Traversing Elements page

Learn how to move from one element to the next and a bit of meta programming.

Overveiw

We will learn about:

  • Creating next/prev/parent/children functions
  • Re-factoring methods

Video

Slides

Setup

Run the following example in CodePen:

<div id="qunit"></div>
<div id="qunit-fixture"></div>
<link rel="stylesheet" href="//code.jquery.com/qunit/qunit-1.12.0.css">
<script src="//code.jquery.com/qunit/qunit-1.12.0.js"></script>
<script src="//bitovi.github.io/academy/static/scripts/jquery-test.js"></script>
<script type="module">
(function() {
  $ = function(selector) {
    if ( !(this instanceof $) ) {
      return new $(selector);
    }
    var elements;
    if (typeof selector === "string") {
      elements = document.querySelectorAll(selector);
    } else if ($.isArrayLike(selector)) {
      elements = selector;
    }
    [].push.apply(this, elements);
  };

  $.extend = function(target, object) {
    for (var prop in object) {
      if (object.hasOwnProperty(prop)) {
        target[prop] = object[prop];
      }
    }
    return target;
  };

  // Static methods
  $.extend($, {
    isArray: function(obj) {
      return Object.prototype.toString.call(obj) === "[object Array]";
    },
    isArrayLike: function(obj) {
      return obj &&
        typeof obj === "object" &&
        (   obj.length === 0 ||
            typeof obj.length === "number" &&
            obj.length > 0 &&
            obj.length - 1 in obj );

    },
    each: function(collection, cb) {
      if ($.isArrayLike(collection)) {
        for (var i = 0; i < collection.length; i++) {
          if (cb.call(this, i, collection[i]) === false) {
            break;
          }
        }
      } else {
        for (var prop in collection) {
          if (collection.hasOwnProperty(prop)) {
            if (cb.call(this, prop, collection[prop]) === false) {
              break;
            }
          }
        }
      }
      return collection;
    },
    makeArray: function(arr) {
      if ($.isArray(arr)) {
        return arr;
      }
      var array = [];
      $.each(arr, function(i, item) {
        array[i] = item;
      });
      return array;
    },
    proxy: function(fn, context) {
      return function() {
        return fn.apply(context, arguments);
      };
    }
  });

  function makeSimpleGetterSetter(prop) {
    return function(value){
      if(arguments.length) {
        return $.each(this, function(i, element) {
          element[prop] = value;
        });
      } else {
        return this[0][prop];
      }
    }
  }

  $.extend($.prototype, {
      html: makeSimpleGetterSetter("innerHTML"),
      val: makeSimpleGetterSetter("value"),
      text: makeSimpleGetterSetter("textContent"),
      find: function(selector) {
        var elements = [];
        $.each(this, function(i, element){
            var result = element.querySelectorAll(selector);
            elements.push(...result);
            // Or elements.push.apply(elements, result);
        })
        return new $(elements);
      },
      parent: function() { },
      next: function() { },
      prev: function() { },
      children: function() { }
  });

})();
</script>

Each exercise builds on the previous exercise. There is a completed solution at the end of this page.

Exercise: collection.parent() -> collection

The problem

collection.parent gets the parent of each element in the current set of matched elements.

<ul class="level-1">
  <li class="item-i">I</li>
  <li class="item-ii">II
    <ul class="level-2">
      <li class="item-a">A</li>
      <li class="item-b">B
        <ul class="level-3">
          <li class="item-1">1</li>
          <li class="item-2">2</li>
          <li class="item-3">3</li>
        </ul>
      </li>
      <li class="item-c">C</li>
    </ul>
  </li>
  <li class="item-iii">III</li>
</ul>
<script type="module">
import "https://unpkg.com/jquery@3/dist/jquery.js";

$( "li.item-a" ).parent().css( "border", "solid 1px red" );
</script>

Click to see test code

QUnit.test('$.fn.parent', function(){
    var $lis = $('#qunit-fixture')
        .html('<ul><li/><li/></ul><ul><li/><li/></ul>')
        .find('li:first-child');

    equal($lis.length, 2, 'got 2 uls');
    equal($lis.parent().length, 2, 'got 2 lis');
});

What you need to know

  • The parentNode property returns an elements parent node.

The solution

Click to see the solution

      parent: function() {
        var parents = [];
        $.each(this, function(i, element){
          parents.push(element.parentNode)
        });
        return $(parents);
      },

Exercise: collection.next() -> collection

The problem

collection.next gets the immediately following sibling of each element in the set of matched elements.

<ul>
  <li>list item 1</li>
  <li>list item 2</li>
  <li class="third-item">list item 3</li>
  <li>list item 4</li>
  <li>list item 5</li>
</ul>
<script type="module">
import "https://unpkg.com/jquery@3/dist/jquery.js";

$( "li.third-item" ).next().css( "border", "solid 1px red" );
</script>

Click to see test code

QUnit.test('$.fn.next', function(){
    var $lis = $('#qunit-fixture')
        .html('<ul> <li></li> <li></li> </ul> <ul> <li></li> <li></li> </ul>')
        .find('li:first-child');

    equal($lis.length, 2, 'got 2 lis');
    equal($lis.next().length, 2, 'got 2 lis');
    equal($lis.next().next().length, 0, 'got 0 lis');
});

What you need to know

  • nextElementSibling returns the element immediately following the specified one in its parent's children list, or null if the specified element is the last one in the list.

The solution

Click to see the solution

      next: function() {
        var nexts = [];
        $.each(this, function(i, element){
          if(element.nextElementSibling) {
            nexts.push( element.nextElementSibling );
          }
        });
        return $(nexts);
      },

Exercise: collection.prev() -> collection

The problem

collection.prev get the immediately preceding sibling of each element in the set of matched elements.

Click to see test code

QUnit.test('$.fn.prev', function(){
    var $lis = $('#qunit-fixture')
        .html('<ul><li/><li/></ul><ul><li/><li/></ul>')
        .find('li:last-child');

    equal($lis.length, 2, 'got 2 uls');
    equal($lis.prev().length, 2, 'got 2 lis');
    equal($lis.prev().prev().length, 0, 'got 2 lis');
});

What you need to know

  • previousElementSibling returns the Element immediately prior to the specified one in its parent's children list, or null if the specified element is the first one in the list.

The solution

Click to see the solution

      prev: function() {
        var prevs = [];
        $.each(this, function(i, element){
          if(element.previousElementSibling) {
            prevs.push( element.previousElementSibling );
          }
        });
        return $(prevs);
      },

Exercise: collection.children() -> collection

The problem

collection.children get the children of each element in the set of matched elements.

<ul class="level-1">
  <li class="item-i">I</li>
  <li class="item-ii">II
    <ul class="level-2">
      <li class="item-a">A</li>
      <li class="item-b">B
        <ul class="level-3">
          <li class="item-1">1</li>
          <li class="item-2">2</li>
          <li class="item-3">3</li>
        </ul>
      </li>
      <li class="item-c">C</li>
    </ul>
  </li>
  <li class="item-iii">III</li>
</ul>
<script type="module">
import "https://unpkg.com/jquery@3/dist/jquery.js";

$( "ul.level-2" ).children().css( "border", "solid 1px red" );
</script>

Click to see test code

QUnit.test('$.fn.children', function(){
    var $ul = $('#qunit-fixture')
        .html('<ul><li/><li/></ul><ul><li/><li/></ul>')
        .children();

    equal($ul.length, 2, 'got 2 uls');
    equal($ul.children().length, 4, 'got four lis');
});

What you need to know

  • children returns the child elements of the node upon which it was called.

The solution

Click to see the solution

      children: function() {
        var children = [];
        $.each(this, function(i, element){
          children.push(...element.children );
        });
        return $(children);
      },

Exercise: eliminate duplicate code in .find, .parent, .next, .prev, and .children.

The problem

Use meta programming techniques to reduce the duplicate code in the .find, .parent, .next, .prev, and .children functions.

      find: function(selector) {
        var elements = [];
        $.each(this, function(i, element){
            var result = element.querySelectorAll(selector);
            elements.push(...result);
            // Or elements.push.apply(elements, result);
        })
        return new $(elements);
      },
      parent: function() {
        var parents = [];
        $.each(this, function(i, element){
          parents.push(element.parentNode)
        });
        return $(parents);
      },
      next: function() {
        var nexts = [];
        $.each(this, function(i, element){
          if(element.nextElementSibling) {
            nexts.push( element.nextElementSibling );
          }
        });
        return $(nexts);
      },
      prev: function() {
        var prevs = [];
        $.each(this, function(i, element){
          if(element.previousElementSibling) {
            prevs.push( element.previousElementSibling );
          }
        });
        return $(prevs);
      },
      children: function() {
        var children = [];
        $.each(this, function(i, element){
          children.push(...element.children );
        });
        return $(children);
      },

Make a makeTraverser(traverse) function that takes a traverse function and produces a function.

The traverse function should be called for each element in the source collection. When calling traverse, its this should be each element.

The traverse functions should be able to return null and element or an array-like object of elements.

What you need to know

You know everything you need to know. You can do it!

The solution

Click to see the solution

  function makeTraverser(traverser) {
    return function() {
      var elements = [], args = arguments;
      $.each(this, function(i, element) {
        var els = traverser.apply(element, args);
        if ($.isArrayLike(els)) {
          elements.push.apply(elements, els);
        } else if (els) {
          elements.push(els);
        }
      });
      return $(elements);
    };
  }
      find: makeTraverser(function(selector) {
        return this.querySelectorAll(selector);
      }),
      parent: makeTraverser(function() {
        return this.parentNode;
      }),
      next: makeTraverser(function() {
        return this.nextElementSibling;
      }),
      prev: makeTraverser(function() {
        return this.previousElementSibling;
      }),
      children: makeTraverser(function() {
        return this.children;
      })

Completed Solution

<div id="qunit"></div>
<div id="qunit-fixture"></div>
<link rel="stylesheet" href="//code.jquery.com/qunit/qunit-1.12.0.css">
<script src="//code.jquery.com/qunit/qunit-1.12.0.js"></script>
<script src="//bitovi.github.io/academy/static/scripts/jquery-test.js"></script>
<script type="module">
(function() {
  $ = function(selector) {
    if ( !(this instanceof $) ) {
      return new $(selector);
    }
    var elements;
    if (typeof selector === "string") {
      elements = document.querySelectorAll(selector);
    } else if ($.isArrayLike(selector)) {
      elements = selector;
    }
    [].push.apply(this, elements);
  };

  $.extend = function(target, object) {
    for (var prop in object) {
      if (object.hasOwnProperty(prop)) {
        target[prop] = object[prop];
      }
    }
    return target;
  };

  // Static methods
  $.extend($, {
    isArray: function(obj) {
      return Object.prototype.toString.call(obj) === "[object Array]";
    },
    isArrayLike: function(obj) {
      return obj &&
        typeof obj === "object" &&
        (   obj.length === 0 ||
            typeof obj.length === "number" &&
            obj.length > 0 &&
            obj.length - 1 in obj );

    },
    each: function(collection, cb) {
      if ($.isArrayLike(collection)) {
        for (var i = 0; i < collection.length; i++) {
          if (cb.call(this, i, collection[i]) === false) {
            break;
          }
        }
      } else {
        for (var prop in collection) {
          if (collection.hasOwnProperty(prop)) {
            if (cb.call(this, prop, collection[prop]) === false) {
              break;
            }
          }
        }
      }
      return collection;
    },
    makeArray: function(arr) {
      if ($.isArray(arr)) {
        return arr;
      }
      var array = [];
      $.each(arr, function(i, item) {
        array[i] = item;
      });
      return array;
    },
    proxy: function(fn, context) {
      return function() {
        return fn.apply(context, arguments);
      };
    }
  });

  function makeTraverser(traverser) {
    return function() {
      var elements = [], args = arguments;
      $.each(this, function(i, element) {
        var els = traverser.apply(element, args);
        if ($.isArrayLike(els)) {
          elements.push.apply(elements, els);
        } else if (els) {
          elements.push(els);
        }
      });
      return $(elements);
    };
  }

  $.extend($.prototype, {
      html: function(newHtml) {
        if(arguments.length) {
          return $.each(this, function(i, element) {
            element.innerHTML = newHtml;
          });
        } else {
          return this[0].innerHTML;
        }
      },
      val: function(newVal) {
        if(arguments.length) {
          return $.each(this, function(i, element) {
            element.value = newVal;
          });
        } else {
          return this[0].value;
        }
      },
      text: function(newText) {
        if (arguments.length) {
          return $.each(this, function(i, element) {
            element.textContent = newText;
          });
        } else {
          return this[0].textContent;
        }
      },
      find: makeTraverser(function(selector) {
        return this.querySelectorAll(selector);
      }),
      parent: makeTraverser(function() {
        return this.parentNode;
      }),
      next: makeTraverser(function() {
        return this.nextElementSibling;
      }),
      prev: makeTraverser(function() {
        return this.previousElementSibling;
      }),
      children: makeTraverser(function() {
        return this.children;
      })
  });

})();
</script>