Web Totals

Question 121 - 130

#121. What's the output?

const config = {
  languages: [],
  set language(lang) {
    return this.languages.push(lang);
  },
};

console.log(config.language);
javascript
  • A: function language(lang) { this.languages.push(lang }
  • B: 0
  • C: []
  • D: undefined
Đáp án

Answer: D

The language method is a setter. Setters don't hold an actual value, their purpose is to modify properties. When calling a setter method, undefined gets returned.

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

console.log(!typeof name === 'object');
console.log(!typeof name === 'string');
javascript
  • A: false true
  • B: true false
  • C: false false
  • D: true true
Đáp án

Answer: C

typeof name returns "string". The string "string" is a truthy value, so !typeof name returns the boolean value false. false === "object" and false === "string" both returnfalse.

(If we wanted to check whether the type was (un)equal to a certain type, we should've written !== instead of !typeof)

#123. What's the output?
const add = (x) => (y) => (z) => {
  console.log(x, y, z);
  return x + y + z;
};

add(4)(5)(6);
javascript
  • A: 4 5 6
  • B: 6 5 4
  • C: 4 function function
  • D: undefined undefined 6
Đáp án

Answer: A

The add function returns an arrow function, which returns an arrow function, which returns an arrow function (still with me?). The first function receives an argument x with the value of 4. We invoke the second function, which receives an argument y with the value 5. Then we invoke the third function, which receives an argument z with the value 6. When we're trying to access the value x, y and z within the last arrow function, the JS engine goes up the scope chain in order to find the values for x and y accordingly. This returns 4 5 6.

#124. What's the output?
async function* range(start, end) {
  for (let i = start; i <= end; i++) {
    yield Promise.resolve(i);
  }
}

(async () => {
  const gen = range(1, 3);
  for await (const item of gen) {
    console.log(item);
  }
})();
javascript
  • A: Promise {1} Promise {2} Promise {3}
  • B: Promise {<pending>} Promise {<pending>} Promise {<pending>}
  • C: 1 2 3
  • D: undefined undefined undefined
Đáp án

Answer: C

The generator function range returns an async object with promises for each item in the range we pass: Promise{1}, Promise{2}, Promise{3}. We set the variable gen equal to the async object, after which we loop over it using a for await ... of loop. We set the variable item equal to the returned Promise values: first Promise{1}, then Promise{2}, then Promise{3}. Since we're awaiting the value of item, the resolved promise, the resolved values of the promises get returned: 1, 2, then 3.

#125. What's the output?
const myFunc = ({ x, y, z }) => {
  console.log(x, y, z);
};

myFunc(1, 2, 3);
javascript
  • A: 1 2 3
  • B: {1: 1} {2: 2} {3: 3}
  • C: { 1: undefined } undefined undefined
  • D: undefined undefined undefined
Đáp án

Answer: D

myFunc expects an object with properties x, y and z as its argument. Since we're only passing three separate numeric values (1, 2, 3) instead of one object with properties x, y and z ({x: 1, y: 2, z: 3}), x, y and z have their default value of undefined.

#126. What's the output?
function getFine(speed, amount) {
  const formattedSpeed = new Intl.NumberFormat('en-US', {
    style: 'unit',
    unit: 'mile-per-hour',
  }).format(speed);

  const formattedAmount = new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency: 'USD',
  }).format(amount);

  return `The driver drove ${formattedSpeed} and has to pay ${formattedAmount}`;
}

console.log(getFine(130, 300));
javascript
  • A: The driver drove 130 and has to pay 300
  • B: The driver drove 130 mph and has to pay $300.00
  • C: The driver drove undefined and has to pay undefined
  • D: The driver drove 130.00 and has to pay 300.00
Đáp án

Answer: B

With the Intl.NumberFormat method, we can format numeric values to any locale. We format the numeric value 130 to the en-US locale as a unit in mile-per-hour, which results in 130 mph. The numeric value 300 to the en-US locale as a currency in USD results in $300.00.

#127. What's the output?
const spookyItems = ['👻', '🎃', '🕸'];
({ item: spookyItems[3] } = { item: '💀' });

console.log(spookyItems);
javascript
  • A: ["👻", "🎃", "🕸"]
  • B: ["👻", "🎃", "🕸", "💀"]
  • C: ["👻", "🎃", "🕸", { item: "💀" }]
  • D: ["👻", "🎃", "🕸", "[object Object]"]
Đáp án

Answer: B

By destructuring objects, we can unpack values from the right-hand object, and assign the unpacked value to the value of the same property name on the left-hand object. In this case, we're assigning the value "💀" to spookyItems[3]. This means that we're modifying the spookyItems array, we're adding the "💀" to it. When logging spookyItems, ["👻", "🎃", "🕸", "💀"] gets logged.

#128. What's the output?
const name = 'Lydia Hallie';
const age = 21;

console.log(Number.isNaN(name));
console.log(Number.isNaN(age));

console.log(isNaN(name));
console.log(isNaN(age));
javascript
  • A: true false true false
  • B: true false false false
  • C: false false true false
  • D: false true false true
Đáp án

Answer: C

With the Number.isNaN method, you can check if the value you pass is a numeric value and equal to NaN. name is not a numeric value, so Number.isNaN(name) returns false. age is a numeric value, but is not equal to NaN, so Number.isNaN(age) returns false.

With the isNaN method, you can check if the value you pass is not a number. name is not a number, so isNaN(name) returns true. age is a number, so isNaN(age) returns false.

#129. What's the output?
const randomValue = 21;

function getInfo() {
  console.log(typeof randomValue);
  const randomValue = 'Lydia Hallie';
}

getInfo();
javascript
  • A: "number"
  • B: "string"
  • C: undefined
  • D: ReferenceError
Đáp án

Answer: D

Variables declared with the const keyword are not referenceable before their initialization: this is called the temporal dead zone. In the getInfo function, the variable randomValue is scoped in the functional scope of getInfo. On the line where we want to log the value of typeof randomValue, the variable randomValue isn't initialized yet: a ReferenceError gets thrown! The engine didn't go down the scope chain since we declared the variable randomValue in the getInfo function.

#130. What's the output?
const myPromise = Promise.resolve('Woah some cool data');

(async () => {
  try {
    console.log(await myPromise);
  } catch {
    throw new Error(`Oops didn't work`);
  } finally {
    console.log('Oh finally!');
  }
})();
javascript
  • A: Woah some cool data
  • B: Oh finally!
  • C: Woah some cool data Oh finally!
  • D: Oops didn't work Oh finally!
Đáp án

Answer: C

In the try block, we're logging the awaited value of the myPromise variable: "Woah some cool data". Since no errors were thrown in the try block, the code in the catch block doesn't run. The code in the finally block always runs, "Oh finally!" gets logged.