Fundamental Concepts
1. What is JavaScript?
JavaScript is a high-level, dynamic, untyped, and interpreted programming language. It is a core technology of the World Wide Web, alongside HTML and CSS. JavaScript enables interactive web pages and is an essential part of web applications. It is an object-oriented language that supports first-class functions, allowing developers to create complex applications.
2. What are the data types in JavaScript?
JavaScript has several built-in data types:
- Primitive Data Types:
1. `String`: Represents a sequence of characters.
2. `Number`: Represents both integer and floating-point numbers.
3. `Boolean`: Represents a logical entity and can have two values: `true` and `false`.
4. `Undefined`: A variable that has been declared but has not yet been assigned a value.
5. `Null`: Represents the intentional absence of any object value.
6. `Symbol`: A unique and immutable primitive value, often used as object keys (introduced in ES6).
- Non-Primitive Data Types:
1. `Object`: A collection of properties and methods, including arrays and functions.
3. Explain the concept of hoisting in JavaScript.
Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase. This means that you can use variables and call functions before they are declared in the code. However, only the declarations are hoisted, not the initializations.
Example:
```javascript
console.log(myVar); // undefined
var myVar = 5;
console.log(myVar); // 5
```
In this example, the variable declaration is hoisted, but the assignment happens at the original line.
Advanced Topics
4. What is the difference between `let`, `const`, and `var`?
- `var`:
- Function-scoped or globally scoped.
- Can be re-declared and updated.
- Hoisted to the top of the function or global context.
- `let`:
- Block-scoped (limited to the block in which it is defined).
- Cannot be re-declared in the same scope but can be updated.
- `const`:
- Block-scoped.
- Cannot be re-declared or updated (constant reference).
- Must be initialized at the time of declaration.
Example:
```javascript
var x = 1;
let y = 2;
const z = 3;
x = 10; // valid
y = 20; // valid
// z = 30; // TypeError: Assignment to constant variable.
```
5. What are promises in JavaScript?
Promises are objects that represent the eventual completion (or failure) of an asynchronous operation and its resulting value. A promise can be in one of three states:
1. Pending: The initial state; neither fulfilled nor rejected.
2. Fulfilled: The operation completed successfully, resulting in a resolved value.
3. Rejected: The operation failed, resulting in a reason for the failure.
Example of a promise:
```javascript
let myPromise = new Promise((resolve, reject) => {
let success = true; // Simulating an operation
if (success) {
resolve("Operation succeeded!");
} else {
reject("Operation failed.");
}
});
myPromise
.then(result => console.log(result))
.catch(error => console.log(error));
```
6. Explain the concept of closures in JavaScript.
A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. Closures are created every time a function is defined, allowing for data encapsulation and private variables.
Example:
```javascript
function outerFunction() {
let outerVariable = "I am outside!";
return function innerFunction() {
console.log(outerVariable); // Accessing outerVariable
};
}
const innerFunc = outerFunction();
innerFunc(); // Outputs: I am outside!
```
Practical Coding Challenges
7. Write a function to reverse a string.
Here’s a simple function to reverse a string in JavaScript:
```javascript
function reverseString(str) {
return str.split('').reverse().join('');
}
console.log(reverseString("hello")); // Outputs: "olleh"
```
8. How do you remove duplicates from an array?
You can use the `Set` object to remove duplicates from an array:
```javascript
function removeDuplicates(arr) {
return [...new Set(arr)];
}
console.log(removeDuplicates([1, 2, 2, 3, 4, 4])); // Outputs: [1, 2, 3, 4]
```
9. How to find the largest number in an array?
You can use the `Math.max` function in combination with the spread operator:
```javascript
function findLargestNumber(arr) {
return Math.max(...arr);
}
console.log(findLargestNumber([1, 3, 5, 2, 4])); // Outputs: 5
```
10. Explain how `this` works in JavaScript.
The value of `this` in JavaScript is determined by how a function is called. Here are some key points to understand:
- In a method, `this` refers to the object the method is called on.
- In a function, `this` refers to the global object (or `undefined` in strict mode).
- In an event handler, `this` refers to the element that fired the event.
- In arrow functions, `this` retains the value from the enclosing lexical scope.
Example:
```javascript
const obj = {
value: 42,
method: function() {
console.log(this.value);
}
};
obj.method(); // Outputs: 42
const method = obj.method;
method(); // Outputs: undefined (or throws error in strict mode)
```
Conclusion
Mastering JavaScript interview questions and answers requires a solid understanding of both basic and advanced concepts of the language. Being well-prepared for common questions can significantly increase your chances of performing well in interviews. Practice coding challenges, familiarize yourself with JavaScript’s unique features, and keep updated with the latest language developments to ensure you stand out in the competitive job market.
Frequently Asked Questions
What is a closure in JavaScript?
A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. This allows for private variables and functions.
Explain the difference between '==' and '===' in JavaScript.
'==' is the equality operator that performs type coercion, meaning it converts the operands to the same type before comparing. '===' is the strict equality operator that checks both the value and the type without coercion.
What are Promises in JavaScript?
Promises are a way to handle asynchronous operations in JavaScript. They represent a value that may be available now, or in the future, or never. A Promise can be in one of three states: pending, fulfilled, or rejected.
Can you explain the concept of 'hoisting' in JavaScript?
Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope during the compile phase. This means that variables and function declarations can be referenced before they are declared in the code.
What is the 'this' keyword in JavaScript?
The 'this' keyword refers to the context in which a function is called. Its value can change depending on how the function is invoked, which can lead to confusion in different execution contexts.
What is event delegation in JavaScript?
Event delegation is a technique that involves setting a single event listener on a parent element instead of multiple listeners on child elements. This takes advantage of event bubbling and can improve performance and memory usage.
What are arrow functions and how do they differ from regular functions?
Arrow functions are a more concise syntax for writing function expressions in JavaScript. They do not have their own 'this' context, which means they inherit 'this' from the parent scope, making them useful for methods that need to maintain the context.
