THE IMAGE GALLERY REVISITED in Font

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CHAPTER 6 THE IMAGE GALLERY REVISITED
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If showPic returns false then we assume that the image wasn't updated and return true to allow the default action to occur. The prepareGallery function now looks like this: function prepareGallery() { if (!document.getElementsByTagName) return false; if (!document.getElementById) return false; if (!document.getElementById("imagegallery")) return false; var gallery = document.getElementById("imagegallery"); var links = gallery.getElementsByTagName("a"); for ( var i=0; i < links.length; i++) { links[i].onclick = function() { return !showPic(this); } } } That solves the problem with our test case. If the placeholder image doesn t exist, the browser simply follows the links to the images. You can go ahead and put the placeholder image back into the markup.
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Fine-tuning
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The functions are looking a lot better now. They may have grown in size, but they now assume much less about the markup. I can still see some assumptions being made in the showPic function that I may need to tackle. For instance, I am assuming that the link has a title attribute: var text = whichpic.getAttribute("title"); To find out if there is a title attribute, I can test to see if it isn t equal to null: if (whichpic.getAttribute("title") != null) This if statement will evaluate to true if there is a title attribute. It will return a value of false if there is no title attribute, because the value of whichpic.getAttribute("title") will be equal to null. I can save some space by simply writing if (whichpic.getAttribue("title")) The if statement will still return a value of true as long as there is a title attribute. As a simple fallback, I could set the value of text to be empty if there is no title attribute: if (whichpic.getAttribute("title")) { var text = whichpic.getAttribute("title"); } else { var text = ""; } Here s another way of doing the same thing: var text = whichpic.getAttribute("title") whichpic.getAttribute("title") : ""; The getAttribute test is followed by a question mark. This is called a ternary operator. Two possible values for the variable text are provided after the question mark. The first value will be assigned if
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CHAPTER 6 THE IMAGE GALLERY REVISITED
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getAttribute("title") returns a value. The second value will be returned if getAttribute("title") returns a value of null: variable = condition if true : if false; If a title attribute exists, the variable text will contain whichpic.getAttribute("title"). If there is no title attribute, the variable text will be an empty string, . A ternary operator is just another way of performing if/else statements. It s shorter, but if you find it confusing, you can always use the more verbose if/else statement. Try removing the title attribute from one of the links in the imagegallery list. If you click on that link, the description element will be filled with an empty string. If I wanted to be really thorough, I could introduce checks for just about everything. For example, I m checking for the existence of an element called placeholder, but I m just assuming that it is an image. I could run a further check, using the nodeName property, to verify this: if (placeholder.nodeName != "IMG") return false; Notice that in HTML documents, nodeName always a returns an uppercase value, even if the element is lowercase in the markup. There are further checks I could introduce. I m assuming that the firstChild of the description element is a text node, but I should really check to make sure. I can use the nodeType property for this test. If you recall, text nodes have a nodeType value of 3: if (description.firstChild.nodeType == 3) { description.firstChild.nodeValue = text; } This is how the showPic function would look with these extra tests: function showPic(whichpic) { if (!document.getElementById("placeholder")) return false; var source = whichpic.getAttribute("href"); var placeholder = document.getElementById("placeholder"); if (placeholder.nodeName != "IMG") return false; placeholder.setAttribute("src",source); if (document.getElementById("description")) { var text = whichpic.getAttribute("title") whichpic.getAttribute("title") : ""; var description = document.getElementById("description"); if (description.firstChild.nodeType == 3) { description.firstChild.nodeValue = text; } } return true; } The function is much more verbose when it s done in this way. In a real-world situation, you may decide that all these checks aren t necessary. They are intended to account for situations where the markup might be beyond your control. Ideally, your scripts shouldn t assume too much about the content and structure of the markup. That said, these kinds of decisions need to be made on a case-by-case basis.
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