Javascript Callback Function

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You’ve probably seen that some snippets use the word " callback " in a function. Callbacks are a special type of function that is passed inside another function.

Callbacks are often used in event handlers to execute a block of code. In this guide, we’ll talk about what callbacks are and how they work. We’ll look at an example callback function to help you learn how to use them in your code.

What is a callback function?

A callback function is a function passed as a parameter to another function.

Callback functions are executed inside the function in which they are declared. When executing a function, its callback function, if specified, will be executed. When executed, the callback function returns a response to the main function.

Callback functions work because in JavaScript each function is an object. This means that we can work with them like any other object. We can assign functions to variables or pass them as arguments, just as we would with any other value.

Let’s use a simple example to show how callbacks work. We are going to create a function that prints the name of a user for a video game on the console, followed by the type of character. Let’s start by declaring a function:

This is an anonymous JavaScript function that contains a callback. Anonymous functions are unnamed functions. They usually appear inside other functions, as in the example above.

This callback is the second parameter of our code. When this function is performed, our player’s name is printed on the console. Then the contents of our callback () function are executed.

Now we are going to use this function:

We called our function printName () in this code. We specified "Purple" as the player name. We have specified a function that prints the font as a reminder. Let’s run this code and see what happens:

Player name: Viola

Character type: Wizard

The contents of the printName () function are executed, followed by the contents of our callback function. be declared when you pass them as a parameter. We could refactor our code so that our callback function is only:

This code will return the same response. We’ve created a function called printCharacterType () which prints a player’s font to the console. This function is executed when we execute printName (), because it is specified as a callback.

Using callbacks with events

Callbacks are commonly used with https:/ / ‚" id =" image ‚" />

This defines an image on the web page that a user can click. Then, insert the following JavaScript code into a

Looking for a challenge ? Write a callback function that runs when a button is pressed on a web page. If you have already done so, try writing a function that commands a list of items and uses a function callback to print them all to the console.

You are now ready to start using JavaScript callbacks for asynchronous programming like a pro!

Javascript Callback Function insert: Questions

How to insert newlines on argparse help text?

5 answers

I"m using argparse in Python 2.7 for parsing input options. One of my options is a multiple choice. I want to make a list in its help text, e.g.

from argparse import ArgumentParser

parser = ArgumentParser(description="test")

parser.add_argument("-g", choices=["a", "b", "g", "d", "e"], default="a",
    help="Some option, where
         " a = alpha
         " b = beta
         " g = gamma
         " d = delta
         " e = epsilon")


However, argparse strips all newlines and consecutive spaces. The result looks like

~/Downloads:52$ python2.7 -h
usage: [-h] [-g {a,b,g,d,e}]


optional arguments:
  -h, --help      show this help message and exit
  -g {a,b,g,d,e}  Some option, where a = alpha b = beta g = gamma d = delta e
                  = epsilon

How to insert newlines in the help text?


Answer #1

Try using RawTextHelpFormatter:

from argparse import RawTextHelpFormatter
parser = ArgumentParser(description="test", formatter_class=RawTextHelpFormatter)

Is a Python list guaranteed to have its elements stay in the order they are inserted in?

5 answers

If I have the following Python code

>>> x = []
>>> x = x + [1]
>>> x = x + [2]
>>> x = x + [3]
>>> x
[1, 2, 3]

Will x be guaranteed to always be [1,2,3], or are other orderings of the interim elements possible?


Answer #1

Yes, the order of elements in a python list is persistent.

Inserting image into IPython notebook markdown

5 answers

I am starting to depend heavily on the IPython notebook app to develop and document algorithms. It is awesome; but there is something that seems like it should be possible, but I can"t figure out how to do it:

I would like to insert a local image into my (local) IPython notebook markdown to aid in documenting an algorithm. I know enough to add something like <img src="image.png"> to the markdown, but that is about as far as my knowledge goes. I assume I could put the image in the directory represented by (or some subdirectory) to be able to access it, but I can"t figure out where that directory is. (I"m working on a mac.) So, is it possible to do what I"m trying to do without too much trouble?


Answer #1

Most of the answers given so far go in the wrong direction, suggesting to load additional libraries and use the code instead of markup. In Ipython/Jupyter Notebooks it is very simple. Make sure the cell is indeed in markup and to display a image use:

![alt text](imagename.png "Title")

Further advantage compared to the other methods proposed is that you can display all common file formats including jpg, png, and gif (animations).

Javascript Callback Function JavaScript: Questions

JSON datetime between Python and JavaScript

4 answers

kevin By kevin

I want to send a datetime.datetime object in serialized form from Python using JSON and de-serialize in JavaScript using JSON. What is the best way to do this?


Answer #1

You can add the "default" parameter to json.dumps to handle this:

date_handler = lambda obj: (
    if isinstance(obj, (datetime.datetime,
    else None
json.dumps(, default=date_handler)

Which is ISO 8601 format.

A more comprehensive default handler function:

def handler(obj):
    if hasattr(obj, "isoformat"):
        return obj.isoformat()
    elif isinstance(obj, ...):
        return ...
        raise TypeError, "Object of type %s with value of %s is not JSON serializable" % (type(obj), repr(obj))

Update: Added output of type as well as value.
Update: Also handle date

What blocks Ruby, Python to get Javascript V8 speed?

4 answers

Are there any Ruby / Python features that are blocking implementation of optimizations (e.g. inline caching) V8 engine has?

Python is co-developed by Google guys so it shouldn"t be blocked by software patents.

Or this is rather matter of resources put into the V8 project by Google.


Answer #1

What blocks Ruby, Python to get Javascript V8 speed?


Well, okay: money. (And time, people, resources, but if you have money, you can buy those.)

V8 has a team of brilliant, highly-specialized, highly-experienced (and thus highly-paid) engineers working on it, that have decades of experience (I"m talking individually – collectively it"s more like centuries) in creating high-performance execution engines for dynamic OO languages. They are basically the same people who also created the Sun HotSpot JVM (among many others).

Lars Bak, the lead developer, has been literally working on VMs for 25 years (and all of those VMs have lead up to V8), which is basically his entire (professional) life. Some of the people writing Ruby VMs aren"t even 25 years old.

Are there any Ruby / Python features that are blocking implementation of optimizations (e.g. inline caching) V8 engine has?

Given that at least IronRuby, JRuby, MagLev, MacRuby and Rubinius have either monomorphic (IronRuby) or polymorphic inline caching, the answer is obviously no.

Modern Ruby implementations already do a great deal of optimizations. For example, for certain operations, Rubinius"s Hash class is faster than YARV"s. Now, this doesn"t sound terribly exciting until you realize that Rubinius"s Hash class is implemented in 100% pure Ruby, while YARV"s is implemented in 100% hand-optimized C.

So, at least in some cases, Rubinius can generate better code than GCC!

Or this is rather matter of resources put into the V8 project by Google.

Yes. Not just Google. The lineage of V8"s source code is 25 years old now. The people who are working on V8 also created the Self VM (to this day one of the fastest dynamic OO language execution engines ever created), the Animorphic Smalltalk VM (to this day one of the fastest Smalltalk execution engines ever created), the HotSpot JVM (the fastest JVM ever created, probably the fastest VM period) and OOVM (one of the most efficient Smalltalk VMs ever created).

In fact, Lars Bak, the lead developer of V8, worked on every single one of those, plus a few others.

Django Template Variables and Javascript

4 answers

When I render a page using the Django template renderer, I can pass in a dictionary variable containing various values to manipulate them in the page using {{ myVar }}.

Is there a way to access the same variable in Javascript (perhaps using the DOM, I don"t know how Django makes the variables accessible)? I want to be able to lookup details using an AJAX lookup based on the values contained in the variables passed in.


Answer #1

The {{variable}} is substituted directly into the HTML. Do a view source; it isn"t a "variable" or anything like it. It"s just rendered text.

Having said that, you can put this kind of substitution into your JavaScript.

<script type="text/javascript"> 
   var a = "{{someDjangoVariable}}";

This gives you "dynamic" javascript.


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