Nov 3, 2020

4 min read

“Expert JavaScript ”

1 . Discuss about Error handling

in error handling using Four keyword
try
catch
finally
throw

2. why use error handling?

Then if we create large apps suppose 1000 line code in the run time has found anyone line error we can not see our output for that reason using error handling. if we use try catch it’s ignore the error code and display the output.

declaration of try..catch

try{

code….

}

catch(){

}

catch is a function it work is before find execution error handle

try work where error have a possibility that’s place

Example:

try{

alert(“Hello everyone”);

alert(“Bye Bye”);

}

catch(){

alert(“Inside catch block”)

}

if try block code is ok catch block does not work but in try block have identify error then catch is work and ignore the error and the output

3. try..catch only work for run time error

JavaScript engine first read the code and then run

2. try catch use finally block

finally block always work if error have or no error it give the output

try{

alert(“Hello everyone”);

alert(“Bye Bye”);

}

catch(err){

alert(“Inside catch block”)

}

finally{

alert(“God bless you”);

}

1. throw statement

throw statement create custom error.

The throw operator generates an error.

throw <error object>

There may be no catch section or no finally, so shorter constructor and try..catch are also valid.

3 .Coding Style

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

coding style is the actually the art of programming

Curly Braces
line lentgh
Indents :
semicolon
nesting lavel
function placement
style guide
automated linters

this is very important for the coding style

4 .JavaScript comments

well programmer maintain all code decoration comment is one of them

// this is single-line comment

/*…….*/ multiline comment

those use the visual studio code or VSCode use

ctrl +slash

5. ES6/ECMAScript

block statement or compound statement in block statement code execute only inside block not other

example :

var x = 1;
let y = 1;

if (true) {
var x = 2;
let y = 2;
}

console.log(x);
expected output: 2

console.log(y);
expected output: 1

6.Hoisting

hoisting is like scope where we declare variable it’s define and when we declare variable how javaScript deal that variable that is hoisting

Hoisting Bangla meaning Uttolon or Uporer dike tola

7. Block-level declaration, Block-Binding

Block-level declarations are those that declare variables that are inaccessible outside of a given block scope. Block scopes are created:

Inside of a function
Inside of a block (indicated by the { and } characters)

var/let/const

var

it al time can use

2. var is a function scope inside the function area it can call

example:

if(true){

var myVariable=“This is var”;

}

console.log(myVariable);

let

if have possibility to change the variable name then use the let l

example :

let patient=20;

patient=40;

if(true){

let letVariable=“this is let”;

console.log(letVariable);

}

2. let is block scope it only work in block area not other

const

1 .if variable value do not have to chance to change in next that time use the

const.

2. const fix no change

example:

const country=“Bangladesh”;

console.log(country);

8 .Function default parameter

when call the function if do not given parameter but we want to run the code that’s why use default parameter

example:

9 .Spread Operator

spread operator (…)

The spread operator allows an expression to be expanded in places where multiple arguments (for function calls) or multiple elements (for array literals) are expected.

2. using spread operator do concatenate work easily

example: