ananyaneogi / javascript-interview-questions

Common JavaScript Interview Questions and Answers

Common JavaScript Interview Questions and Answers

Q1. Write a method sum that would work with the following syntax?

console.log(sum(2,3));   // Outputs 5
console.log(sum(2)(3));  // Outputs 5

Solution

function sum(x) {
  if (arguments.length == 2) {
    return arguments[0] + arguments[1];
  } else {
    return function(y) { return x + y; };
  }
}

---

Q2. What gets logged in the following code?

(function() {
    console.log(1); 
    setTimeout(function(){console.log(2)}, 1000); 
    setTimeout(function(){console.log(3)}, 0); 
    console.log(4);
})();

Solution

1
4
3
2

---

Q3. What will be the output? How can we fix this?

for (var i = 0; i < 5; i++) {
	setTimeout(function() { console.log(i); }, i * 1000 );
}

Solution

Output:

5
5
5
5
5

The fix:

1. Using let

for (let i = 0; i < 5; i++) {
	setTimeout(function() { console.log(i); }, i * 1000 );
}

2. Using closures:

for (let i = 0; i < 5; i++) {
	setTimeout(function() { console.log(i); }, i * 1000 );
}

---

Q4. What is the output?

var x = 21;
var foo = function () {
    console.log(x);
    var x = 20;
};
foo();

Solution

undefined

---

Q5: What will be the output?

console.log(1 < 2 < 3);
console.log(3 > 2 > 1);

Solution

true
false

---

Q6: What will be the output?

var b = 1;
function outer(){
   	var b = 2
    function inner(){
        b++;
        var b = 3;
        console.log(b)
    }
    inner();
}
outer();

Solution

3

---

Q7. Memoize a function.

Solution

function memoize(fn) {
  const cache = {}
  return function() {
    const args = JSON.stringify(arguments);
    if (cache[args]) {
      return cache[args]
    }
    const evaluatedValue = fn.apply(this, arguments);
    cache[args] = evaluatedValue;
    return evaluatedValue;
  }
}

// example use
function factorial(n) {
   if(n === 0 || n === 1) {
     return 1
   }
   return factorial(n-1) * n; 
}
const memoizedFactorial = memoize(factorial)
factorial(1000) // slow
memoizedFactorial(1000) // faster