Web Totals

Question 91 - 100

#91. What's the output?

let newList = [1, 2, 3].push(4);

console.log(newList.push(5));
javascript
  • A: [1, 2, 3, 4, 5]
  • B: [1, 2, 3, 5]
  • C: [1, 2, 3, 4]
  • D: Error
Answer

Answer: D

The .push method returns the new length of the array, not the array itself! By setting newList equal to [1, 2, 3].push(4), we set newList equal to the new length of the array: 4.

Then, we try to use the .push method on newList. Since newList is the numerical value 4, we cannot use the .push method: a TypeError is thrown.

#92. What's the output?
function giveLydiaPizza() {
  return 'Here is pizza!';
}

const giveLydiaChocolate = () => "Here's chocolate... now go hit the gym already.";

console.log(giveLydiaPizza.prototype);
console.log(giveLydiaChocolate.prototype);
javascript
  • A: { constructor: ...} { constructor: ...}
  • B: {} { constructor: ...}
  • C: { constructor: ...} {}
  • D: { constructor: ...} undefined
Answer

Answer: D

Regular functions, such as the giveLydiaPizza function, have a prototype property, which is an object (prototype object) with a constructor property. Arrow functions however, such as the giveLydiaChocolate function, do not have this prototype property. undefined gets returned when trying to access the prototype property using giveLydiaChocolate.prototype.

#93. What's the output?
const person = {
  name: 'Lydia',
  age: 21,
};

for (const [x, y] of Object.entries(person)) {
  console.log(x, y);
}
javascript
  • A: name Lydia and age 21
  • B: ["name", "Lydia"] and ["age", 21]
  • C: ["name", "age"] and undefined
  • D: Error
Answer

Answer: A

Object.entries(person) returns an array of nested arrays, containing the keys and objects:

[ [ 'name', 'Lydia' ], [ 'age', 21 ] ]

Using the for-of loop, we can iterate over each element in the array, the subarrays in this case. We can destructure the subarrays instantly in the for-of loop, using const [x, y]. x is equal to the first element in the subarray, y is equal to the second element in the subarray.

The first subarray is [ "name", "Lydia" ], with x equal to "name", and y equal to "Lydia", which get logged. The second subarray is [ "age", 21 ], with x equal to "age", and y equal to 21, which get logged.

#94. What's the output?
function getItems(fruitList, ...args, favoriteFruit) {
  return [...fruitList, ...args, favoriteFruit]
}

getItems(["banana", "apple"], "pear", "orange")
javascript
  • A: ["banana", "apple", "pear", "orange"]
  • B: [["banana", "apple"], "pear", "orange"]
  • C: ["banana", "apple", ["pear"], "orange"]
  • D: SyntaxError
Answer

Answer: D

...args is a rest parameter. The rest parameter's value is an array containing all remaining arguments, and can only be the last parameter! In this example, the rest parameter was the second parameter. This is not possible, and will throw a syntax error.

function getItems(fruitList, favoriteFruit, ...args) {
  return [...fruitList, ...args, favoriteFruit];
}

getItems(['banana', 'apple'], 'pear', 'orange');
javascript

The above example works. This returns the array [ 'banana', 'apple', 'orange', 'pear' ]

#95. What's the output?
function nums(a, b) {
  if (a > b) console.log('a is bigger');
  else console.log('b is bigger');
  return;
  a + b;
}

console.log(nums(4, 2));
console.log(nums(1, 2));
javascript
  • A: a is bigger, 6 and b is bigger, 3
  • B: a is bigger, undefined and b is bigger, undefined
  • C: undefined and undefined
  • D: SyntaxError
Answer

Answer: B

In JavaScript, we don't have to write the semicolon (;) explicitly, however the JavaScript engine still adds them after statements. This is called Automatic Semicolon Insertion. A statement can for example be variables, or keywords like throw, return, break, etc.

Here, we wrote a return statement, and another value a + b on a new line. However, since it's a new line, the engine doesn't know that it's actually the value that we wanted to return. Instead, it automatically added a semicolon after return. You could see this as:

return;
a + b;
javascript

This means that a + b is never reached, since a function stops running after the return keyword. If no value gets returned, like here, the function returns undefined. Note that there is no automatic insertion after if/else statements!

#96. What's the output?
class Person {
  constructor() {
    this.name = 'Lydia';
  }
}

Person = class AnotherPerson {
  constructor() {
    this.name = 'Sarah';
  }
};

const member = new Person();
console.log(member.name);
javascript
  • A: "Lydia"
  • B: "Sarah"
  • C: Error: cannot redeclare Person
  • D: SyntaxError
Answer

Answer: B

We can set classes equal to other classes/function constructors. In this case, we set Person equal to AnotherPerson. The name on this constructor is Sarah, so the name property on the new Person instance member is "Sarah".

#97. What's the output?
const info = {
  [Symbol('a')]: 'b',
};

console.log(info);
console.log(Object.keys(info));
javascript
  • A: {Symbol('a'): 'b'} and ["{Symbol('a')"]
  • B: {} and []
  • C: { a: "b" } and ["a"]
  • D: {Symbol('a'): 'b'} and []
Answer

Answer: D

A Symbol is not enumerable. The Object.keys method returns all enumerable key properties on an object. The Symbol won't be visible, and an empty array is returned. When logging the entire object, all properties will be visible, even non-enumerable ones.

This is one of the many qualities of a symbol: besides representing an entirely unique value (which prevents accidental name collision on objects, for example when working with 2 libraries that want to add properties to the same object), you can also "hide" properties on objects this way (although not entirely. You can still access symbols using the Object.getOwnPropertySymbols() method).

#98. What's the output?
const getList = ([x, ...y]) => [x, y]
const getUser = user => { name: user.name, age: user.age }

const list = [1, 2, 3, 4]
const user = { name: "Lydia", age: 21 }

console.log(getList(list))
console.log(getUser(user))
javascript
  • A: [1, [2, 3, 4]] and SyntaxError
  • B: [1, [2, 3, 4]] and { name: "Lydia", age: 21 }
  • C: [1, 2, 3, 4] and { name: "Lydia", age: 21 }
  • D: Error and { name: "Lydia", age: 21 }
Answer

Answer: A

The getList function receives an array as its argument. Between the parentheses of the getList function, we destructure this array right away. You could see this as:

[x, ...y] = [1, 2, 3, 4]

With the rest parameter ...y, we put all "remaining" arguments in an array. The remaining arguments are 2, 3 and 4 in this case. The value of y is an array, containing all the rest parameters. The value of x is equal to 1 in this case, so when we log [x, y], [1, [2, 3, 4]] gets logged.

The getUser function receives an object. With arrow functions, we don't have to write curly brackets if we just return one value. However, if you want to instantly return an object from an arrow function, you have to write it between parentheses, otherwise everything between the two braces will be interpreted as a block statement. In this case the code between the braces is not a valid JavaScript code, so a SyntaxError gets thrown.

The following function would have returned an object:

const getUser = user => ({ name: user.name, age: user.age })

#99. What's the output?
const name = 'Lydia';

console.log(name());
javascript
  • A: SyntaxError
  • B: ReferenceError
  • C: TypeError
  • D: undefined
Answer

Answer: C

The variable name holds the value of a string, which is not a function, thus cannot invoke.

TypeErrors get thrown when a value is not of the expected type. JavaScript expected name to be a function since we're trying to invoke it. It was a string however, so a TypeError gets thrown: name is not a function!

SyntaxErrors get thrown when you've written something that isn't valid JavaScript, for example when you've written the word return as retrun. ReferenceErrors get thrown when JavaScript isn't able to find a reference to a value that you're trying to access.

#100. What's the value of output?
// 🎉✨ This is my 100th question! ✨🎉

const output = `${[] && 'Im'}possible!
You should${'' && `n't`} see a therapist after so much JavaScript lol`;
javascript
  • A: possible! You should see a therapist after so much JavaScript lol
  • B: Impossible! You should see a therapist after so much JavaScript lol
  • C: possible! You shouldn't see a therapist after so much JavaScript lol
  • D: Impossible! You shouldn't see a therapist after so much JavaScript lol
Answer

Answer: B

[] is a truthy value. With the && operator, the right-hand value will be returned if the left-hand value is a truthy value. In this case, the left-hand value [] is a truthy value, so "Im' gets returned.

"" is a falsy value. If the left-hand value is falsy, nothing gets returned. n't doesn't get returned.