;
			var original;

			testElement.classList.add( 'c1', 'c2' );

			// Polyfill for IE 10/11 and Firefox <26, where classList.add and
			// classList.remove exist but support only one argument at a time.
			if ( !testElement.classList.contains( 'c2' ) ) {
				createMethod = function( method ) {
					var original = DOMTokenList.prototype[method];

					DOMTokenList.prototype[method] = function( token ) {
						var i;
						var len = arguments.length;

						for ( i = 0; i < len; i++ ) {
							token = arguments[i];
							original.call( this, token );
						}
					};
				};
				createMethod( 'add' );
				createMethod( 'remove' );
			}

			testElement.classList.toggle( 'c3', false );

			// Polyfill for IE 10 and Firefox <24, where classList.toggle does not
			// support the second argument.
			if ( testElement.classList.contains( 'c3' ) ) {
				original = DOMTokenList.prototype.toggle;

				DOMTokenList.prototype.toggle = function( token, force ) {
					if ( 1 in arguments && !this.contains( token ) === !force ) {
						return force;
					}
					else {
						return original.call( this, token );
					}
				};
			}

			testElement = null;
		}() );

	}
}

Straight from https://purl.eligrey.com/github/classList.js/blob/master/classList.js but rewritten without comma first notation, shortcuts for protoshizzle and with hoisted vars.