Javascript Foreach

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The JavaScript forEach loop is an Array method that performs a custom callback function on each element of an array. The forEach loop can only be used on arrays, sets, and maps.

If you’ve spent any time with a programming language, you should have seen a "per loop." Using a for loop, you can execute a dataset or a function more than once. For example, you can have a list of names, also known as an array, and a for loop will go through 100 of those names.

There is a more efficient way to write a for loop if you are working with a collection, such as a list or a set. This is where JavaScript forEach loops come in. A forEach loop will perform a JavaScript callback for each element of a list. Then the loop stops.

JavaScript for Loop Refresher

The JavaScript for loop executes a function a predefined number of times. For loops are useful if you need to run the same block of code multiple times. Using a for loop instead of copying code helps reduce redundancy. This improves the readability of a code base.

We write a for loop that prints a value to the console ten times:

This loop will execute the line of code console.log (" test ‚") ten times, once for each execution of the loop.

How does it work? The for loop is broken down into three components.

The first component is i = 0. This is where we define a JavaScript variable which registers the number of times our loop has been executed.

Then i < 10 defines how many times the loop should be executed (in this case, 10). The i ++ component runs after each iteration. This component adds one to the variable counter "i" command .

The JavaScript loop forEach

forEach is a JavaScript Array method. It is used to perform a function on each element of an array. Lists, sets, and all other list-type objects support the forEach method.

We’ll write a loop that will display a list of companies on the console. Instead of using a for loop, we’ll use a forEach loop. This is because we are working with a list of companies.

The following code prints each value of a list of companies to the console:

Our output for our code is as follows:

For each item in our "companies" list , we print the item on the console. This is different from how a normal for loop works.

JavaScript forEach vs for Loop

In a traditional for loop, you should access each item in the "companies" list for

ForEach loops accept a function of callback while for loops don’t. In a for loop, all your code is included in the body p main loop. In a forEach loop, you need to write a function that will be executed for each item in the list that you iterate over.

JavaScript forEach Callback

The forEach method accepts a function callback. This is a function passed to a another function as argument. Callbacks are executed in the function they appear in.

Let’s go back to our last example:

Our callback comes after the arrow (=>). This arrow indicates an arrow function. But callbacks don’t need to be arrow functions.

We can define the function we want to run elsewhere:

In this code, we’ve defined a new function called printValue. We passed this function to our forEach loop as an argument.

This function is defined independently of our forEach loop. Our code works because the forEach loop accepts any type of function. You do not need to specify an arrow function.

The advantage of this approach is that our code is easier to read. It is clear that the printValue () function is executed for each element of the "companies" list .

Tracking current index

We can track the index value of the item in the ’list that we are displaying with a forEach method. We can do this by specifying an "index" variable in our callback function.

We have decided that we want to see both the index of the company and the content of the item we are viewing. This can be achieved by adding an "index" variable:

Let’s run our code and see what happens:

0. Apple

1. Google

2. Facebook

We can see both the name of each company and its index value. The "index" variable represents each index value.

"index" can be called whatever you want, as long as it comes after the first variable in the callback. The first variable is reserved for tracking the element on which the loop is iterated.

Uses of JavaScript forEach

You must use the forEach method if you want to scroll through the array elements. The map () and reduce () methods are more efficient if you need to calculate a result based on the values ‚Äã‚Äãin a list.

JavaScript forEach loops are very useful when you need to do something with every element of an array in JavaScript, not just some.

The same goes for for… in loops. If you tried to iterate through all the objects that are n an array, a for ... in loop would work.

On the other hand, if you need to write something more customizable, perhaps with multiple rules, a "for; it could be better.

Additionally, forEach calls are part of JavaScript 1.6. This means that the forEach method supports browsers in most major browsers.

There you go: forEach JavaScript looping in a nutshell !

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Javascript Foreach 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

Javascript Foreach JavaScript: Questions


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.

Javascript Foreach JavaScript: Questions


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.


Python"s equivalent of && (logical-and) in an if-statement

5 answers

delete By delete

Here"s my code:

def front_back(a, b):
  # +++your code here+++
  if len(a) % 2 == 0 && len(b) % 2 == 0:
    return a[:(len(a)/2)] + b[:(len(b)/2)] + a[(len(a)/2):] + b[(len(b)/2):] 
    #todo! Not yet done. :P

I"m getting an error in the IF conditional.
What am I doing wrong?


Answer #1

You would want and instead of &&.


Answer #2

Python uses and and or conditionals.


if foo == "abc" and bar == "bac" or zoo == "123":
  # do something


How do you get the logical xor of two variables in Python?

5 answers

Zach Hirsch By Zach Hirsch

How do you get the logical xor of two variables in Python?

For example, I have two variables that I expect to be strings. I want to test that only one of them contains a True value (is not None or the empty string):

str1 = raw_input("Enter string one:")
str2 = raw_input("Enter string two:")
if logical_xor(str1, str2):
    print "ok"
    print "bad"

The ^ operator seems to be bitwise, and not defined on all objects:

>>> 1 ^ 1
>>> 2 ^ 1
>>> "abc" ^ ""
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for ^: "str" and "str"

Answer #1

If you"re already normalizing the inputs to booleans, then != is xor.

bool(a) != bool(b)


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