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# Question 101 - 110

### 101. What's the value of output?​

- A: `false` `null` `[]`
- B: `null` `""` `true`
- C: `{}` `""` `[]`
- D: `null` `null` `true`

Answer

#### Answer: C​

With the `||` operator, we can return the first truthy operand. If all values are falsy, the last operand gets returned.

`(false || {} || null)`: the empty object `{}` is a truthy value. This is the first (and only) truthy value, which gets returned. `one` is equal to `{}`.

`(null || false || "")`: all operands are falsy values. This means that the last operand, `""` gets returned. `two` is equal to `""`.

`([] || 0 || "")`: the empty array`[]` is a truthy value. This is the first truthy value, which gets returned. `three` is equal to `[]`.

### 102. What's the value of output?​

- A: `I have resolved!`, `second` and `I have resolved!`, `second`
- B: `second`, `I have resolved!` and `second`, `I have resolved!`
- C: `I have resolved!`, `second` and `second`, `I have resolved!`
- D: `second`, `I have resolved!` and `I have resolved!`, `second`

Answer

#### Answer: D​

With a promise, we basically say I want to execute this function, but I'll put it aside for now while it's running since this might take a while. Only when a certain value is resolved (or rejected), and when the call stack is empty, I want to use this value.

We can get this value with both `.then` and the `await` keyword in an `async` function. Although we can get a promise's value with both `.then` and `await`, they work a bit differently.

In the `firstFunction`, we (sort of) put the myPromise function aside while it was running, but continued running the other code, which is `console.log('second')` in this case. Then, the function resolved with the string `I have resolved`, which then got logged after it saw that the callstack was empty.

With the await keyword in `secondFunction`, we literally pause the execution of an async function until the value has been resolved before moving to the next line.

This means that it waited for the `myPromise` to resolve with the value `I have resolved`, and only once that happened, we moved to the next line: `second` got logged.

### 103. What's the value of output?​

- A: `3`, `NaN`, `NaN`
- B: `3`, `7`, `NaN`
- C: `3`, `Lydia2`, `[object Object]2`
- D: `"12"`, `Lydia2`, `[object Object]2`

Answer

#### Answer: C​

The `+` operator is not only used for adding numerical values, but we can also use it to concatenate strings. Whenever the JavaScript engine sees that one or more values are not a number, it coerces the number into a string.

The first one is `1`, which is a numerical value. `1 + 2` returns the number 3.

However, the second one is a string `"Lydia"`. `"Lydia"` is a string and `2` is a number: `2` gets coerced into a string. `"Lydia"` and `"2"` get concatenated, which results in the string `"Lydia2"`.

`{ name: "Lydia" }` is an object. Neither a number nor an object is a string, so it stringifies both. Whenever we stringify a regular object, it becomes `"[object Object]"`. `"[object Object]"` concatenated with `"2"` becomes `"[object Object]2"`.

### 104. What's its value?​

- A: `5`
- B: `Promise {<pending>: 5}`
- C: `Promise {<fulfilled>: 5}`
- D: `Error`

Answer

#### Answer: C​

We can pass any type of value we want to `Promise.resolve`, either a promise or a non-promise. The method itself returns a promise with the resolved value (`<fulfilled>`). If you pass a regular function, it'll be a resolved promise with a regular value. If you pass a promise, it'll be a resolved promise with the resolved value of that passed promise.

In this case, we just passed the numerical value `5`. It returns a resolved promise with the value `5`.

### 105. What's its value?​

- A: `Not the same!`
- B: `They are the same!`
- C: `ReferenceError`
- D: `SyntaxError`

Answer

#### Answer: B​

Objects are passed by reference. When we check objects for strict equality (`===`), we're comparing their references.

We set the default value for `person2` equal to the `person` object, and passed the `person` object as the value for `person1`.

This means that both values have a reference to the same spot in memory, thus they are equal.

The code block in the `else` statement gets run, and `They are the same!` gets logged.

### 106. What's its value?​

- A: `true`
- B: `false`
- C: `undefined`
- D: `TypeError`

Answer

#### Answer: D​

In JavaScript, we have two ways to access properties on an object: bracket notation, or dot notation. In this example, we use dot notation (`colorConfig.colors`) instead of bracket notation (`colorConfig["colors"]`).

With dot notation, JavaScript tries to find the property on the object with that exact name. In this example, JavaScript tries to find a property called `colors` on the `colorConfig` object. There is no property called `colors`, so this returns `undefined`. Then, we try to access the value of the first element by using `[1]`. We cannot do this on a value that's `undefined`, so it throws a `TypeError`: `Cannot read property '1' of undefined`.

JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket `[` and keeps going until it finds the closing bracket `]`. Only then, it will evaluate the statement. If we would've used `colorConfig[colors[1]]`, it would have returned the value of the `red` property on the `colorConfig` object.

### 107. What's its value?​

- A: `true`
- B: `false`

Answer

#### Answer: A​

Under the hood, emojis are unicodes. The unicodes for the heart emoji is `"U+2764 U+FE0F"`. These are always the same for the same emojis, so we're comparing two equal strings to each other, which returns true.

### 108. Which of these methods modifies the original array?​

- A: `All of them`
- B: `map` `reduce` `slice` `splice`
- C: `map` `slice` `splice`
- D: `splice`

Answer

#### Answer: D​

With `splice` method, we modify the original array by deleting, replacing or adding elements. In this case, we removed 2 items from index 1 (we removed `'🥑'` and `'😍'`) and added the ✨ emoji instead.

`map`, `filter` and `slice` return a new array, `find` returns an element, and `reduce` returns a reduced value.

### 109. What's the output?​

- A: `['🍕', '🍫', '🥑', '🍔']`
- B: `['🍝', '🍫', '🥑', '🍔']`
- C: `['🍝', '🍕', '🍫', '🥑', '🍔']`
- D: `ReferenceError`

Answer

#### Answer: A​

We set the value of the `favoriteFood` property on the `info` object equal to the string with the pizza emoji, `'🍕'`. A string is a primitive data type. In JavaScript, primitive data types don't interact by reference.

In JavaScript, primitive data types (everything that's not an object) interact by value. In this case, we set the value of the `favoriteFood` property on the `info` object equal to the value of the first element in the `food` array, the string with the pizza emoji in this case (`'🍕'`). A string is a primitive data type, and interact by value (see my blogpost if you're interested in learning more)

Then, we change the value of the `favoriteFood` property on the `info` object. The `food` array hasn't changed, since the value of `favoriteFood` was merely a copy of the value of the first element in the array, and doesn't have a reference to the same spot in memory as the element on `food[0]`. When we log food, it's still the original array, `['🍕', '🍫', '🥑', '🍔']`.

### 110. What does this method do?​

- A: Parses JSON to a JavaScript value
- B: Parses a JavaScript object to JSON
- C: Parses any JavaScript value to JSON
- D: Parses JSON to a JavaScript object only

Answer

#### Answer: A​

With the `JSON.parse()` method, we can parse JSON string to a JavaScript value.