Javascript Syntax

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If you’re looking to learn how to write JavaScript, you’ve come to the right place.

Each programming language has its own rules, just like English. Think about it. When you were in elementary school, you learned grammar rules to structure sentences.

Much like English, programming languages ‚Äã‚Äãfollow rules to ensure that everyone can understand what is being said. Imagine if we could all create our own rules when using English. No one could communicate. Likewise, programming languages ‚Äã‚Äãare strict on the rules so that the code you write can be executed.

In this guide, we’ll cover JavaScript syntax. Syntax refers to the set of rules that define how code is written in JavaScript.

Why is syntax important?

In JavaScript, you must follow certain syntax rules. For example, if you don’t close a hook after opening one, an error will be returned. The computer cannot continue to run your program because it does not understand what you are telling it to do.

This deserves an important distinction between computers and humans: while humans may be able to identify the occasional error and skip it, computers cannot do the same when executing code in any programming language. This is why it is important to write code using the syntax of the programming language you are working with.

The syntax also makes your code more readable. When everyone uses the same syntax, it is easy to interpret each other’s programs. Just like in English, once you know the basic rules there are no more lines of code that you cannot read.

In this guide, we’ll focus on the following syntax features:

  • Whitespace
  • Naming of variables
  • Withdrawal
  • Comments
  • Semicolon

Let’s go!


Programmers are constantly discussing whitespaces. It is a very controversial subject. Putting these debates aside for a minute, there is one key rule you must remember when it comes to JavaScript syntax: you should have spaces between variables and before and after brackets

Suppose you write a loop for:

This loop prints all numbers between 0 and 4. as you can see it There is a space between our and the support for opening our loop. There is a space between our closing brace (() and the opening brace ({). There is also a space between all statements in parentheses:

  • var i = 0;
  • i <4;

When assigning a variable outside of a loop, you will use the same approach:

var biscuit = "raspberry Chocolate Chip";

Adding spaces between yo Our variables make it easier to read code creation.

Unlike other programming languages, you can write a single line JavaScript program. Not a good idea - think how hard it would be to read your code - but you can still do it.

Any statement that is inside a block of code - such as a conditional if statement or a class - must be indented. Indent refers to the add two spaces, four spaces or a tab at the beginning t code. There is a lot of debate among programmers as to which is best, but as long as you keep your code consistent, you won’t have a problem.

Here is an indented for loop:

You can see that the term console.log () has been indented with a tab. This is because it is contained in our braces. This helps distinguish the content of our for loop from the rest of our code. If we had another block of code in our code, like an if statement, we would indent its contents further:

This code prints the value of i if it is equal to three. Otherwise nothing happens. In this snippet, our if statement is in removal of a tab because it is in our for loop. Our console.log () statement is indented by two tabs because it is contained in our statement if, which in turn is contained in our for statement.


Comments are statements written by the developers, for the developers. technically, they are read by JavaScript, but will not be executed.

Lo The purpose of comments is to help developers keep track of their code. If you are implementing a complex function, you can write some comments so that you know what each part of the function does. Comments are often used in collaborative projects as although every developer can understand a program, even if they haven’t written themselves

in JavaScript, comments are written using double slashes , followed by the comment: ..

All subsequent characters after the double slash are treated as a comment. To create a multiline comment, you can use this syntax:

Any text between / * and * / will be ignored by the JavaScript compiler.

Naming variables

There are many different ways to name a variable. The three most common methods use upper and lower case letters and underscores.

In JavaScript, most developers prefer to use lowercase letters to name variables:

firstName, surname, isAdmin

The variable declaration looks like JavaScript like this:

var firstname = "Tony";

we follow the type of variable you use (in this case var) by the name of the variable, then by an equal sign, then by the value you want to store in the variable.

The first word of a variable must start with a lowercase letter, even if the variable is only one word long. The second word and all subsequent words must start with an uppercase letter, as you can see above.

Although you can write variables using other methods (such as "first_name" in underscore or "FirstName" in uppercase camel), it is not common to do so. in JavaScript.

Variables in JavaScript are case sensitive . this means that firstname and firstname, while containing the same basic characters, would be treated differently.

There are a few words called "reserved words‚" that you cannot use as variable names . these are words they have one. special function in JavaScript for example, you cannot name a variable "class" or "for" because they are already used in language


You can find a list of reserved JavaScript keywords on target Mozilla Developer Network .


In English, each sentence ends with a period ; a point. tells you when to stop reading one sentence and start another. If you think back to your elementary school years, you’ve probably been told that sentences are a place to pause and catch your breath.

Computer programs. don’t "don’t need to catch your breath, but they do need to know how instructions are broken down into a program in JavaScript, most instructions end with a semicolon:

var cookie = "Chocolate fondant"

It is not necessary to specify a semicolon after each declaration block . These are statements like class, switch, do, if, and for. These statements use curly braces to store their code:

In this code, you can see that we ended our return statement with a semicolon. It is because it is in our function. Our function does not end with a semicolon.


There are many rules in the JavaScript programming language. Each of these rules ensures that you are writing code that you and the browser can read. If you are working in a team, it is especially important to follow the rules of JavaScript syntax.

Here is a summary of the main rules we covered:

You are now ready to start coding in JavaScript as a professional developer !

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Javascript Syntax find: Questions


Finding the index of an item in a list

5 answers

Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index (1) in Python?


Answer #1

>>> ["foo", "bar", "baz"].index("bar")

Reference: Data Structures > More on Lists

Caveats follow

Note that while this is perhaps the cleanest way to answer the question as asked, index is a rather weak component of the list API, and I can"t remember the last time I used it in anger. It"s been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about list.index follow. It is probably worth initially taking a look at the documentation for it:

list.index(x[, start[, end]])

Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item.

The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

Linear time-complexity in list length

An index call checks every element of the list in order, until it finds a match. If your list is long, and you don"t know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give index a hint. For instance, in this snippet, l.index(999_999, 999_990, 1_000_000) is roughly five orders of magnitude faster than straight l.index(999_999), because the former only has to search 10 entries, while the latter searches a million:

>>> import timeit
>>> timeit.timeit("l.index(999_999)", setup="l = list(range(0, 1_000_000))", number=1000)
>>> timeit.timeit("l.index(999_999, 999_990, 1_000_000)", setup="l = list(range(0, 1_000_000))", number=1000)

Only returns the index of the first match to its argument

A call to index searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.

>>> [1, 1].index(1)
>>> [i for i, e in enumerate([1, 2, 1]) if e == 1]
[0, 2]
>>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
>>> next(g)
>>> next(g)

Most places where I once would have used index, I now use a list comprehension or generator expression because they"re more generalizable. So if you"re considering reaching for index, take a look at these excellent Python features.

Throws if element not present in list

A call to index results in a ValueError if the item"s not present.

>>> [1, 1].index(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 2 is not in list

If the item might not be present in the list, you should either

  1. Check for it first with item in my_list (clean, readable approach), or
  2. Wrap the index call in a try/except block which catches ValueError (probably faster, at least when the list to search is long, and the item is usually present.)


Answer #2

One thing that is really helpful in learning Python is to use the interactive help function:

>>> help(["foo", "bar", "baz"])
Help on list object:

class list(object)

 |  index(...)
 |      L.index(value, [start, [stop]]) -> integer -- return first index of value

which will often lead you to the method you are looking for.


Answer #3

The majority of answers explain how to find a single index, but their methods do not return multiple indexes if the item is in the list multiple times. Use enumerate():

for i, j in enumerate(["foo", "bar", "baz"]):
    if j == "bar":

The index() function only returns the first occurrence, while enumerate() returns all occurrences.

As a list comprehension:

[i for i, j in enumerate(["foo", "bar", "baz"]) if j == "bar"]

Here"s also another small solution with itertools.count() (which is pretty much the same approach as enumerate):

from itertools import izip as zip, count # izip for maximum efficiency
[i for i, j in zip(count(), ["foo", "bar", "baz"]) if j == "bar"]

This is more efficient for larger lists than using enumerate():

$ python -m timeit -s "from itertools import izip as zip, count" "[i for i, j in zip(count(), ["foo", "bar", "baz"]*500) if j == "bar"]"
10000 loops, best of 3: 174 usec per loop
$ python -m timeit "[i for i, j in enumerate(["foo", "bar", "baz"]*500) if j == "bar"]"
10000 loops, best of 3: 196 usec per loop


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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