DEV Community: Ashraful Islam

10 thing you should know about javascript

Ashraful Islam — Sat, 08 May 2021 15:19:57 +0000

What is the purpose of the array slice method: The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end.

2.What is the purpose of the array splice method:
The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the optional second argument indicates the number of elements to be deleted. Each additional argument is added to the array.

3.How do you compare Object and Map:
Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.

The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually

4.What is the difference between == and === operators:
JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,

Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value. There are two special cases in this,
NaN is not equal to anything, including NaN.
Positive and negative zeros are equal to one another.

5.What are lambda or arrow functions:
An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.

6.What is a first class function:
In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.

For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener

7.What is a first order function:
First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.

8.What is a higher order function:
Higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.

9.What is a pure function:
A Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value.

10.What is the purpose of the let keyword:
The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope.

Comments

Ashraful Islam — Thu, 06 May 2021 15:31:21 +0000

Comments
As we know from the chapter Code structure, comments can be single-line: starting with // and multiline: /* ... */.

We normally use them to describe how and why the code works.

At first sight, commenting might be obvious, but novices in programming often use them wrongly.:

Curly Braces

Ashraful Islam — Thu, 06 May 2021 15:24:26 +0000

Curly Braces
In most JavaScript projects curly braces are written in “Egyptian” style with the opening brace on the same line as the corresponding keyword – not on a new line. There should also be a space before the opening bracket, like this:

Coding Style

Ashraful Islam — Thu, 06 May 2021 15:18:37 +0000

Coding Style
Our code must be as clean and easy to read as possible.

That is actually the art of programming – to take a complex task and code it in a way that is both correct and human-readable. A good code style greatly assists in that.

Syntax
Here is a cheat sheet with some suggested rules (see below for more detail

How to throwing our own errors

Ashraful Islam — Thu, 06 May 2021 15:15:56 +0000

Throwing our own errors
What if json is syntactically correct, but doesn’t have a required name property?

Error object

Ashraful Islam — Thu, 06 May 2021 15:03:25 +0000

Error object
When an error occurs, JavaScript generates an object containing the details about it. The object is then passed as an argument to catch

try...catch works synchronously

Ashraful Islam — Thu, 06 May 2021 15:01:49 +0000

try...catch works synchronously
If an exception happens in “scheduled” code, like in setTimeout, then try...catch won’t catch it:

When try...catch dosen't work

Ashraful Islam — Thu, 06 May 2021 14:57:39 +0000

try...catch only works for runtime errors
For try...catch to work, the code must be runnable. In other words, it should be valid JavaScript.

It won’t work if the code is syntactically wrong, for instance it has unmatched curly braces:

Error Handling(try...catch)

Ashraful Islam — Thu, 06 May 2021 14:53:20 +0000

Hey todays topic is very intersting, today we will learn about error handling.
Accualy it dosen't matter how great we are at programming , sometime our script have error. Error may occur because of our mistake , an unknown code, an unexpected user input , and for a thousand reason we can have error. Usually a script stop due to error.
The “try…catch” syntax
The try...catch construct has two main blocks: try, and then catch:

try {

// code...

} catch (err) {

// error handling

}
It works like this:

First the try code is executed. if there is no error then code is run avoiding the catch error.
if try code executed and get some problem , cod stop and catch function catch the error with out kill the script so we have chance to handle it.

Types of Values

Ashraful Islam — Thu, 06 May 2021 13:57:22 +0000

As a javascript learner you should have proper knowledge about values. After Almost twenty five yeas of studying JavaScript, the scientists have only discovered nine such type of values , that you should remember always.
There are two type of values

Primitive Values
Objects And Functions

Primitive Values:
● Undefined (undefined), used for unintentionally missing values.
● Null (null), used for intentionally missing values.
● Booleans (true and false), used for logical operations.
● Numbers (-100, 3.14, and others), used for math calculations.
● Strings ("hello", "abracadabra", and others), used for text.
● Symbols (uncommon), used to hide implementation details.
● BigInts (uncommon and new), used for math on big numbers.

Objects And Functions:
● Objects ({} and others), used to group related data and code.
● Functions (x => x * 2 and others), used to refer to code.

thank you for reading.
happy coading(.)

Expressions

Ashraful Islam — Thu, 06 May 2021 12:06:48 +0000

There are many questions javascript can't answer. As if you want to know about weather , javascript wont be abele to answer it . But there are some questions that javascript can answer it, or you can say javascript would make you happy by answering your questions. This tipe of questions have a name , it calls expressions. Let's see some example:
if we ask the expressions 2+2 javascript will answer with value 4.
CONSOL.LOG(2+2); // 4
one thing you shuld know that, EXPRESSIONS is a questions that javascript can answer , and it always result in a single value.