JavaScript Interview Questions and Answers.

Q1. What is JavaScript?

JavaScript (JS) is a lightweight, interpreted, or just-in-time compiled programming language with first-class functions. JavaScript is a prototype-based, multi-paradigm, single-threaded, dynamic language, supporting object-oriented, imperative, and declarative (e.g. functional programming) styles. 

Q2. What are the different data types in JavaScript?

The set of types in the JavaScript language consists of primitive values and objects.

  • Primitive values
    • Boolean
    • Null
    • Undefined
    • Number
    • BigInt
    • String
    • Symbol
  • Objects (collection of properties)

Q3. What are the JavaScript Falsy values?

falsy value is a value that is considered false when encountered in a Boolean context.

falseThe keyword false
0The Number zero
-0The Number negative zero
0nThe BigInt zero
“”, ”,“Empty string value
nullnull – the absence of any value
undefinedundefined – the primitive value
NaNNaN – not a number

Q3. Explain Type Coercion.

Type coercion is the automatic or implicit conversion of values from one data type to another (such as strings to numbers). Type conversion is similar to type coercion because they both convert values from one data type to another with one key difference — type coercion is implicit whereas type conversion can be either implicit or explicit.

Example:

const value1 = '1';
const value2 = 2;
let sum = value1 + value2;

console.log(sum);

In the above example, JavaScript has coerced the 2 from a number into a string and then concatenated the two values together, resulting in a string of 12.

Q4. Explain Variable Declaration in JavaScript.

In JavaScript variables can be declared as:

  • var – Declares a variable, optionally initializing it to a value.
  • let – Declares a block scope local variable, optionally initializing it to a value.
  • const – Declares a read-only named constant.

Q5. Explain difference Iterations in JavaScript.

  • do...while – Creates a loop that executes a specified statement until the test condition evaluates to false. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.
  • for – Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement executed in the loop.
  • for...in – Iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.
  • for...of – Iterates over iterable objects (including arrays, array-like objects, iterators and generators), invoking a custom iteration hook with statements to be executed for the value of each distinct property.
  • for await...of – Iterates over async iterable objects, array-like objects, iterators and generators, invoking a custom iteration hook with statements to be executed for the value of each distinct property.
  • while – Creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated before executing the statement.

Q6. Explain various control flow statements in JavaScript.

Block – A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.

break – Terminates the current loop, switch, or label statement and transfers program control to the statement following the terminated statement.

continue – Terminates execution of the statements in the current iteration of the current or labeled loop, and continues execution of the loop with the next iteration.

Empty – An empty statement is used to provide no statement, although the JavaScript syntax would expect one.

if...else – Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.

switch – Evaluates an expression, matching the expression’s value to a case clause, and executes statements associated with that case.

throw – Throws a user-defined exception.

try...catch – Marks a block of statements to try, and specifies a response, should an exception be thrown.

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