Answer: A Arguments are passed by _value_, unless their value is an object, then they're passed by _reference_. `birthYear` is passed by value, since it's a string, not an object. When we pass arguments by value, a _copy_ of that value is created (see question 46). The variable `birthYear` has a reference to the value `"1997"`. The argument `year` also has a reference to the value `"1997"`, but it's not the same value as `birthYear` has a reference to. When we update the value of `year` by setting `year` equal to `"1998"`, we are only updating the value of `year`. `birthYear` is still equal to `"1997"`. The value of `person` is an object. The argument `member` has a (copied) reference to the _same_ object. When we modify a property of the object `member` has a reference to, the value of `person` will also be modified, since they both have a reference to the same object. `person`'s `name` property is now equal to the value `"Lydia"`

Answer: D With the `throw` statement, we can create custom errors. With this statement, you can throw exceptions. An exception can be a string, a number, a boolean or an object. In this case, our exception is the string `'Hello world!'`. With the `catch` statement, we can specify what to do if an exception is thrown in the `try` block. An exception is thrown: the string `'Hello world!'`. `e` is now equal to that string, which we log. This results in `'Oh an error: Hello world!'`.

Answer: B When you return a property, the value of the property is equal to the _returned_ value, not the value set in the constructor function. We return the string `"Maserati"`, so `myCar.make` is equal to `"Maserati"`.

Answer: A `let x = (y = 10);` is actually shorthand for: ```javascript y = 10; let x = y; ``` When we set `y` equal to `10`, we actually add a property `y` to the global object (`window` in browser, `global` in Node). In a browser, `window.y` is now equal to `10`. Then, we declare a variable `x` with the value of `y`, which is `10`. Variables declared with the `let` keyword are _block scoped_, they are only defined within the block they're declared in; the immediately invoked function expression (IIFE) in this case. When we use the `typeof` operator, the operand `x` is not defined: we are trying to access `x` outside of the block it's declared in. This means that `x` is not defined. Values who haven't been assigned a value or declared are of type `"undefined"`. `console.log(typeof x)` returns `"undefined"`. However, we created a global variable `y` when setting `y` equal to `10`. This value is accessible anywhere in our code. `y` is defined, and holds a value of type `"number"`. `console.log(typeof y)` returns `"number"`.

Answer: A We can delete properties from objects using the `delete` keyword, also on the prototype. By deleting a property on the prototype, it is not available anymore in the prototype chain. In this case, the `bark` function is not available anymore on the prototype after `delete Dog.prototype.bark`, yet we still try to access it. When we try to invoke something that is not a function, a `TypeError` is thrown. In this case `TypeError: pet.bark is not a function`, since `pet.bark` is `undefined`.

Answer: D The `Set` object is a collection of _unique_ values: a value can only occur once in a set. We passed the iterable `[1, 1, 2, 3, 4]` with a duplicate value `1`. Since we cannot have two of the same values in a set, one of them is removed. This results in `{1, 2, 3, 4}`.

Answer: C An imported module is _read-only_: you cannot modify the imported module. Only the module that exports them can change its value. When we try to increment the value of `myCounter`, it throws an error: `myCounter` is read-only and cannot be modified.

Answer: A The `delete` operator returns a boolean value: `true` on a successful deletion, else it'll return `false`. However, variables declared with the `var`, `const` or `let` keyword cannot be deleted using the `delete` operator. The `name` variable was declared with a `const` keyword, so its deletion is not successful: `false` is returned. When we set `age` equal to `21`, we actually added a property called `age` to the global object. You can successfully delete properties from objects this way, also the global object, so `delete age` returns `true`.

Answer: C We can unpack values from arrays or properties from objects through destructuring. For example: ```javascript [a, b] = [1, 2]; ``` The value of `a` is now `1`, and the value of `b` is now `2`. What we actually did in the question, is: ```javascript [y] = [1, 2, 3, 4, 5]; ``` This means that the value of `y` is equal to the first value in the array, which is the number `1`. When we log `y`, `1` is returned.

Answer: B It's possible to combine objects using the spread operator `...`. It lets you create copies of the key/value pairs of one object, and add them to another object. In this case, we create copies of the `user` object, and add them to the `admin` object. The `admin` object now contains the copied key/value pairs, which results in `{ admin: true, name: "Lydia", age: 21 }`.

Answer: B With the `defineProperty` method, we can add new properties to an object, or modify existing ones. When we add a property to an object using the `defineProperty` method, they are by default _not enumerable_. The `Object.keys` method returns all _enumerable_ property names from an object, in this case only `"name"`. Properties added using the `defineProperty` method are immutable by default. You can override this behavior using the `writable`, `configurable` and `enumerable` properties. This way, the `defineProperty` method gives you a lot more control over the properties you're adding to an object.

Answer: A The second argument of `JSON.stringify` is the _replacer_. The replacer can either be a function or an array, and lets you control what and how the values should be stringified. If the replacer is an _array_, only the property names included in the array will be added to the JSON string. In this case, only the properties with the names `"level"` and `"health"` are included, `"username"` is excluded. `data` is now equal to `"{"level":19, "health":90}"`. If the replacer is a _function_, this function gets called on every property in the object you're stringifying. The value returned from this function will be the value of the property when it's added to the JSON string. If the value is `undefined`, this property is excluded from the JSON string.

Answer: A The unary operator `++` _first returns_ the value of the operand, _then increments_ the value of the operand. The value of `num1` is `10`, since the `increaseNumber` function first returns the value of `num`, which is `10`, and only increments the value of `num` afterwards. `num2` is `10`, since we passed `num1` to the `increasePassedNumber`. `number` is equal to `10`(the value of `num1`. Again, the unary operator `++` _first returns_ the value of the operand, _then increments_ the value of the operand. The value of `number` is `10`, so `num2` is equal to `10`.

Answer: C In ES6, we can initialize parameters with a default value. The value of the parameter will be the default value, if no other value has been passed to the function, or if the value of the parameter is `"undefined"`. In this case, we spread the properties of the `value` object into a new object, so `x` has the default value of `{ number: 10 }`. The default argument is evaluated at _call time_! Every time we call the function, a _new_ object is created. We invoke the `multiply` function the first two times without passing a value: `x` has the default value of `{ number: 10 }`. We then log the multiplied value of that number, which is `20`. The third time we invoke multiply, we do pass an argument: the object called `value`. The `*=` operator is actually shorthand for `x.number = x.number * 2`: we modify the value of `x.number`, and log the multiplied value `20`. The fourth time, we pass the `value` object again. `x.number` was previously modified to `20`, so `x.number *= 2` logs `40`.

Answer: D The first argument that the `reduce` method receives is the _accumulator_, `x` in this case. The second argument is the _current value_, `y`. With the reduce method, we execute a callback function on every element in the array, which could ultimately result in one single value. In this example, we are not returning any values, we are simply logging the values of the accumulator and the current value. The value of the accumulator is equal to the previously returned value of the callback function. If you don't pass the optional `initialValue` argument to the `reduce` method, the accumulator is equal to the first element on the first call. On the first call, the accumulator (`x`) is `1`, and the current value (`y`) is `2`. We don't return from the callback function, we log the accumulator and current value: `1` and `2` get logged. If you don't return a value from a function, it returns `undefined`. On the next call, the accumulator is `undefined`, and the current value is `3`. `undefined` and `3` get logged. On the fourth call, we again don't return from the callback function. The accumulator is again `undefined`, and the current value is `4`. `undefined` and `4` get logged.

Answer: B In a derived class, you cannot access the `this` keyword before calling `super`. If you try to do that, it will throw a ReferenceError: 1 and 4 would throw a reference error. With the `super` keyword, we call that parent class's constructor with the given arguments. The parent's constructor receives the `name` argument, so we need to pass `name` to `super`. The `Labrador` class receives two arguments, `name` since it extends `Dog`, and `size` as an extra property on the `Labrador` class. They both need to be passed to the constructor function on `Labrador`, which is done correctly using constructor 2.

Answer: B With the `import` keyword, all imported modules are _pre-parsed_. This means that the imported modules get run _first_, the code in the file which imports the module gets executed _after_. This is a difference between `require()` in CommonJS and `import`! With `require()`, you can load dependencies on demand while the code is being run. If we would have used `require` instead of `import`, `running index.js`, `running sum.js`, `3` would have been logged to the console.

Answer: A Every Symbol is entirely unique. The purpose of the argument passed to the Symbol is to give the Symbol a description. The value of the Symbol is not dependent on the passed argument. As we test equality, we are creating two entirely new symbols: the first `Symbol('foo')`, and the second `Symbol('foo')`. These two values are unique and not equal to each other, `Symbol('foo') === Symbol('foo')` returns `false`.

Answer: C With the `padStart` method, we can add padding to the beginning of a string. The value passed to this method is the _total_ length of the string together with the padding. The string `"Lydia Hallie"` has a length of `12`. `name.padStart(13)` inserts 1 space at the start of the string, because 12 + 1 is 13. If the argument passed to the `padStart` method is smaller than the length of the array, no padding will be added.

Answer: A With the `+` operator, you can concatenate strings. In this case, we are concatenating the string `"🥑"` with the string `"💻"`, resulting in `"🥑💻"`.

Answer: C A generator function "pauses" its execution when it sees the `yield` keyword. First, we have to let the function yield the string "Do you love JavaScript?", which can be done by calling `game.next().value`. Every line is executed, until it finds the first `yield` keyword. There is a `yield` keyword on the first line within the function: the execution stops with the first yield! _This means that the variable `answer` is not defined yet!_ When we call `game.next("Yes").value`, the previous `yield` is replaced with the value of the parameters passed to the `next()` function, `"Yes"` in this case. The value of the variable `answer` is now equal to `"Yes"`. The condition of the if-statement returns `false`, and `JavaScript loves you back ❤️` gets logged.

Answer: C `String.raw` returns a string where the escapes (`\n`, `\v`, `\t` etc.) are ignored! Backslashes can be an issue since you could end up with something like: `` const path = `C:\Documents\Projects\table.html` `` Which would result in: `"C:DocumentsProjects able.html"` With `String.raw`, it would simply ignore the escape and print: `C:\Documents\Projects\table.html` In this case, the string is `Hello\nworld`, which gets logged.

Answer: C An async function always returns a promise. The `await` still has to wait for the promise to resolve: a pending promise gets returned when we call `getData()` in order to set `data` equal to it. If we wanted to get access to the resolved value `"I made it"`, we could have used the `.then()` method on `data`: `data.then(res => console.log(res))` This would've logged `"I made it!"`

Answer: B The `.push()` method returns the _length_ of the new array! Previously, the array contained one element (the string `"banana"`) and had a length of `1`. After adding the string `"apple"` to the array, the array contains two elements, and has a length of `2`. This gets returned from the `addToList` function. The `push` method modifies the original array. If you wanted to return the _array_ from the function rather than the _length of the array_, you should have returned `list` after pushing `item` to it.

Answer: B `Object.freeze` makes it impossible to add, remove, or modify properties of an object (unless the property's value is another object). When we create the variable `shape` and set it equal to the frozen object `box`, `shape` also refers to a frozen object. You can check whether an object is frozen by using `Object.isFrozen`. In this case, `Object.isFrozen(shape)` returns true, since the variable `shape` has a reference to a frozen object. Since `shape` is frozen, and since the value of `x` is not an object, we cannot modify the property `x`. `x` is still equal to `10`, and `{ x: 10, y: 20 }` gets logged.

Answer: D When we unpack the property `name` from the object on the right-hand side, we assign its value `"Lydia"` to a variable with the name `myName`. With `{ name: myName }`, we tell JavaScript that we want to create a new variable called `myName` with the value of the `name` property on the right-hand side. Since we try to log `name`, a variable that is not defined, a ReferenceError gets thrown.

Answer: A A pure function is a function that _always_ returns the same result, if the same arguments are passed. The `sum` function always returns the same result. If we pass `1` and `2`, it will _always_ return `3` without side effects. If we pass `5` and `10`, it will _always_ return `15`, and so on. This is the definition of a pure function.

Answer: C The `add` function is a _memoized_ function. With memoization, we can cache the results of a function in order to speed up its execution. In this case, we create a `cache` object that stores the previously returned values. If we call the `addFunction` function again with the same argument, it first checks whether it has already gotten that value in its cache. If that's the case, the caches value will be returned, which saves on execution time. Else, if it's not cached, it will calculate the value and store it afterwards. We call the `addFunction` function three times with the same value: on the first invocation, the value of the function when `num` is equal to `10` isn't cached yet. The condition of the if-statement `num in cache` returns `false`, and the else block gets executed: `Calculated! 20` gets logged, and the value of the result gets added to the cache object. `cache` now looks like `{ 10: 20 }`. The second time, the `cache` object contains the value that gets returned for `10`. The condition of the if-statement `num in cache` returns `true`, and `'From cache! 20'` gets logged. The third time, we pass `5 * 2` to the function which gets evaluated to `10`. The `cache` object contains the value that gets returned for `10`. The condition of the if-statement `num in cache` returns `true`, and `'From cache! 20'` gets logged.

Answer: A With a _for-in_ loop, we can iterate over **enumerable** properties. In an array, the enumerable properties are the "keys" of array elements, which are actually their indexes. You could see an array as: `{0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}` Where the keys are the enumerable properties. `0` `1` `2` `3` get logged. With a _for-of_ loop, we can iterate over **iterables**. An array is an iterable. When we iterate over the array, the variable "item" is equal to the element it's currently iterating over, `"☕"` `"💻"` `"🍷"` `"🍫"` get logged.

Answer: C Array elements can hold any value. Numbers, strings, objects, other arrays, null, boolean values, undefined, and other expressions such as dates, functions, and calculations. The element will be equal to the returned value. `1 + 2` returns `3`, `1 * 2` returns `2`, and `1 / 2` returns `0.5`.

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`

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.

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.

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.

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

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`.

Answer: B In order to get an character on a specific index in 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 which index is 0, the character `"I'`, which gets logged. Note that this method is not supported in IE7 and below. In that case, use `.charAt()`

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.

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.

Answer: C Classes are syntactical sugar for function constructors. The equivalent of the `Person` class as a function constructor would be: ```javascript function Person() { this.name = name; } ``` 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"`.

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.

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`.

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.

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. ```javascript function getItems(fruitList, favoriteFruit, ...args) { return [...fruitList, ...args, favoriteFruit]; } getItems(['banana', 'apple'], 'pear', 'orange'); ``` The above example works. This returns the array `[ 'banana', 'apple', 'orange', 'pear' ]`

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: ```javascript return; a + b; ``` 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!

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"`.

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).

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 return an _object_ from an arrow function, you have to write it between parentheses, otherwise no value gets returned! The following function would have returned an object: `const getUser = user => ({ name: user.name, age: user.age })` Since no value gets returned in this case, the function returns `undefined`.

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.

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.