Answer: B
All objects have prototypes, except for the base object. The base object is the object created by the user, or an object that is created using the new keyword. The base object has access to some methods and properties, such as .toString. This is the reason why you can use built-in JavaScript methods! All of such methods are available on the prototype. Although JavaScript can't find it directly on your object, it goes down the prototype chain and finds it there, which makes it accessible for you.

Object can created without prototype using Object.create(null, {}) with passing null args

18. What's the output?

function checkAge(data) {
if (data === { age: 18 }) {
console.log('You are an adult!');
} else if (data == { age: 18 }) {
console.log('You are still an adult.');
} else {
console.log(Hmm.. You don't have an age I guess);
}
}

checkAge({ age: 18 });

A: You are an adult!
B: You are still an adult.
C: Hmm.. You don't have an age I guess

Answer: C
When testing equality, primitives are compared by their value, while objects are compared by their reference. JavaScript checks if the objects have a reference to the same location in memory.
The two objects that we are comparing don't have that: the object we passed as a parameter refers to a different location in memory than the object we used in order to check equality.
This is why both { age: 18 } === { age: 18 } and { age: 18 } == { age: 18 } return false.

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Answer: C
The rest parameter (...args) lets us "collect" all remaining arguments into an array. An array is an object, so typeof args returns "object"

Answer: C
With "use strict", you can make sure that you don't accidentally declare global variables. We never declared the variable age, and since we use "use strict", it will throw a reference error. If we didn't use "use strict", it would have worked, since the property age would have gotten added to the global object.

Answer: A
eval evaluates codes that's passed as a string. If it's an expression, like in this case, it evaluates the expression. The expression is 10 * 10 + 5. This returns the number 105.

Answer: B
The data stored in sessionStorage is removed after closing the tab.
If you used localStorage, the data would've been there forever, unless for example localStorage.clear() is invoked.

What's the output?

const obj = { 1: 'a', 2: 'b', 3: 'c' };
const set = new Set([1, 2, 3, 4, 5]); obj.hasOwnProperty('1'); obj.hasOwnProperty(1); set.has('1');
set.has(1);

A: false true false true
B: false true true true
C: true true false true
D: true true true true

Answer: C

All object keys (excluding Symbols) are strings under the hood, even if you don't type it yourself as a string. This is why obj.hasOwnProperty('1') also returns true.
It doesn't work that way for a set. There is no '1' in our set: set.has('1') returns false. It has the numeric type 1, set.has(1) returns true.

Answer: C
If you have two keys with the same name, the key will be replaced. It will still be in its first position, but with the last specified value.