Use In Javascript

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JavaScript is a programming language used to create dynamic content on websites.

Take a moment to think about the web features you use every day. Think about how the new content loads when you keep scrolling on Twitter or how YouTube updates when you click the like button. This is JavaScript at work. JavaScript can be used to create interactive forms, animate images, create responsive elements and more to meet your needs

When building a website, there are three main web technologies that you will use. HTML, CSS and JavaScript. You will use HTML to determine the structure of a web page and CSS to control how a web page looks . JavaScript is the third element of web development you can use to make your site dynamic.

And that’s not all JavaScript can do. If you want to create more than one static web page, we recommend that you use JS. But JavaScript can also be used in back - end web development , or to develop games or to create mobile applications.

In this guide, we’ll go over some - some of the operations most common that you might encounter in learning JavaScript. We will discuss the basics of JavaScript, including sub - chains, test code using try ... catch blocks, using if statements and more. After covering the basics, we’ll also explore some more advanced JavaScript features.

If you are new to JavaScript and are looking for an introductory tutorial , look no further than this guide! Let ’s start by exploring some of the most common operations in JavaScript.

Filter and shrink JavaScript

When working with an array in JavaScript, you can filter your objects or perform a calculation on all elements. to the interior of the matrix. This is where the filter () and reduce () functions come into play, respectively.

Filter () The function can be used to get values ‚Äã‚Äãthat meet particular criteria from an array. Here is an example of a filter function that returns all students 1st grade:

reduce () method can be used to reduce an array to a single value. For example, if we want to get the total age of students in a class, we could use this code:

See our full guide to these functions for more information on how they work and how to use them in your code.

JavaScript string contains

strings are used in programming for r display and with text. When working with a string, you can check whether this string contains a certain value or not. For example, you can check if someone’s name contains Jones or not.

You can use the built-in understands () in JavaScript to check if a string contains a substring. Here is an example of the include () method used to check if a string contains a substring:

JavaScript Split and Slice

Often you can have a string that you want to split into a substring or an array. To do this, you can use the JavaScript functions split () and slice ()

the split () function allows you to split a string into an array. For example, if you want to convert a user’s full name to an array of strings, you can use the. split () function Here is an example of split () in action:

slice () returns all characters between two index numbers in a string, allowing you to create a substring Here is an example of a slice in action.

JavaScript code splice () can be used to add or remove an element from any position in an array. For example, if you have a list of employee names and you want to remove the second one from the list, you can use splice ().

Here is an example of splice () used to remove the name Paul, which has the value of index 3 , from an array:

JavaScript Substring

The JavaScript code substring ( ) method allows you to extract parts of a string and create a substring there. For example, if you want to get the last two characters in someone’s name, you can use the substring ().

Here is an example of a substring used to remove the first two characters from a string:

JavaScript Try Catch

Every developer makes mistakes when they are coding and it is very important to make sure that if an error appears , it is treated appropriately. Try / catch blocks are used in JavaScript for handle errors.

When a try / catch block is used, the code will execute and return a custom response if an error is found in the code. Here is an example try / capture procedure in action:

JavaScript If Else

Conditional statements are used when you want your code to run only when a certain set of criteria are met. For example, you might want your program to print the username if they are entered in a valid name and return a message to the user in the web browser if their name is invalid.

The if / else statement can be used to execute certain code if one statement is true and to execute another block of code if one statement is false. Here is the syntax of an if / else Declaration:

Replace JavaScript string

the JavaScript < the> replace () code can be used to replace certain characters - or sequences of characters - in a text string. For example, you can use replace () to change the email address of each occurrence of a user to a string if the user changed their email

Here is an example of how you can use replace () in JavaScript to replace text:

JavaScript onclick

When you create a web page you may want an element of do something when a user clicks on it. for example, you can a user to get a message if they click a button or an image to enlarge it is clicked. This is where the onclick () function comes in .

onclick () allows you to make an interactive webpage so that if a user clicks on an item, something will happen. Read our guide to the onclick () event to learn more about how it works and how you can use it in your code.

JavaScript Sort array

When working with an array, you can decide to sort the array by a special way. For example, you can sort a list of names alphabetically or a list of votes in ascending order.

The JavaScript sort () function can be used to organize an array in a particular way. Here is an example of the () sort function used to sort an array of student names in alphabetical order:

The sort () function can also be used to perform advanced sorting if you define a custom function in the genre

JavaScript operator ternary

JavaScript ternary operators are if ... else which controls whether a condition is met and execute a block of code. For example if you want to check if they are over 18 and false an age of the user and return true if they are under 18 you can use a ternary operator.

Ternary operators are often used in place of simple if ... else because they are compact and on one line.

Here is the syntax for a ternary operator in JavaScript:

JavaScript forEach

When programming in JavaScript, you may want to iterate through all the items in a list. For example, you might have a list of vendor names that you want to print for the user. You can use the forEach loop to create this function

forEach loop is a form of a loop that performs a function once on each element of an array. Here is the syntax for a forEach statement that iterates through a list of companies and prints each on a new line:

JavaScript functions

Functions that are part of most programming languages, allow you to write code once per process, and use that code multiple times in your program.

For example, if you have a block of code that checks if a form field is valid, you might want to make it a function, so I don’t have to repeat a block of code for each field form on a web page. <

p> Here is an example of a JavaScript function:

By itself our function does nothing, but when called, the function will print Google to the console. To call our function, we use the following code:

JavaScript Either

The keyword JavaScript let allows to declare a block-field, locale variable which can be reassigned. Unlike var variables let variables are only accessible within a given code block, but can be reassigned as var Variables.

Declaring a variable using let works the same as var:

Read our full article on JavaScript let to learn the differences between var, const and let, as well as how to use them all correctly.

JavaScript array Press

When working with an array,you can add an element to this array. This is where the push () JavaScript comes in. You can use push () to append an element to the end of an array. Here is an example of push () method used:

Output:

You can also use unshift () to add an element to the from an array. unshift () works the same as push () and accepts the same argument.

Conclusion

JavaScript is a programming tool language used to create dynamic web pages. In this guide, we discussed some - some of the operations most common JavaScript used by beginners. We’ve covered everything from onClick to string replacement, cutting data to meet your needs.

This manual isn’t just for beginners, however. As you continue your journey of learning JavaScript, you should keep coming back to this guide and use it to help you if you get lost. We’ll keep this article updated so that if you have a question about a common JavaScript operation, you’ll probably find it here.

Use In Javascript exp: Questions

exp

How do I merge two dictionaries in a single expression (taking union of dictionaries)?

5 answers

Carl Meyer By Carl Meyer

I have two Python dictionaries, and I want to write a single expression that returns these two dictionaries, merged (i.e. taking the union). The update() method would be what I need, if it returned its result instead of modifying a dictionary in-place.

>>> x = {"a": 1, "b": 2}
>>> y = {"b": 10, "c": 11}
>>> z = x.update(y)
>>> print(z)
None
>>> x
{"a": 1, "b": 10, "c": 11}

How can I get that final merged dictionary in z, not x?

(To be extra-clear, the last-one-wins conflict-handling of dict.update() is what I"m looking for as well.)

5839

Answer #1

How can I merge two Python dictionaries in a single expression?

For dictionaries x and y, z becomes a shallowly-merged dictionary with values from y replacing those from x.

  • In Python 3.9.0 or greater (released 17 October 2020): PEP-584, discussed here, was implemented and provides the simplest method:

    z = x | y          # NOTE: 3.9+ ONLY
    
  • In Python 3.5 or greater:

    z = {**x, **y}
    
  • In Python 2, (or 3.4 or lower) write a function:

    def merge_two_dicts(x, y):
        z = x.copy()   # start with keys and values of x
        z.update(y)    # modifies z with keys and values of y
        return z
    

    and now:

    z = merge_two_dicts(x, y)
    

Explanation

Say you have two dictionaries and you want to merge them into a new dictionary without altering the original dictionaries:

x = {"a": 1, "b": 2}
y = {"b": 3, "c": 4}

The desired result is to get a new dictionary (z) with the values merged, and the second dictionary"s values overwriting those from the first.

>>> z
{"a": 1, "b": 3, "c": 4}

A new syntax for this, proposed in PEP 448 and available as of Python 3.5, is

z = {**x, **y}

And it is indeed a single expression.

Note that we can merge in with literal notation as well:

z = {**x, "foo": 1, "bar": 2, **y}

and now:

>>> z
{"a": 1, "b": 3, "foo": 1, "bar": 2, "c": 4}

It is now showing as implemented in the release schedule for 3.5, PEP 478, and it has now made its way into the What"s New in Python 3.5 document.

However, since many organizations are still on Python 2, you may wish to do this in a backward-compatible way. The classically Pythonic way, available in Python 2 and Python 3.0-3.4, is to do this as a two-step process:

z = x.copy()
z.update(y) # which returns None since it mutates z

In both approaches, y will come second and its values will replace x"s values, thus b will point to 3 in our final result.

Not yet on Python 3.5, but want a single expression

If you are not yet on Python 3.5 or need to write backward-compatible code, and you want this in a single expression, the most performant while the correct approach is to put it in a function:

def merge_two_dicts(x, y):
    """Given two dictionaries, merge them into a new dict as a shallow copy."""
    z = x.copy()
    z.update(y)
    return z

and then you have a single expression:

z = merge_two_dicts(x, y)

You can also make a function to merge an arbitrary number of dictionaries, from zero to a very large number:

def merge_dicts(*dict_args):
    """
    Given any number of dictionaries, shallow copy and merge into a new dict,
    precedence goes to key-value pairs in latter dictionaries.
    """
    result = {}
    for dictionary in dict_args:
        result.update(dictionary)
    return result

This function will work in Python 2 and 3 for all dictionaries. e.g. given dictionaries a to g:

z = merge_dicts(a, b, c, d, e, f, g) 

and key-value pairs in g will take precedence over dictionaries a to f, and so on.

Critiques of Other Answers

Don"t use what you see in the formerly accepted answer:

z = dict(x.items() + y.items())

In Python 2, you create two lists in memory for each dict, create a third list in memory with length equal to the length of the first two put together, and then discard all three lists to create the dict. In Python 3, this will fail because you"re adding two dict_items objects together, not two lists -

>>> c = dict(a.items() + b.items())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: "dict_items" and "dict_items"

and you would have to explicitly create them as lists, e.g. z = dict(list(x.items()) + list(y.items())). This is a waste of resources and computation power.

Similarly, taking the union of items() in Python 3 (viewitems() in Python 2.7) will also fail when values are unhashable objects (like lists, for example). Even if your values are hashable, since sets are semantically unordered, the behavior is undefined in regards to precedence. So don"t do this:

>>> c = dict(a.items() | b.items())

This example demonstrates what happens when values are unhashable:

>>> x = {"a": []}
>>> y = {"b": []}
>>> dict(x.items() | y.items())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: "list"

Here"s an example where y should have precedence, but instead the value from x is retained due to the arbitrary order of sets:

>>> x = {"a": 2}
>>> y = {"a": 1}
>>> dict(x.items() | y.items())
{"a": 2}

Another hack you should not use:

z = dict(x, **y)

This uses the dict constructor and is very fast and memory-efficient (even slightly more so than our two-step process) but unless you know precisely what is happening here (that is, the second dict is being passed as keyword arguments to the dict constructor), it"s difficult to read, it"s not the intended usage, and so it is not Pythonic.

Here"s an example of the usage being remediated in django.

Dictionaries are intended to take hashable keys (e.g. frozensets or tuples), but this method fails in Python 3 when keys are not strings.

>>> c = dict(a, **b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: keyword arguments must be strings

From the mailing list, Guido van Rossum, the creator of the language, wrote:

I am fine with declaring dict({}, **{1:3}) illegal, since after all it is abuse of the ** mechanism.

and

Apparently dict(x, **y) is going around as "cool hack" for "call x.update(y) and return x". Personally, I find it more despicable than cool.

It is my understanding (as well as the understanding of the creator of the language) that the intended usage for dict(**y) is for creating dictionaries for readability purposes, e.g.:

dict(a=1, b=10, c=11)

instead of

{"a": 1, "b": 10, "c": 11}

Response to comments

Despite what Guido says, dict(x, **y) is in line with the dict specification, which btw. works for both Python 2 and 3. The fact that this only works for string keys is a direct consequence of how keyword parameters work and not a short-coming of dict. Nor is using the ** operator in this place an abuse of the mechanism, in fact, ** was designed precisely to pass dictionaries as keywords.

Again, it doesn"t work for 3 when keys are not strings. The implicit calling contract is that namespaces take ordinary dictionaries, while users must only pass keyword arguments that are strings. All other callables enforced it. dict broke this consistency in Python 2:

>>> foo(**{("a", "b"): None})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() keywords must be strings
>>> dict(**{("a", "b"): None})
{("a", "b"): None}

This inconsistency was bad given other implementations of Python (PyPy, Jython, IronPython). Thus it was fixed in Python 3, as this usage could be a breaking change.

I submit to you that it is malicious incompetence to intentionally write code that only works in one version of a language or that only works given certain arbitrary constraints.

More comments:

dict(x.items() + y.items()) is still the most readable solution for Python 2. Readability counts.

My response: merge_two_dicts(x, y) actually seems much clearer to me, if we"re actually concerned about readability. And it is not forward compatible, as Python 2 is increasingly deprecated.

{**x, **y} does not seem to handle nested dictionaries. the contents of nested keys are simply overwritten, not merged [...] I ended up being burnt by these answers that do not merge recursively and I was surprised no one mentioned it. In my interpretation of the word "merging" these answers describe "updating one dict with another", and not merging.

Yes. I must refer you back to the question, which is asking for a shallow merge of two dictionaries, with the first"s values being overwritten by the second"s - in a single expression.

Assuming two dictionaries of dictionaries, one might recursively merge them in a single function, but you should be careful not to modify the dictionaries from either source, and the surest way to avoid that is to make a copy when assigning values. As keys must be hashable and are usually therefore immutable, it is pointless to copy them:

from copy import deepcopy

def dict_of_dicts_merge(x, y):
    z = {}
    overlapping_keys = x.keys() & y.keys()
    for key in overlapping_keys:
        z[key] = dict_of_dicts_merge(x[key], y[key])
    for key in x.keys() - overlapping_keys:
        z[key] = deepcopy(x[key])
    for key in y.keys() - overlapping_keys:
        z[key] = deepcopy(y[key])
    return z

Usage:

>>> x = {"a":{1:{}}, "b": {2:{}}}
>>> y = {"b":{10:{}}, "c": {11:{}}}
>>> dict_of_dicts_merge(x, y)
{"b": {2: {}, 10: {}}, "a": {1: {}}, "c": {11: {}}}

Coming up with contingencies for other value types is far beyond the scope of this question, so I will point you at my answer to the canonical question on a "Dictionaries of dictionaries merge".

Less Performant But Correct Ad-hocs

These approaches are less performant, but they will provide correct behavior. They will be much less performant than copy and update or the new unpacking because they iterate through each key-value pair at a higher level of abstraction, but they do respect the order of precedence (latter dictionaries have precedence)

You can also chain the dictionaries manually inside a dict comprehension:

{k: v for d in dicts for k, v in d.items()} # iteritems in Python 2.7

or in Python 2.6 (and perhaps as early as 2.4 when generator expressions were introduced):

dict((k, v) for d in dicts for k, v in d.items()) # iteritems in Python 2

itertools.chain will chain the iterators over the key-value pairs in the correct order:

from itertools import chain
z = dict(chain(x.items(), y.items())) # iteritems in Python 2

Performance Analysis

I"m only going to do the performance analysis of the usages known to behave correctly. (Self-contained so you can copy and paste yourself.)

from timeit import repeat
from itertools import chain

x = dict.fromkeys("abcdefg")
y = dict.fromkeys("efghijk")

def merge_two_dicts(x, y):
    z = x.copy()
    z.update(y)
    return z

min(repeat(lambda: {**x, **y}))
min(repeat(lambda: merge_two_dicts(x, y)))
min(repeat(lambda: {k: v for d in (x, y) for k, v in d.items()}))
min(repeat(lambda: dict(chain(x.items(), y.items()))))
min(repeat(lambda: dict(item for d in (x, y) for item in d.items())))

In Python 3.8.1, NixOS:

>>> min(repeat(lambda: {**x, **y}))
1.0804965235292912
>>> min(repeat(lambda: merge_two_dicts(x, y)))
1.636518670246005
>>> min(repeat(lambda: {k: v for d in (x, y) for k, v in d.items()}))
3.1779992282390594
>>> min(repeat(lambda: dict(chain(x.items(), y.items()))))
2.740647904574871
>>> min(repeat(lambda: dict(item for d in (x, y) for item in d.items())))
4.266070580109954
$ uname -a
Linux nixos 4.19.113 #1-NixOS SMP Wed Mar 25 07:06:15 UTC 2020 x86_64 GNU/Linux

Resources on Dictionaries

5839

Answer #2

In your case, what you can do is:

z = dict(list(x.items()) + list(y.items()))

This will, as you want it, put the final dict in z, and make the value for key b be properly overridden by the second (y) dict"s value:

>>> x = {"a":1, "b": 2}
>>> y = {"b":10, "c": 11}
>>> z = dict(list(x.items()) + list(y.items()))
>>> z
{"a": 1, "c": 11, "b": 10}

If you use Python 2, you can even remove the list() calls. To create z:

>>> z = dict(x.items() + y.items())
>>> z
{"a": 1, "c": 11, "b": 10}

If you use Python version 3.9.0a4 or greater, then you can directly use:

x = {"a":1, "b": 2}
y = {"b":10, "c": 11}
z = x | y
print(z)
{"a": 1, "c": 11, "b": 10}

5839

Answer #3

An alternative:

z = x.copy()
z.update(y)

Use In Javascript filter: Questions

filter

List comprehension vs. lambda + filter

5 answers

I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items.

My code looked like this:

my_list = [x for x in my_list if x.attribute == value]

But then I thought, wouldn"t it be better to write it like this?

my_list = filter(lambda x: x.attribute == value, my_list)

It"s more readable, and if needed for performance the lambda could be taken out to gain something.

Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way‚Ñ¢ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)?

957

Answer #1

It is strange how much beauty varies for different people. I find the list comprehension much clearer than filter+lambda, but use whichever you find easier.

There are two things that may slow down your use of filter.

The first is the function call overhead: as soon as you use a Python function (whether created by def or lambda) it is likely that filter will be slower than the list comprehension. It almost certainly is not enough to matter, and you shouldn"t think much about performance until you"ve timed your code and found it to be a bottleneck, but the difference will be there.

The other overhead that might apply is that the lambda is being forced to access a scoped variable (value). That is slower than accessing a local variable and in Python 2.x the list comprehension only accesses local variables. If you are using Python 3.x the list comprehension runs in a separate function so it will also be accessing value through a closure and this difference won"t apply.

The other option to consider is to use a generator instead of a list comprehension:

def filterbyvalue(seq, value):
   for el in seq:
       if el.attribute==value: yield el

Then in your main code (which is where readability really matters) you"ve replaced both list comprehension and filter with a hopefully meaningful function name.

957

Answer #2

This is a somewhat religious issue in Python. Even though Guido considered removing map, filter and reduce from Python 3, there was enough of a backlash that in the end only reduce was moved from built-ins to functools.reduce.

Personally I find list comprehensions easier to read. It is more explicit what is happening from the expression [i for i in list if i.attribute == value] as all the behaviour is on the surface not inside the filter function.

I would not worry too much about the performance difference between the two approaches as it is marginal. I would really only optimise this if it proved to be the bottleneck in your application which is unlikely.

Also since the BDFL wanted filter gone from the language then surely that automatically makes list comprehensions more Pythonic ;-)

filter

How do I do a not equal in Django queryset filtering?

5 answers

MikeN By MikeN

In Django model QuerySets, I see that there is a __gt and __lt for comparative values, but is there a __ne or != (not equals)? I want to filter out using a not equals. For example, for

Model:
    bool a;
    int x;

I want to do

results = Model.objects.exclude(a=True, x!=5)

The != is not correct syntax. I also tried __ne.

I ended up using:

results = Model.objects.exclude(a=True, x__lt=5).exclude(a=True, x__gt=5)
784

Answer #1

You can use Q objects for this. They can be negated with the ~ operator and combined much like normal Python expressions:

from myapp.models import Entry
from django.db.models import Q

Entry.objects.filter(~Q(id=3))

will return all entries except the one(s) with 3 as their ID:

[<Entry: Entry object>, <Entry: Entry object>, <Entry: Entry object>, ...]

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