How can I return a variable from a $.getJSON function

How do I include a JavaScript file in another JavaScript file?

by Blogger 4:27:00 AM file import Javascript

How do I include a JavaScript file in another JavaScript file?

Is there something in JavaScript similar to @import in CSS that allows you to include a JavaScript file inside another JavaScript file?

Solution:

JavaScript has no import, include, or require. (Update 2015: it does now with ES6 modules) There are other ways for JavaScript to include external JavaScript contents, though.

Ajax Loading

Load an additional script with an Ajax call and then use eval. This is the most straightforward way, but it is limited to your domain because of the JavaScript sandbox security model. Using eval also opens the door to bugs and hacks.

jQuery Loading

The jQuery library provides loading functionality in one line:

$.getScript("my_lovely_script.js", function(){

   alert("Script loaded but not necessarily executed.");

});

Dynamic Script Loading

Add a script tag with the script URL in the HTML. To avoid the overhead of jQuery, this is an ideal solution.

The script can even reside on a different server. Furthermore, the browser evaluates the code. The <script> tag can be injected into either the web page <head>, or inserted just before the closing </body> tag.

Both of these solutions are discussed and illustrated in JavaScript Madness: Dynamic Script Loading.

Detecting when the script has been executed

Now, there is a big issue you must know about. Doing that implies that you remotely load the code. Modern web browsers will load the file and keep executing your current script because they load everything asynchronously to improve performance. (This applies to both the jQuery method and the manual dynamic script loading method.)

It means that if you use these tricks directly, you won't be able to use your newly loaded code the next line after you asked it to be loaded, because it will be still loading.

For example: my_lovely_script.js contains MySuperObject:

var js = document.createElement("script");

js.type = "text/javascript";
js.src = jsFilePath;

document.body.appendChild(js);

var s = new MySuperObject();

Error : MySuperObject is undefined

Then you reload the page hitting F5. And it works! Confusing...

So what to do about it ?

Well, you can use the hack the author suggests in the link I gave you. In summary, for people in a hurry, he uses an event to run a callback function when the script is loaded. So you can put all the code using the remote library in the callback function. For example:

function loadScript(url, callback)
{
    // Adding the script tag to the head as suggested before
    var head = document.getElementsByTagName('head')[0];
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = url;

    // Then bind the event to the callback function.
    // There are several events for cross browser compatibility.
    script.onreadystatechange = callback;
    script.onload = callback;

    // Fire the loading
    head.appendChild(script);
}

Then you write the code you want to use AFTER the script is loaded in a lambda function:

var myPrettyCode = function() {

   // Here, do what ever you want
};

Then you run all that:

loadScript("my_lovely_script.js", myPrettyCode);

Note that the script may execute after the DOM has loaded, or before, depending on the browser and whether you included the line script.async = false;. There's a great article on Javascript loading in general which discusses this.

Source Code Merge

Another solution is to combine the two files into a single file. This can be used with minification to produce a single, minimally sized JavaScript file to include as normal.

source

How to implement file upload progress bar on web?

by Blogger 4:23:00 AM .Net Ajax Javascript

Why does Google prepend while(1); to their JSON responses?

by Blogger 4:05:00 AM Ajax Google Javascript JSON

Why does ++[[]][+[]]+[+[]] return the string “10”?

by Blogger 4:03:00 AM Javascript string Syntax

Why does ++[[]][+[]]+[+[]] return the string “10”?

This is valid and returns the string "10" in JavaScript (more examples here):

++[[]][+[]]+[+[]]

Why? What is happening here?

Solution:

If we split it up, the mess is equal to:

++[[]][+[]]
+
[+[]]

In JavaScript, it is true that +[] === 0. + converts something into a number, and in this case it will come down to +"" or 0 (see specification details below).

Therefore, we can simplify it (++ has precendence over +):

++[[]][0]
+
[0]

Because [[]][0] means: get the first element from [[]], it is true that:

[[]][0] returns the inner array ([]). Due to references it's wrong to say [[]][0] === [], but let's call the inner array A to avoid the wrong notation.
++[[]][0] == A + 1, since ++ means 'increment by one'.
++[[]][0] === +(A + 1); in other words, it will always be a number (+1 does not necessarily return a number, whereas ++ always does - thanks to Tim Down for pointing this out).

Again, we can simplify the mess into something more legible. Let's substitute [] back for A:

+([] + 1)
+
[0]

In JavaScript, this is true as well: [] + 1 === "1", because [] == "" (joining an empty array), so:

+([] + 1) === +("" + 1), and
+("" + 1) === +("1"), and
+("1") === 1

Let's simplify it even more:

1
+
[0]

Also, this is true in JavaScript: [0] == "0", because it's joining an array with one element. Joining will concatenate the elements separated by ,. With one element, you can deduce that this logic will result in the first element itself.

So, in the end we obtain (number + string = string):

1
+
"0"

=== "10" // Yay!

Specification details for +[]:

This is quite a maze, but to do +[], first it is being converted to a string because that's what + says:

11.4.6 Unary + Operator

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

Let expr be the result of evaluating UnaryExpression.

Return ToNumber(GetValue(expr)).

ToNumber() says:

Object

Apply the following steps:

Let primValue be ToPrimitive(input argument, hint String).

Return ToString(primValue).

ToPrimitive() says:

Object

Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.

[[DefaultValue]] says:

8.12.8 [[DefaultValue]] (hint)

When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:

Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".

If IsCallable(toString) is true then,

a. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.

b. If str is a primitive value, return str.

The .toString of an array says:

15.4.4.2 Array.prototype.toString ( )

When the toString method is called, the following steps are taken:

Let array be the result of calling ToObject on the this value.

Let func be the result of calling the [[Get]] internal method of array with argument "join".

If IsCallable(func) is false, then let func be the standard built-in method Object.prototype.toString (15.2.4.2).

Return the result of calling the [[Call]] internal method of func providing array as the this value and an empty arguments list.

So +[] comes down to +"", because [].join() === "".

Again, the + is defined as:

11.4.6 Unary + Operator

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

Let expr be the result of evaluating UnaryExpression.

Return ToNumber(GetValue(expr)).

ToNumber is defined for "" as:

The MV of StringNumericLiteral ::: [empty] is 0.

So +"" === 0, and thus +[] === 0.

All In One Script

How can I return a variable from a $.getJSON function