Most asked Javascript Interview Questions

JavaScript, one of the most well-liked computer languages, is widely utilized in the creation of websites. Whether you're a novice or a seasoned developer, you'll probably run into a few questions about JavaScript that are frequently asked. We'll address the top 5 questions about JavaScript in this blog

1. What are JavaScript promises?
   Ans - Programming constructs like JavaScript Promises make it easier and more logical for programmers to create asynchronous code. They give you a mechanism to handle asynchronous tasks like reading a file from a disk or getting data from a server without disrupting the main thread of processing.

   Promises stand for a value that might not be accessible right away but will be in the future. One of three states can contain them.

   Fulfilled: The promise has been resolved successfully and has a value associated with it.

   Pending: The initial state, when the promise is neither fulfilled nor rejected.

   Rejected: The promise has been rejected and has a reason for the rejection.

const myPromise = new Promise((resolve, reject) => {
  // Do some asynchronous operation here
  // If operation is successful, call resolve with a value
  // If operation fails, call reject with a reason
});

myPromise.then((value) => {
  // Do something with the value
}).catch((reason) => {
  // Handle the rejection reason
});

1. What is the difference between == and === in JavaScript?

   Ans - The double equals (==) and triple equals (===) in JavaScript are used to check for equality, however, they behave a little differently.

   In order to compare two operands, the double equals (==) operator attempts to convert both operands to the same type first. JavaScript will make an effort to change one or both of the operands to a compatible type if the two operands are of different types.
   For example, if one operand is a string and the other is a number, JavaScript will attempt to convert the string to a number before comparing them. This can sometimes lead to unexpected results, as the conversion process may not be what you expect.

   On the other hand, the triple equals (===) is a strict equality operator,
   which indicates that it evaluates the operands' value and type. The outcome is always false if the types of the two operands differ. Because of this, the triple equals operator is less prone to mistakes and is more predictable than the double equals operator.

    console.log(1 == '1'); // true (loose equality, It doesn't check type)
    console.log(1 === '1'); // false (strict equality, different types)
   
    console.log(null == undefined); // true (loose equality, It doesn't check type)
    console.log(null === undefined); // false (strict equality, different types)
   

2. What is a callback function in JavaScript?

   Ans: In JavaScript, a callback function is a function that is passed as an argument to another function, and is intended to be called or executed after some specific event or condition occurs.
   In asynchronous programming, callbacks are frequently used when a function needs to wait for an operation to finish before continuing.

    function addNumbers(a, b, callback) {
      var result = a + b;
      callback(result);
    }
   
    function printResult(result) {
      console.log('The result is: ' + result);
    }
   
    // Call the addNumbers function with a callback function
    addNumbers(2, 3, printResult); // Output: "The result is: 5"
   

   In this example, the addNumbers function takes three arguments: a, b, and callback. The function adds a and b together and stores the result in a variable called result. The callback function is then called with result as its argument.

   The printResult function is a callback function that simply logs the result to the console.

   When we call the addNumbers function with the arguments 2, 3, and printResult, the addNumbers function adds 2 and 3 together to get 5, and then calls the printResult function with 5 as its argument. The printResult function then logs "The result is: 5" to the console.

3. What is hoisting?

   Ans: In JavaScript, hoisting is a behavior where variable and function declarations are moved to the top of their respective scopes during the compilation phase before the code is actually executed.

   This means that regardless of where the variable or function is declared in the code, it can be used anywhere in that scope (i.e., within the same function or global scope).

   However, it's important to note that only the declaration of the variable or function is hoisted, not the initialization or assignment. If a variable is declared but not assigned a value, its value will be undefined until it is assigned a value later in the code.

    function example() {
      console.log(myVar); // undefined (declaration is hoisted, but not assignment)
      var myVar = 'Hello, world!';
      console.log(myVar); // 'Hello, world!' (variable has been assigned a value)
    }
   
    example();
   

   In this example, the example function declares a variable called myVar using the var keyword. However, when the function is called and the first console.log statement is executed, the value of myVar is undefined, because the assignment has not yet happened. The second console.log statement logs the actual value of myVar after it has been assigned.

   Hoisting can be a useful feature, as it allows us to use variables and functions before they are declared. However, it can also lead to unexpected behavior and bugs if we're not careful. It's generally a good practice to declare variables and functions at the beginning of their respective scopes, to make the code more readable and avoid any potential hoisting-related issues.

   1. what is memoization in javascript?

      Ans: Memoization is a technique in computer science and programming where the result of a function call is cached so that subsequent calls with the same inputs can be returned quickly from the cache instead of recomputing the result.

      In JavaScript, memoization can be implemented using closures and higher-order functions.

      This is going to be difficult for few people to understand, So please go through the code and explanation under the code

       function memoize(func) {
         var cache = {};
         return function() {
           var args = JSON.stringify(arguments);
           if (cache[args]) {
             return cache[args];
           } else {
             var result = func.apply(this, arguments);
             cache[args] = result;
             return result;
           }
         };
       }
      
       // Example function to be memoized
       function fibonacci(n) {
         if (n <= 1) {
           return n;
         }
         return fibonacci(n - 1) + fibonacci(n - 2);
       }
      
       // Memoize the fibonacci function
       var memoizedFibonacci = memoize(fibonacci);
      
       // Call the memoized function with the same input multiple times
       console.log(memoizedFibonacci(10)); // Output: 55
       console.log(memoizedFibonacci(10)); // Output: 55 (returned from cache)
      

      In this example, we define a memoize function that takes another function func as its argument. The memoize function returns a new function that wraps the original func function.

      When the memoized function is called with some arguments, the memoize function first checks if the result for those arguments is already cached. If it is, it returns the cached result. If not, it calls the original function with those arguments, caches the result, and returns the result.

      In the example, we use the memoize function to cache the results of the fibonacci function, which calculates the Fibonacci sequence. We call the memoized function with the same input multiple times, and the second call returns the result from the cache instead of recomputing it.

      Memoization can be a powerful optimization technique, especially when working with complex functions or when making expensive calculations. However, it should be used with caution, as caching results can increase memory usage and may not always be appropriate for all situations.

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