Introduction:
JavaScript is a high-level programming language that is used to make web pages interactive and dynamic. It is the most popular language on the web, and is widely used for front-end and back-end web development. JavaScript was created in 1995 by Brendan Eich, and has since become one of the most important languages in modern web development. In this post, we will explore the history of JavaScript, and provide some interview questions with tabular differences that can help you better understand the language.
History of JavaScript:
JavaScript was created in 1995 by Brendan Eich, while he was working at Netscape Communications Corporation. Eich was tasked with creating a language that could be used to add interactivity to web pages. He created a prototype of the language in just 10 days, and it was first released as part of the Netscape Navigator 2.0 browser in September of that year. Originally, the language was called Mocha, but it was later renamed to LiveScript, and then finally to JavaScript.
JavaScript was quickly adopted by other web browsers, and became a de facto standard for client-side scripting on the web. In 1996, Microsoft created a rival language called JScript, which was designed to be compatible with JavaScript but had a few differences. The two languages were eventually standardized by the ECMA International organization, which created the ECMAScript standard.
The first version of ECMAScript was released in 1997, and it included the core features of the language, such as variables, functions, and loops. Over the years, new versions of ECMAScript have been released, each adding new features and improving the language. The most recent version of ECMAScript is ECMAScript 2022, which was released in June 2022.
Interview Questions with Tabular Differences:
To help you better understand JavaScript, we have compiled a list of interview questions with tabular differences. These questions cover a range of topics, from the basics of the language to more advanced concepts.
- What is the difference between var, let, and const in JavaScript?
var | let | const |
---|
Function-scoped | Block-scoped | Block-scoped |
Can be redeclared | Cannot be redeclared | Cannot be redeclared |
Can be reassigned | Can be reassigned | Cannot be reassigned |
- What is the difference between == and === in JavaScript?
== | === |
---|
Loose equality comparison | Strict equality comparison |
Does type coercion | Does not do type coercion |
"1" == 1 is true | "1" === 1 is false |
- What is the difference between a function declaration and a function expression in JavaScript?
Function declaration | Function expression |
---|
Can be called before it is defined | Must be defined before it is called |
Starts with the keyword function | Can start with any expression that evaluates to a function |
Has a name | Can be anonymous |
- What is the difference between synchronous and asynchronous code in JavaScript?
Synchronous code | Asynchronous code |
---|
Executes in a single thread | Executes in multiple threads |
Blocks the execution until it completes | Does not block the execution |
Easy to reason about | Difficult to reason about |
- What is the difference between callback, promise, and async/await in JavaScript?
Callback | Promise | Async/await |
---|
Uses a function as a parameter | Returns a promise object | Uses the async and await keywords |
Callback hell can occur | No callback hell | No callback hell |
Difficult to read and understand | Easier to read and understand than |
- What is the difference between null and undefined in JavaScript?
null | undefined |
---|
A value that represents the absence of any object value | A value that represents the absence of any value |
Can be explicitly set | Is the default value of uninitialized variables |
typeof null returns "object" | typeof undefined returns "undefined" |
- What is the difference between a for loop and a for...in loop in JavaScript?
for loop | for...in loop |
---|
Used to iterate over arrays and objects with known length | Used to iterate over object properties |
Uses a counter to keep track of the current index | Uses a variable to store the current property name |
Can be used with break and continue statements | Should not be used with break and continue statements |
- What is the difference between call() and apply() methods in JavaScript?
call() method | apply() method |
---|
Takes individual arguments as parameters | Takes an array of arguments as a parameter |
Used to invoke a function with a specified context | Used to invoke a function with a specified context |
function.call(context, arg1, arg2, ...) | function.apply(context, [arg1, arg2, ...]) |
- What is the difference between the spread operator and the rest parameter in JavaScript?
Spread operator | Rest parameter |
---|
Used to spread the elements of an array or object | Used to gather all remaining arguments into an array |
Used in function calls and array literals | Used in function parameters |
...[1, 2, 3] returns 1, 2, 3 | function(...args) {...} |
- What is the difference between a class and a constructor function in JavaScript?
Class | Constructor function |
---|
Introduced in ES6 | Pre-ES6 way of creating objects |
Can use class inheritance and static methods | Cannot use class inheritance or static methods |
Uses the keyword "class" to define a template | Uses a function and "new" keyword to create an object |
class Person {...} | function Person(...) {...} |
- What is the difference between a closure and a callback function in JavaScript?
Closure | Callback function |
---|
A function that has access to variables in its outer scope | A function that is passed as an argument to another function and is invoked when a specific event occurs |
Used for data hiding, encapsulation, and to create private variables | Used to handle asynchronous operations and events |
function outer() {
let x = 10;
function inner() {
console.log(x);
}
return inner;
} | function fetchData(url, callback) {
fetch(url).then(response => callback(response));
}
|
- What is the difference between synchronous and asynchronous code execution in JavaScript?
Synchronous code | Asynchronous code |
---|
Code that executes one line at a time, in the order it appears in the program | Code that executes out of order, based on the completion of a specific event or task |
Can cause the program to "block" if a long-running operation is performed | Does not block the program, allowing other operations to be performed concurrently |
function sync() {
console.log("start");
longRunningOperation();
console.log("end");
}
| function async() {
console.log("start");
setTimeout(() => {
console.log("timeout");
}, 1000);
console.log("end");
}
|
- What is the difference between an arrow function and a regular function in JavaScript?
Arrow function | Regular function |
---|
Introduced in ES6 | Pre-ES6 way of defining functions |
Does not have its own "this" context | Has its own "this" context |
Cannot be used as a constructor | Can be used as a constructor |
(x, y) => x + y | function add(x, y) {
return x + y;
}
|
- What is the difference between let and var in JavaScript?
let | var |
---|
Introduced in ES6 | Pre-ES6 way of declaring variables |
Has block scope | Has function scope |
Cannot be redeclared in the same scope | Can be redeclared in the same scope |
let x = 10; | var x = 10; |
- What is the difference between a promise and an observable in JavaScript?
Promise | Observable |
---|
Used to handle asynchronous operations | Used to handle streams of asynchronous data |
Can only return a single value or an error | Can return multiple values over time |
Promise.resolve(42).then(value => console.log(value)); | const observable = new Observable(observer => {
observer.next(1);
observer.next(2);
observer.next(3);
});
|
Conclusion:
By preparing for JavaScript job interviews with these interview questions with tabular differences, you can deepen your knowledge of the language and be better equipped to answer technical questions about its features and concepts. JavaScript is a powerful and widely used language that is constantly evolving
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