Tag: JavaScript
Two Google Maps Tips

Two Google Maps Tips

Centre a map on a collection of pins

The basic way to centre a Google Map is to give it co-ordinates of where you want the map to centre. But what if you have a collection of pin/markers and you want to show all of them but don't know beforehand where they will be.

The solution is to create a LatLngBounds object and for each of your pins call the extend method of your bounds object. Once this is done call fitBounds on your map.

1var bounds = new google.maps.LatLngBounds();
2 
3$.each(mapMarkers(), function(index, value) {
4    bounds.extend(value.marker.position);
5});
6 
7map.fitBounds(bounds);

Loading a map in a hidden div

The reason for doing this could be that you have a set of tabs and a non-visible one contains the Google Map. If you instantiate a Google Map when it isn't visible you end up with the smallest map size possible.

One popular solution for this is to only create the map when the tab is being displayed, which is a good option as it means the map is only loaded when it's viewed. However if your using something like Knockout to bind you've views to a model it may not be possible to attach an event to the tab change.

Google Maps actually have an event handler for just this scenario called resize. You simply need to trigger it at the point in which you can size the map.

1google.maps.event.trigger(map, 'resize')

HTML5 Series - Array

If your going to do any sort of JavaScript programming it's not going to be long until you have to work with an array, so there's a few array functions you need to know about.

Push and Pop
Push and Pop are functions that add and remove items from an array. The easiest way to understand what they are doing is to imagine your array as a stack of paper. When you push and item onto the array it's the same as putting it on the top of your pile. When you Pop an item off it's the same as taking the top item from the pile.

1var myArray = [] // Declare a new array
2myArray.push("Red");
3myArray.push("Blue");
4 
5var value1 = myArray.pop();
6var value2 = myArray.pop();
7 
8alert(value1); // should alert the value Blue
9alert(value2); // should alert the value Red

Foreach loop
Probably one of the most useful functions for an array is the for each loop. A for each loop is essentially calling a function for each item in the array (hence for each). This is particularly useful in all kinds of scenarios.

1var myArray = ["blue", "red", "green"];
2 
3myArray.forEach(function (x) {
4    alert(x);
5});

Filter
As the name suggests filtering is a way to find items in your array. If you know C# then it works in a similar way to a lambda expression. The filter function takes a parameter of a function. Like the forEach loop the function is called on each item in the array, the function must then returns either true or false depending on if the filter criteria matched.

In this example notice that the result of myArray.filter is being assigned to another variable. This is because applying the filter wont actually remove items from the myArray array.

1var myArray = ["blue", "red", "green"];
2 
3var results = myArray.filter(function(x) {
4    if (x == "blue")
5        return true;
6    else
7        return false;
8})
9 
10results.forEach(function (x) {
11    alert(x);
12});

Some, Every
The some and every functions can be used to see if some items in the array match a criteria of if all of them do. They return either true or false.

Like the filter function, a function is passed as the criteria and returns either true or false.

1var myArray = ["blue", "red", "green"];
2 
3alert(myArray.some(function (x) {
4    if (x == "blue")
5        return true;
6    else
7        return false;
8})); // Alerts true as 1 item in the array is blue
9 
10alert(myArray.every(function (x) {
11    if (x == "blue")
12        return true;
13    else
14        return false;
15})); // Alerts false as not every item in the array is blue

Concat
Concat is used to combine 2 arrays into 2 new array.

1var myArray = ["blue", "red", "green"];
2var myArray2 = ["yellow", "orange"];
3 
4var myArray3 = myArray.concat(myArray2);

Slice
Slice lets you create a new array from an existing by letting you specify the start and end item. Those items and the others between then form the new array.

1var myArray = ["blue", "red", "green"];
2var myArray2 = myArray.slice(2, 3); // selects red and green
3

Splice
Splice can be used to add and remove items in an array. The function has the syntax:

arrayName.splice(index, how many, items to add);

Note: the index value starts at 0.

For example in our colour array we could add yellow and orange in between red and green with the following:

1var myArray = ["blue", "red", "green"];
2myArray.splice(2, 0, "yellow", "orange");
3 
4myArray.forEach(function (x) {
5    alert(x);
6});

Alternatively we could replace red and green with yellow and orange.

1var myArray = ["blue", "red", "green"];
2myArray.splice(1, 2, "yellow", "orange");
3 
4myArray.forEach(function (x) {
5    alert(x);
6});

A couple of others

Sort - Sorts the array into alphabetical order

Reverse - Reverses the order of the array

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);