JS fundamental part-1
These are the important topics that accelerate JavaScript concept.
Some of the questions with answers for the JS interview.
Why do we call JavaScript a dynamic language?
JavaScript is a dynamic language that means data types of the variables can change during the runtime.
example
var x = 90;
x++;
console.log(typeof (x)); // number
x = "string";
console.log(typeof (x)); // string
... so on.
how does JavaScript determine data types?
JS determines data types depending on the value assigned during run time.
Example
let x = true;
let datatype1 = typeof (x);
let y = "string";
let datatype2 = typeof (y);
let z = 0;
let datatype3 = typeof (z);
console.log("1." + datatype1, "2." + datatype2, "3." + datatype3);
What are different datatypes in JavaScript?
Explain Undefined Data types and null?
Undefined means the variable has been declared but no value is assigned to it.
Example
var x = 100;
var word = "Hello";
// These above are defined with some values
var y;
console.log(x);
console.log(word);
console.log(y);
output:
100
Hello
undefined
Null indicates the intentional absence of data. Null indicates its not ZERO, it's not empty it's just an absence of data.
Example
var x = 100;
var word = "Hello";
// These above are defined with some values
var y = null;
console.log(x);
console.log(word);
console.log(y);
output:
100
Hello
null
Undefined Vs Null
clear with example
var x = 100;
var word = "Hello";
// These above are defined with some values
var y;
var n = null
console.log(x);
console.log(word);
console.log(y);
console.log(n);
output:
100
Hello
undefined -> Here variable has been created but the value is not assigned.
null -> we assign the value Null, which indicates an absence of data.
What is Hoisting?
It’s a mechanism where variable and function declarations are at the top of the scope.
Demo
console.log(y); // before execution: undefined
var y = "Ram";
console.log(y); // after execution: Ram
// for a function
getName(); // fuction declared here
console.log(getName);
function getName() {
console.log("Curry");
}
var VS let
Demo
var name = "John";
let name1 = "Doe";
Above confusion about function and block scope.
First of all, What is Scope?
-
Scope
‘scope’ means which variable is accessible and up to which point it is accessible.
-
Local scope and global scope
-
Global scope
In JS whole document is the global scope which contains other functions and variables.
-
Local scope
variables declared inside the functions are considered to be of the local scope and it is further divided into function scoped and block scope.
-
Function Scope
When a variable is declared inside a function, it is only accessible within that function and cannot be used outside that function.
var is called function scope.
-
Block Scope
A variable when declared inside the if or switch conditions or inside for or while loops, are accessible within that particular condition or loop. To be concise the variables declared inside the curly braces are called within block scope.
let & const are also called block scope.
-
-
function Demo() {
if (true) {
var a = "hello";
let b = 90;
const c = "world";
console.log("inside scope");
console.log(a);
console.log(b);
console.log(c);
}
console.log("outside scope");
console.log(a); // function scope
console.log(b);
console.log(c);
}
console.log(a); // outside function it is not defined
Demo();
output:
inside scope
hello
90
world
outside scope
hello
d:\Js\frontEnd\jsInterview.js:14
console.log(c);
^
ReferenceError: c is not defined
d:\Js\frontEnd\jsInterview.js:15
console.log(c);
^
ReferenceError: c is not defined
As the output shows when we try to access the variables b and c outside the if statement it gives an error as not defined. same for a variable when trying access outside the function.
What are Closures and why do we need Closures?
Closures are functions inside function and it makes a normal function stateful. closure gives you access to an outer function’s scope from an inner function.
function sum(a) {
return function (b) {
return function (c) {
// outer functions scope
return function (d) {
// local scope
return a + b + c + d ;
};
};
};
}
console.log(sum(1)(2)(3)(4)); // 10
Closures are
- Self-contained modules.
- Self-contained state.
IIFE (Immediately Invoked Function Expression)
This is nothing but an anonymous function which invoked immediately. This is also called a function without a name.
demo
var x = 0;
(function(){
var y = 30;
console.log("I am anonymous function");
})();
// any variable within iife is local to it.
What is name collision and how can you remove it?
Name collision happens when the same name function names and
variable names are declared.
demo
function hello(a,b){
var sum = 0;
console.log(sum+a+b);
}
var hello = "hey result is ";
hello();
// error: hello() is not a function
By using the anonymous function we can easily remove it.
(function (a,b){
var sum = 0;
console.log(sum+a+b);
})();
var hello = "hey result is ";
// error: hello() is not a function
What are the various ways to create JavaScript objects?
We can create an object in three different ways:
- Using object literal
- By creating an instance of an Object directly
- By using the constructor function
- ES6 class
1. Using object literal
Literals are smaller and simpler ways to define objects. We simply define the property and values inside curly braces.
Example
const student = {
name: "xyz",
faculty:"cse",
department: function(){
console.log("from computer department");
}
}
console.log(student);
console.log(student.name);
2. By creating an instance of an object directly
The Object. create() method creates a new object, using an existing object as the prototype of the newly created object.
Example
const student = {
name: "xyz",
faculty:"cse",
department: function(){
console.log("from computer department");
}
}
const s1 = Object.create(student);
console.log(s1);
console.log(s1.name);
3. By using the constructor function
One of the easiest way to instantiate an object in JavaScript. A constructor is nothing but a function and with help of a new keyword, the constructor function allows to create of multiple objects of the same flavor as shown below:
Example
const student = {
name: "xyz",
faculty:"cse",
department: function(){
console.log("from computer department");
}
}
const s1 = new student();
console.log(s1);
console.log(s1.department);
//simple function with argument
function vehicle(name,maker,engine){
this.name = name;
this.maker = maker;
this.engine = engine;
}
//new keyword to create an object
let car = new vehicle('GT','BMW','1998cc');
//property accessors
console.log(car.name);
console.log(car.maker);
console.log(car['engine']);
4. ES6 class
ES6 supports class concepts like any other Statically typed or object-oriented language.
Example
class Student {
constructor(name,age,faculty){
this.name = name;
this.age = age;
this.faculty = faculty;
}
}
const s1 = new Student("xyz",21,"cse");
console.log(s1);
Remaining Concept you will learn.
- OOP Concepts
- How can we do inheritance in JavaScript ?
- What is prototype in JavaScript ?
- Explain Prototype chaining ?
- What is Let Keyword?
- Are Let variables hoisted ?
Explain Temporal Dead Zone?
Temporal Dead Zone (TDZ) is a state of variables where variables are named in memory, but they are not initialized with any value.
console.log(x); // error can't access 'x' before initialize
// This is TDZ
// This is TDZ
let x = 10; // End of TDZ
Thankyou for reading.
Your comments appreciates me to improve my writing. Hope this helpful. Next part will soon... stay tuned.
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