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A list of questions based on uncommon or tricky concepts in Javascript.

1. What's the result?
Number(undefined)
Answer

NaN

2. What's the result?
Number("   123    ")
Number("12 3")
Answer

123, NaN (only whitespace from the start and end are removed)

3. What's the result?
Boolean(" ")
Boolean("0")
Answer

true, true

Only values that are intuitively “empty”, like 0, "", null, undefined, and NaN, become false.

4. What's the result?
typeof null
Answer

object

That’s an officially recognized error in typeof behavior, coming from the early days of JavaScript and kept for compatibility.

Definitely, null is not an object. It is a special value with a separate type of its own.

5. What's the result?
2 ** 2
Answer

2² = 4

6. What's the result?
2 + 2 + '1'
'1' + 2 + 2
Answer

4 + '1' >> '41'

'12' + 2 >> '122'

7. What's the result?
+true
+""
+"2" + +"3"
Answer

1, 0, 5

The plus + exists in two forms: the binary and the unary form.

The unary plus doesn’t do anything to numbers. But if the operand is not a number, the unary plus converts it into a number.

8. What's the result?
let a = 1;
let b = 2;

let c = 3 - (a = b + 1);  
// Good to understand how it works. Please don’t write the code like that.
Answer

0

All operators in JavaScript return a value. That’s obvious for + and -, but also true for =

The call x = value writes the value into x and then returns it.

9. What's the result?
let a = (1 + 2, 3 + 4);
Answer

a = 7

The comma operator allows us to evaluate several expressions, dividing them with a comma ,. Each of them is evaluated but only the result of the last one is returned.

Why do we need an operator that throws away everything except the last expression?

Sometimes, people use it in more complex constructs to put several actions in one line.

For example:

// three operations in one line
for (a = 1, b = 3, c = a * b; a < 10; a++) {
 ...
}
// doesn't improve code readability so we should think well before using this.
10. What's the result?
let y = "5";
let x = y++; // number or string 5?
Answer

number 5

JavaScript first coerces the string to a number then assigns the value to the variable x and then increments the value.

11. What's the result?
let a = NaN;
let b = NaN;

a === b;
a == b;
Answer

false, false

NaN is the only value in JavaScript that is not equal to itself. So we can check for a NaN value by checking if the value is equal to itself. According to IEEE standard NaN is not equal to NaN.

12. What's the result?
isNaN("this is a string not a NaN value");
Number.isNaN("this is a string not a NaN value");
Answer

true, false

isNaN tries to coerce the value into a Number before checking if it's a NaN value. This issue has been fixed in Number.isNaN().

13. What's the result?
let negativeZero = -0;

negativeZero.toString();
negativeZero === 0;
negativeZero < 0;
negativeZero > 0;

Object.is(negativeZero, -0)
Object.is(negativeZero, 0)
Answer

"0", true, false, false

When we try to use these operations we get some unexpected behaviour because language developers decided negative zero isn't needed.

true, false This was fixed in Object.is() method.

Use case of -0: To show direction when something is stationary

14. What's the result?
let string = 'orange';

function changeToApple(string) {
  string = 'apple';
}

changeToApple(string);

console.log(string);  // ??
Answer

orange

15. What's the result?
const promise = new Promise((resolve, reject) => {
  console.log(1);
  setTimeout(() => {
    console.log("timerStart");
    resolve("success");
    console.log("timerEnd");
  }, 0);
  console.log(2);
});
promise.then(console.log);
console.log(3);
Answer

1 2 3 timerStart timerEnd success

16. What's the result?
let fruit = prompt("Which fruit to buy?", "apple");

let bag = {
  [fruit]: 5,
};
Answer

That’s called computed properties. The name of the property is taken from the variable fruit.

17. What's the result?
let obj = {
  for: 1,
  let: 2,
  return: 3
};

console.log( obj.for + obj.let + obj.return );
Answer

It will print 6. There are no limitations on property names. They can be any strings or symbols. Other types are automatically converted to strings.

18. What's the result?
let obj = {
  test: undefined
};

if(obj.test){
  console.log("hi);
}
Answer

It won't log hi because the property exists but the value is undefined. So we can use the `in` operator to check if a property is defined of not.

if("test" in obj){ // returns true console.log("hi"); }

19. What's the result?
let codes = {
  "49": "Germany",
  "41": "Switzerland",
  "44": "Great Britain",
  // ..,
  "1": "USA"
};

for (let code in codes) {
  console.log(code);
}
Answer

1, 41, 44, 49 integer properties are sorted (UTF-16 code units order), others appear in creation order.

20. What's the result?
let a = {};
let b = a;

log( a == b );
log( a === b );
Answer

true, true Both refer to the same object.

21. What's does the User function return?
function User(name) {
  this.name = name;
  this.isAdmin = false;
}

let user = new User("Jack");
Answer

The value of this is returned implicitly.

22. What's the result?
function User() {

  if(new.target){
    console.log("Hi");
  }
}

User();
new User();
Answer

1. undefined, 2. Hi Inside a function, we can check whether it was called with new or without it, using a special new.target property.

23. What's the result?
function BigUser() {

  this.name = "John";

  return { name: "Godzilla" };
}

function SmallUser() {

  this.name = "John";

  return "Rick";
}

console.log( new BigUser().name );
console.log( new SmallUser().name );
Answer

  1. Godzilla, 2. Johm`

If return is called with an object, then the object is returned instead of this If return is called with a primitive, it’s ignored.

24. Is the constructor call valid?
function User(){
  this.name = "Admin"
}
let user = new User;
Answer

We can omit parentheses after new, if it has no arguments. Omitting parentheses here is not considered a “good style”, but the syntax is permitted by specification.

25. Modify the code of up, down and showStep to make the calls chainable?
let ladder = {
  step: 0,
  up() {
    this.step++;
  },
  down() {
    this.step--;
  },
  showStep: function() {
    alert( this.step );
  }
};

// ladder.up().up().down().showStep(); // 1
Answer

We can return `this` in every method ```javascript let ladder = { step: 0, up() { this.step++; return this; }, down() { this.step--; return this; }, showStep() { alert( this.step ); return this; } };```

26. Is it possible to create functions A and B so that new A() == new B()??
function A() { ... }
function B() { ... }

let a = new A;
let b = new B;

alert( a == b ); // true
Answer

Yes, it’s possible.

If a function returns an object then new returns it instead of this.

So they can, for instance, return the same externally defined object obj

  let obj = {};

  function A() { return obj; }
  function B() { return obj; }

  alert( new A() == new B() ); // true
27. What's the result?
let user = {
  address: null
};

log( user?.name?.first );
log( user?.address?.street )
Answer

1. undefined 2. undefined

The optional chaining ?. stops the evaluation if the value before ?. is undefined or null and returns undefined

28. What's the result?
log(user?.address);

let userAdmin = {
  admin() {
    alert("I am admin");
  }
};

let userGuest = {};

log(userAdmin.admin?.());

log(userGuest.admin?.());
Answer

1. ReferenceError: user is not defined. If there’s no variable user at all, then user?.anything triggers an error

  1. I am admin 3. nothing happens

The optional chaining ?. is not an operator, but a special syntax construct, that also works with functions and square brackets. ?.(), ?.[]

29. Are the two statements valid?
delete user?.name;
user?.name = "John";
Answer

1. valid 2. invalid

We can use ?. for safe reading and deleting, but not writing

30. What's is valid in the below code?
 let numbers = {
    0: 0
 }
 
 numbers.1 = 1;
Answer

the first part is valid the number 0 will automatically be converted to string. By specification, object property keys may be either of string type, or of symbol type.

the numbers.1 = 1; is invalid

31. What's the result?
 let numbers = {
    0: 0
 }
 
log(numbers."0");
log(numbers[0]);
Answer

1. error 2. returns 0

32. What's the result?
alert( 1 || 0 );
alert( null || 1 );
alert( null || 0 || 1 );
alert( undefined || null || 0 )
Answer

1, 1, 1, 0

|| returns the first truthy value (without any conversion) or the last one if no truthy value is found

33. What's the result?
alert( 1 && 0 );
alert( 1 && 5 );
alert( null && 5 );
alert( 0 && "no matter what" );
Answer

0, 5, null, 0

AND returns the first falsy value(without any conversion) or the last value if none were found

34. What's the result?
alert( !!"non-empty string" );
alert( !!null );
Answer

true, false

double NOT !! is sometimes used for converting a value to boolean type. The first NOT converts the value to boolean and returns the inverse, and the second NOT inverses it again. In the end, we have a plain value-to-boolean conversion.

35. What's the result?
NaN ** 0
Answer

1

36. What's the result?
let n = 2;
n *= 3 + 5
Answer

16

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A list of questions based on uncommon or tricky concepts in Javascript.

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