Node js Developer Top Interview Questions

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Is Node.js a JavaScript framework or library?

Node.js is not a JavaScript framework or library. Node.js is a runtime environment that allows you to run JavaScript code outside of a web browser. It is built on the V8 JavaScript engine, the same engine that powers Google Chrome, and provides additional features and APIs for server-side development.

Node.js allows you to build scalable and high-performance network applications using JavaScript. It provides an event-driven, non-blocking I/O model, which makes it efficient for handling concurrent requests and performing tasks asynchronously. Node.js also includes a built-in module system and a vast ecosystem of libraries and packages available through npm (Node Package Manager).

What is the difference between const, let and var?

In JavaScript, const, let, and var are used for variable declarations, but they have some differences in terms of scope, hoisting, and reassignment capabilities.

1. var: Prior to the introduction of let and const in ECMAScript 2015 (ES6), var was the primary keyword used for variable declaration. Variables declared with var have function-level scope, which means they are accessible within the function they are defined in, or they can be accessed globally if declared outside any function. Variables declared with var are hoisted, meaning they are moved to the top of their scope during the compilation phase, allowing them to be accessed before they are declared in the code. Additionally, var variables can be redeclared and reassigned within their scope.

Example 1: Using const

   const PI = 3.14159;
   console.log(PI); // Output: 3.14159
   /// Attempting to reassign a const variable will result in an error
   PI = 3.14; Error: Assignment to constant variable.

In this example, PI is declared as a constant variable using const. The value of PI cannot be reassigned because it is a constant.

1. let: Introduced in ES6, let allows you to declare block-scoped variables. Block scope refers to variables being limited to the block of code (enclosed within curly braces) where they are defined, such as within a loop or conditional statement. Unlike var, let variables are not hoisted to the top of their scope and can only be accessed after they have been declared. let variables cannot be redeclared within the same scope, but they can be reassigned new values. This means that you can modify the value of a let variable, but you cannot use the let keyword to create a new variable with the same name within the same block scope.

Example 2: Using let

let count = 5;
console.log(count); // Output: 5

// Modifying the value of a let variable
count = 10;
console.log(count); // Output: 10

// Block scope with let
if (true) {
  let count = 20;
  console.log(count); // Output: 20
}
console.log(count); // Output: 10

In this example, count is declared using let. The variable can be reassigned with a new value. It also demonstrates block scoping, where the let variable declared within the if block has a different scope than the outer count variable.

1. const: Also introduced in ES6, const is used to declare variables that are block-scoped and have a constant value. const variables, once assigned a value, cannot be reassigned or redeclared within the same scope. They have the same block scope rules as let variables. It's important to note that while a const variable is immutable in terms of reassignment, if the variable holds a reference to an object or an array, the properties of the object or elements of the array can still be modified. This behavior is referred to as "const correctness."

Example 3: Using var

var x = 10;
console.log(x); // Output: 10

// Hoisting with var
console.log(y); // Output: undefined
var y = 5;

// Function scope with var
function varExample() {
  var z = 15;
  console.log(z); // Output: 15
}
console.log(z); // Error: z is not defined

In this example, x and y are declared using var. The var variables are hoisted to the top of their scope, which is why y can be accessed before it's declared (but it will have an initial value of undefined). Additionally, var has function-level scope, so the variable z declared within the varExample function is not accessible outside of the function.

In general, it is recommended to use const when you know the value of a variable should not change, let when you need to reassign a variable, and minimize the use of var due to its behavior and potential scoping issues.

What is the difference between apply, call and bind?

In JavaScript, apply, call, and bind are methods that allow you to explicitly set the value of this and invoke functions in different contexts. They differ in how they pass arguments to the function and how they handle function invocation.

1. apply: The apply method allows you to invoke a function and specify the value of this and an array-like object or an array as the arguments to be passed to the function.

Syntax:

function.apply(thisArg, [argsArray])

Example:

function greet(name) {
  console.log(`Hello, ${name}!`);
}

greet.apply(null, ['John']);
// Output: Hello, John!

In the above example, apply is used to invoke the greet function with 'John' as the argument.

1. call: Similar to apply, the call method allows you to invoke a function and explicitly set the value of this. However, instead of passing an array-like object or an array as arguments, you pass individual arguments directly to the function.

Syntax:

function.call(thisArg, arg1, arg2, ...)

Example:

function greet(name) {
  console.log(`Hello, ${name}!`);
}

greet.call(null, 'John');
// Output: Hello, John!

In this example, call is used to invoke the greet function with 'John' as the argument.

1. bind: The bind method creates a new function that, when called, has a specified this value and, optionally, prepends any provided arguments to the original function's arguments list.

Syntax:

function.bind(thisArg, arg1, arg2, ...)

Example:

function greet(name) {
  console.log(`Hello, ${name}!`);
}

const greetJohn = greet.bind(null, 'John');
greetJohn();
// Output: Hello, John!

What is the behavior of this in normal vs arrow function?

The behavior of this differs between normal functions and arrow functions in JavaScript. The value of this in a function is determined by how the function is called or invoked.

Normal Functions:
In a normal function, the value of this is dynamically determined at runtime based on the context of the function's invocation. It can vary depending on how the function is called. The following are some common scenarios:

1. Function Invocation: When a function is called as a standalone function, without any specific context, this refers to the global object (window in a browser or global in Node.js) in non-strict mode, or it is undefined in strict mode.

2. Method Invocation: When a function is called as a method of an object, this refers to the object that owns the method. The value of this is set to the object before the dot notation when calling the method.

3. Constructor Invocation: When a function is used as a constructor function with the new keyword, this refers to the newly created instance of the object.

4. Explicitly Binding this: The value of this can be explicitly bound using call(), apply(), or bind() methods. These methods allow you to specify the value of this within a function.

Arrow Functions:

Arrow functions, introduced in ES6, have a different behavior regarding this. In arrow functions, the value of this is lexically or statically scoped. This means that the value of this is inherited from the enclosing scope, where the arrow function is defined. It does not have its own this context.

In other words, within an arrow function, this retains the value of the surrounding scope, and it is not affected by how the function is invoked. It will always refer to the this value of the enclosing context where the arrow function is defined.

Arrow functions are especially useful when working with callbacks or nested functions, as they avoid potential confusion and issues related to the dynamic binding of this in normal functions.

Here's an example to illustrate the difference between normal functions and arrow functions:

const obj = {
  name: 'Example',
  normalFunction: function() {
    console.log(this.name);
  },
  arrowFunction: () => {
    console.log(this.name);
  }
};

obj.normalFunction(); // Output: "Example" (referring to obj)
obj.arrowFunction();  // Output: undefined (referring to the global object)

In the example above, obj.normalFunction() logs the name property of the obj object since this refers to the object itself. However, obj.arrowFunction() logs undefined because arrow functions do not have their own this context and instead inherit the value of this from the surrounding scope (in this case, the global object).

It's important to be aware of these differences in behavior between normal functions and arrow functions when working with this in JavaScript.

Is Node.js synchronous or asynchronous in nature?

Node.js is primarily designed to be asynchronous in nature. It uses an event-driven, non-blocking I/O model, which means that it can handle multiple concurrent operations without blocking the execution of other code. This asynchronous nature allows Node.js to efficiently handle high levels of concurrency and perform tasks without waiting for previous operations to complete. However, Node.js also provides synchronous versions of certain functions for situations where synchronous behavior is necessary or preferred.

These functions are typically suffixed with Sync to differentiate them from their asynchronous counterparts. Here are a few examples:

• fs.readFileSync(): This function is the synchronous version of fs.readFile(), allowing you to read the contents of a file synchronously.
• fs.writeFileSync(): It is the synchronous counterpart of fs.writeFile(), used for writing data to a file synchronously.
• fs.readdirSync(): This function is the synchronous alternative to fs.readdir(), used to read the contents of a directory synchronously.
• fs.statSync(): It is the synchronous version of fs.stat(), used to retrieve information about a file or directory synchronously.
• fs.existsSync(): This function is synchronous and used to check the existence of a file or directory synchronously.

These synchronous versions can be useful in certain scenarios, it's generally recommended to use the asynchronous counterparts in most cases, as they allow for better scalability and responsiveness in Node.js applications.

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