Web Totals

Question 81 - 90

#81. What is the output?

function sayHi(name) {
  return `Hi there, ${name}`;
}

console.log(sayHi());
javascript
  • A: Hi there,
  • B: Hi there, undefined
  • C: Hi there, null
  • D: ReferenceError
Đáp án

Answer: B

By default, arguments have the value of undefined, unless a value has been passed to the function. In this case, we didn't pass a value for the name argument. name is equal to undefined which gets logged.

In ES6, we can overwrite this default undefined value with default parameters. For example:

function sayHi(name = "Lydia") { ... }

In this case, if we didn't pass a value or if we passed undefined, name would always be equal to the string Lydia

#82. What is the output?
var status = '😎';

setTimeout(() => {
  const status = '😍';

  const data = {
    status: '🥑',
    getStatus() {
      return this.status;
    },
  };

  console.log(data.getStatus());
  console.log(data.getStatus.call(this));
}, 0);
javascript
  • A: "🥑" and "😍"
  • B: "🥑" and "😎"
  • C: "😍" and "😎"
  • D: "😎" and "😎"
Đáp án

Answer: B

The value of the this keyword is dependent on where you use it. In a method, like the getStatus method, the this keyword refers to the object that the method belongs to. The method belongs to the data object, so this refers to the data object. When we log this.status, the status property on the data object gets logged, which is "🥑".

With the call method, we can change the object to which the this keyword refers. In functions, the this keyword refers to the the object that the function belongs to. We declared the setTimeout function on the global object, so within the setTimeout function, the this keyword refers to the global object. On the global object, there is a variable called status with the value of "😎". When logging this.status, "😎" gets logged.

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

let city = person.city;
city = 'Amsterdam';

console.log(person);
javascript
  • A: { name: "Lydia", age: 21 }
  • B: { name: "Lydia", age: 21, city: "Amsterdam" }
  • C: { name: "Lydia", age: 21, city: undefined }
  • D: "Amsterdam"
Đáp án

Answer: A

We set the variable city equal to the value of the property called city on the person object. There is no property on this object called city, so the variable city has the value of undefined.

Note that we are not referencing the person object itself! We simply set the variable city equal to the current value of the city property on the person object.

Then, we set city equal to the string "Amsterdam". This doesn't change the person object: there is no reference to that object.

When logging the person object, the unmodified object gets returned.

#84. What is the output?
function checkAge(age) {
  if (age < 18) {
    const message = "Sorry, you're too young.";
  } else {
    const message = "Yay! You're old enough!";
  }

  return message;
}

console.log(checkAge(21));
javascript
  • A: "Sorry, you're too young."
  • B: "Yay! You're old enough!"
  • C: ReferenceError
  • D: undefined
Đáp án

Answer: C

Variables with the const and let keyword are block-scoped. A block is anything between curly brackets ({ }). In this case, the curly brackets of the if/else statements. You cannot reference a variable outside of the block it's declared in, a ReferenceError gets thrown.

#85. What kind of information would get logged?
fetch('https://www.website.com/api/user/1')
  .then((res) => res.json())
  .then((res) => console.log(res));
javascript
  • A: The result of the fetch method.
  • B: The result of the second invocation of the fetch method.
  • C: The result of the callback in the previous .then().
  • D: It would always be undefined.
Đáp án

Answer: C

The value of res in the second .then is equal to the returned value of the previous .then. You can keep chaining .thens like this, where the value is passed to the next handler.

#86. Which option is a way to set hasName equal to true, provided you cannot pass true as an argument?
function getName(name) {
  const hasName = //
}
javascript
  • A: !!name
  • B: name
  • C: new Boolean(name)
  • D: name.length
Đáp án

Answer: A

With !!name, we determine whether the value of name is truthy or falsy. If name is truthy, which we want to test for, !name returns false. !false (which is what !!name practically is) returns true.

By setting hasName equal to name, you set hasName equal to whatever value you passed to the getName function, not the boolean value true.

new Boolean(true) returns an object wrapper, not the boolean value itself.

name.length returns the length of the passed argument, not whether it's true.

#87. What's the output?
console.log('I want pizza'[0]);
javascript
  • A: """
  • B: "I"
  • C: SyntaxError
  • D: undefined
Đáp án

Answer: B

In order to get a character at a specific index of a string, you can use bracket notation. The first character in the string has index 0, and so on. In this case, we want to get the element with index 0, the character "I', which gets logged.

Note that this method is not supported in IE7 and below. In that case, use .charAt().

#88. What's the output?
function sum(num1, num2 = num1) {
  console.log(num1 + num2);
}

sum(10);
javascript
  • A: NaN
  • B: 20
  • C: ReferenceError
  • D: undefined
Đáp án

Answer: B

You can set a default parameter's value equal to another parameter of the function, as long as they've been defined before the default parameter. We pass the value 10 to the sum function. If the sum function only receives 1 argument, it means that the value for num2 is not passed, and the value of num1 is equal to the passed value 10 in this case. The default value of num2 is the value of num1, which is 10. num1 + num2 returns 20.

If you're trying to set a default parameter's value equal to a parameter which is defined after (to the right), the parameter's value hasn't been initialized yet, which will throw an error.

#89. What's the output?
// module.js
export default () => 'Hello world';
export const name = 'Lydia';

// index.js
import * as data from './module';

console.log(data);
javascript
  • A: { default: function default(), name: "Lydia" }
  • B: { default: function default() }
  • C: { default: "Hello world", name: "Lydia" }
  • D: Global object of module.js
Đáp án

Answer: A

With the import * as name syntax, we import all exports from the module.js file into the index.js file as a new object called data is created. In the module.js file, there are two exports: the default export, and a named export. The default export is a function which returns the string "Hello World", and the named export is a variable called name which has the value of the string "Lydia".

The data object has a default property for the default export, other properties have the names of the named exports and their corresponding values.

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

const member = new Person('John');
console.log(typeof member);
javascript
  • A: "class"
  • B: "function"
  • C: "object"
  • D: "string"
Đáp án

Answer: C

Classes are syntactical sugar for function constructors. The equivalent of the Person class as a function constructor would be:

function Person() {
  this.name = name;
}
javascript

Calling a function constructor with new results in the creation of an instance of Person, typeof keyword returns "object" for an instance. typeof member returns "object".