Is there an easy way to create dynamic variables w

问题:

I've built a data-driven google map with different icons that get assigned to the map depending on the type of item located. So if I have 5 types of landmark, and each gets a different icon (store, library, hospital, etc.)-- what I'd like to do is generate the google icon objects dynamically. I was thinking something like this:

types = array('hospital','church','library','store',etc);
var i=0;
while (i<=types.length) {

    var landmark + i = new google.maps.Icon();
    landmark.image = "icon" + i + ".png";
    i++;
    } 

however, as you've probably guessed, this doesn't work. I also tried using eval, like this:

while (i<=types.length) {
        doIcon(i);
        i++;
    }   

    function doIcon(i){ 
        eval("var landmark" + i + " = new.google.maps.Icon();");
        return eval("landmark" + i);
    }

but it didn't work either-- I'd appreciate any pointers on generating javascript variables dynamically. It's got to be pure js, I could do it in PHP but that's not an option here.

Thanks!

回答1:

It's really easy to do: object["variablename"] = whatever;

So for example you could have an object: var Landmarks = {} and you could add to it like so: Landmarks["landmark" + i] = new google.maps.Icon(); and pass it that way.

If you need these variables to be global (why would you?) you can access the global object directly using window.

回答2:

If you're going to do it using a declared object such as Landmark["landmark" + i], you really may as well use an index array rather than an associative, it's much easier for iteration. Objects aren't really used with indexed properties because Arrays do a much better job of it:

var myObj =           // object version
{
   "item0": "blah",
   "item1": "blah"
   // etc
}
var myArr =           // array version
[
   "blah",
   "blah"
   // etc
]

Much more sensible to use the array:

landmarks = []; // new array
types = array('hospital','church','library','store',etc);  
var i=0;  
while (i<=types.length) {  
    landmarks.push(new google.maps.Icon());
    landmarks[i].image = "icon" + i + ".png";
    i++;  
}

It makes more sense to do it that way and for...in loops on objects can get a bit messy with prototyped properties being enumerable, etc.

If you're trying to make a variable global, add it to the window object:

var myCustomVar = "landmark" + i;
window[myCustomVar] = new google.maps.Icon();

alert(landmark0);

But this would be polluting the global namespace with many unnecessary variables. So you'd still be better with an array:

window.landmarks = [];
landmarks.push(new google.maps.Icon());
// etc...

回答3:

Just to answer your question directly (although please note that this is not the solution you want. Check out the other answers. This is just for documentation!), here's a copy-paste from a JavaScript console:

> window["myNamedVar"] = "Hello, World!";
> console.log(myNamedVar);
  "Hello, World!"

回答4:

You'd be better off creating a javascript object which you can use somewhat like an associative array is used in PHP:

var types = ['hospital','church','library','store'];
var landmarks= {};
for (var i in types) {
    landmarks[types[i]]= new google.maps.Icon();
    landmarks[types[i]].image = "icon" + i + ".png";
} 
alert(landmarks['hospital'].image);  // displays "icon0.png"

回答5:

Do you really need those variables? Can't you do with this:

var types = ['hospital','church','library','store'];    
for(var i =0; i < types.length; i += 1) (new google.maps.Icon()).image = "icon" + i + ".png";

Modifications done based on comment:

icon name pattern changed from icon + index + .png to icon + type + .png and saving the results of the loop.

types = ['hospital','church','library','store'];
var landmarks = {};


// images names are of the form icon + type + .png
function createIcon(type) 
{ 
    var icon = new google.maps.Icon();
    icon.image = "icon" + type + ".png";
    return icon;
}

// mapping of landamarks by type and icon
for (var i = 0, len = types.length; i < len; i++)
{
    landmarks[types[i]] = createIcon(types[i]);
}

the result is : { hospital : icon, church : icon, ... }

where icon is a google map icon that has an image attribute that is a string of the form "icon{type}.png" , e.g, iconhostpital.png, iconchurch.png.

To use the icons write landmarks.type where type is one the names in the array of types, e.g. landmarks.hospital.

if the image names are of the form icon + number + .png, and the number for each type is equivalent to its index in the array replace the call createIcon(type[i]) for createIcon(i).