Quick Js Operations — Javascript

Interviewers sometimes throw in quick, unexpected JavaScript questions to gauge a candidate’s presence of mind and ability to think on their feet. These questions may seem straightforward in hindsight, but in the moment, they can catch us off guard — especially if we panic or overthink.

Since we’re diving into advanced JavaScript concepts and interview preparation, here’s a list of commonly asked quick JavaScript operation questions that can help you respond confidently when the pressure is on.

console.log(1 + 2 + 3);
// OutPut 6

• all operands are numbers, so they are added normally

console.log("1" + "2" + "3");
// Output 123

• All operands are strings, so the + operator concatenates them in sequence.

console.log(1 + 2 + "3");
// Output 33 

• 1 + 2 = 3 because they are treated as number and 3 + “3” concatenate because of string

console.log(1 + "2" + "2");
// Output 122

• All operands are strings, so the + operator concatenates them in sequence: "1" + "2" results in "12", and "12" + "2" results in "122".

console.log(1 + 2 + "3" + 4 + 5);
// Output 3345

• 1 + 2 is 3. Then 3 + "3" results in "33" (string concatenation starts here). After that, "33" + 4 and "334" + 5 continue as string concatenations.

console.log(false - 1);
// Output -1

• false is converted to 0, so 0 - 1 is -1.

console.log(true + 1);
// Output 2

• true is converted to 1, so 1 + 1 is 2.

console.log([1] + "abc");
// Output 1abc

• [1] is converted to the string '1' (by .toString()), and then concatenated with "abc".

console.log(1 + "2" + + "2");
// Output 122

• 1 is converted to '1', so '1' + "2" results in "12", then "12" + "2" results in "122".

console.log(1 + +"2" + "2");
// Output 32

• +"2" converts the string "2" to the number 2. So, 1 + 2 is 3, which is then concatenated with "2", resulting in "32".

console.log(1 + -"1" + "2");
// Output 02

• 1 is the number and — convert "1" to the number . So, 1–1 is 0, which is then concatenated with "2", resulting in "02".

console.log(1 + "1" + - "2"); 
//Output 11-2

• 1 is the number and "1" is string so they are concatenated to each other & , "2" is converted to a number because of the — operator , So the output is 11-2.

console.log("A" - "B" + "2");
// Output Nan2

• "A" - "B" results in NaN because subtraction between non-numeric strings is not defined. NaN + "2" treats NaN as a string, so it results in "NaN2".

console.log("A" - "B" + 2);
// Output Nan

• "A" - "B" results in NaN for the same reason as above. NaN + 2 still results in NaN because any arithmetic operation with NaN returns NaN.

const obj = { a: "one", b: "two", a: "three" };
console.log(obj); 
// Output b= two , a = "three

• When you create an object with duplicate keys, the last occurrence of the key overwrites any previous values.
• This behavior occurs because JavaScript objects do not support duplicate keys, so the last value assigned to a key is the one that persists.

const person  = { " Abdul Moiz" };
function sayHi(age){
return '$this.name) is $(age)';
}

console.log(sayHi.call(person , 23)); // Output Abdul Moiz is 23
console.log(sayHi.bind(person , 23)); // Bound Function

• sayHi.call(person, 23) calls the sayHi function immediately, setting this to person and passing 23 as age. The result is "Abdul Moiz is 23".
• sayHi.bind(person, 23) creates a new bound function with this set to person and age fixed at 23. It does not call the function immediately, so it returns a bound function instead.

var x = 1;
function foo() {
  var x = b = 10;
  return;
  function x() {}
}
foo();
console.log(b); // Output 10

• Inside foo, var x = b = 10; is interpreted as:
• var x = 10; (local variable within foo)
• b = 10; (creates a global variable b since it is not declared with var, let, or const)
• The function x() {} declaration inside foo is hoisted, but it doesn't affect x outside foo.
• After calling foo(), b is accessible globally with the value 10.

var info = {
  name: "Ahsan Mumtaz",
  getSecretIdentity: function () {
    return this.name;
  },
};

var stoleSecretIdentity = info.getSecretIdentity;

console.log(stoleSecretIdentity());  // undefined
console.log(info.getSecretIdentity()); // Ahsan Mumtaz

• info.getSecretIdentity() works as expected because this refers to the info object, returning "Ahsan Mumtaz".
• stoleSecretIdentity is a reference to info.getSecretIdentity but not bound to info. When called independently, this refers to the global scope (or undefined in strict mode), so this.name is undefined.

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