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HTML5 Series - Web Storage

One of the best benefits of HTML5 particularly for Web Apps is the ability to store data locally within the users browser. This means in many places you can speed your Web App up by fetching data once and then retrieving it locally in the future, rather than going back to a server each time.

There are 2 types of storage available localStorage and sessionStorage. The simple difference is that localStorage doesn't expire while sessionStorage expires at the end of the users session.

Both types store data in a key/value list and could not be simpler:

1// local storage
2localStorage.variableName = value;
3 
4// session storage
5sessionStorage.variableName = value;

One thing to note though is you can only store text. If you want to store something more complex the simple solution is to convert it to and from JSON. E.g.

1var myColours = [ 'red', 'blue', 'green', 'yellow'];
2 
3// Save to local storage
4localStorage.colours = JSON.stringify(myColours)
5 
6// Retrieve from local storage
7var retrievedColorus = JSON.parse(localStorage.colours);

HTML5 Series - Error Handling

One thing you can say for certain about writing code is at some point your going to write something that error's. It may be picked up in code reviews, or by a tester, and then fixed but for certain as you are human at some point you will make a mistake. What's important though is what happens after the mistake.

If your a C# developer then good news JavaScript has the same try, catch, finally functionality that C# has and it works in the same way.

Wrap you code that could error with a try statement and curly braces and following it add in a catch statement with the code that you want to execute in the event of an error. Finally if you want some code to execute either way after both, place this in the finally block.

1try
2{
3    functionThatDoesntExists();
4}
5catch (err)
6{
7     alert(err.message);
8}
9finally
10{
11    //Code to execute at the end
12}

Notice the catch section is passed a parameter and I am then alerting a property of this called message. The error parameter can be useful for debugging what actually went wrong and contains a few other parameters.

  • Description
  • Message
  • Name
  • Number
  • Stack

Sometimes you may also want to cause an error to be thrown. You may be asking why, but consider people calling your functions may in-fact pass values that are not valid. You could detect this and throw an error containing some useful information before there incorrect parameter values causes your code to error and a less helpful message given.

1var err = new Error(1, "Oh dear, that's not going to work"); // First parameter is the error number, second is the message
2throw err;

HTML5 Series - Grid

There was a time when an HTML page layout mainly consisted of tables, and lots of them. Tables went inside other tables and complex layouts were achieved through the use of colspans and rowspans. The only problem though was it was a mess.

By using tables the HTML mark-up was dictating the design/layout of the page and if you ever wanted to change anything it was a huge task to undertake. Since then the world has moved on and now most webpages consist mainly of div's. But what if you did want to quite a rigid layout with columns and rows?

HTML5 introduces the Grid style through CSS. Your page can still consist mainly of div's (or anything for that matter), but in your css you set a display property of grid (for now in IE it's -ms-grid). You can then specify a grid-columns and grid-rows property to create a grid within your div.

Child items can be placed in grid cells through grid-row and grid-column properties in the CSS, and if you want something to span more than one cell you can use grid-row-span or grid-column-span.

If you've done any XMAL programming (WPF/Silverlight/Win8/Windows Phone) then this should all sound very similar to XMAL's Grid control.

The example below creates something like this:

1<style>
2        /* Set a grid with 3 columns and 2 rows */
3        div {
4            display: -ms-grid;
5            -ms-grid-columns:200px 200px 200px;
6            -ms-grid-rows:200px 200px;
7        }
8 
9        /* Position the child div's in the grid */
10        div:nth-child(1) {
11            -ms-grid-row: 1;
12            -ms-grid-column: 1;
13        }
14        div:nth-child(2) {
15            -ms-grid-row: 2;
16            -ms-grid-column: 1;
17        }
18        div:nth-child(3) {
19            -ms-grid-row: 1;
20            -ms-grid-column: 2;
21            -ms-grid-row-span:2; /* The middle cell spans 2 rows */
22        }
23        div:nth-child(4) {
24            -ms-grid-row: 1;
25            -ms-grid-column: 3;
26        }
27        div:nth-child(5) {
28            -ms-grid-row: 2;
29            -ms-grid-column: 3;
30        }
31 
32        /* Add a border to the cells */
33        div>div {
34            border: solid 1px black;
35        }
36</style>
37<div class="myGrid">
38    <div></div>
39    <div></div>
40    <div></div>
41    <div></div>
42    <div></div>
43</div>

It's also worth noting that the column/row sizes can be specified using fractions rather than fixed sizes. e.g. The following will produce a grid 600px wide where the middle column is twice the width of the other 2.

1<style>
2        /* Set a grid with 3 columns and 2 rows */
3        div {
4            display: -ms-grid;
5            width:600px;
6            -ms-grid-columns:1fr 2fr 1fr;
7            -ms-grid-rows:200px 200px;
8        }
9</style>

For another great example check out this hands on page from Microsoft Hands on: CSS3 Grid