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In-JavaScript-we-trust

Tài liệu luyện tập kiến thức trắc nghiệm về Javascript

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Thông tin

Đây là một Github repository về các thử thách/câu đố/coding dành cho lập trình viên yêu thích JavaScript.

Các bạn có thể truy cập vào Github In-JavaScript-we-trust để xem đầy đủ hơn.

Hãy tặng họ 1 ⭐️ nếu bạn yêu thích repo này !

1. What's the output?

function a(x) {
  x++;
  return function () {
    console.log(++x);
  };
}

a(1)();
a(1)();
a(1)();

let x = a(1);
x();
x();
x();
  • A: 1, 2, 3 and 1, 2, 3
  • B: 3, 3, 3 and 3, 4, 5
  • C: 3, 3, 3 and 1, 2, 3
  • D: 1, 2, 3 and 3, 3, 3
Answer

Answer: B

This question revisits closure - one of the most confusing concepts in JavaScript. Closure allows us to create a stateful function and such a function can access to the variable outside of its scope. In a nutshell, a closure can have access to the global variable (scope), father function scope and its own scope.

We have here, the only one correct answer, 3, 3, 3 and 3, 4, 5 because first we simply call the function a(). It works like a normal function and we have not seen anything so-called stateful yet. In the following code, we declare a variable x and it stores the value of function a(1), that is why we get 3. 4. 5 rather than 3, 3, 3.

This kind of gotcha gives me the feeling of static variable in PHP world.

2. What's the output?

function Name(a, b) {
  this.a = a;
  this.b = b;
}

const me = Name('Vuong', 'Nguyen');

console.log(!(a.length - window.a.length));
  • A: undefined
  • B: NaN
  • C: true
  • D: false
Answer

Answer: C

We get true in the console. The tricky part is when we create an object from the constructor function Name but we DO NOT USE new keywork. That makes the variable a global one and get the value "Vuong". Remember that it is actually a property of the global object window (in the browser) or global in the nodejs.

We then get a.length ~ 5 and window.a.length ~ 5 which return 0. !0 returns true.

Imagine what would happen when we create the instance me with the new keywork. That is an interesting inquire!

3. What's the output?

const x = function (...x) {
  let k = (typeof x).length;
  let y = () => 'freetut'.length;
  let z = { y: y };

  return k - z.y();
};

console.log(Boolean(x()));
  • A: true
  • B: 1
  • C: -1
  • D: false
Answer

Answer: A

The spread operator ...x might help us obtain the parameter in the function in the form of array. Yet, in Javascript the typeof array return "object" rather than "array". It is totally odd if you are coming from PHP.

That is said, we now have the length of the string object which returns 6. z.y() simply returns the length of the string 'freetut' (7).

Be aware that the function x() (in the form of function express or anonymous function (if you are coming from PHP) return -1 when being called and when converted to bool with Boolean(-1) return true instead of false. Noted that Boolean(0) return false.

4. What's the output?

(function js(x) {
  const y = (j) => j * x;

  console.log(y(s()));

  function s() {
    return j();
  }

  function j() {
    return x ** x;
  }
})(3);
  • A: undefined
  • B: 18
  • C: 81
  • D: 12
Answer

Answer: C

The function js() can be automatically executed without calling it and known as IIFE (Immediately Invoked Function Expression). Noted the parameter x of the function js is actuallly passed with the value 3.

The value return of the function is y(s())), meaning calling three other functions y(), s() and j() because the function s() returns j().

j() returns 3^3 = 27 so that s() returns 27.

y(s()) means y(27) which returns 27*3 = 81.

Note that we can call declare function BEFORE the function is actually declared but not with expression function.

5. What's the output?

var tip = 100;

(function () {
  console.log('I have $' + husband());

  function wife() {
    return tip * 2;
  }

  function husband() {
    return wife() / 2;
  }

  var tip = 10;
})();
  • A: "I have $10";
  • B: "I have $100";
  • C: "I have $50";
  • D: "I have $NaN";
Answer

Answer: D

We have here an IIFE (Immediately Invoked Function Expression). It means we do not have to call it but it will be excuted automatically when declared. The flow is as: husband() returns wife()/2 and wife() returns tip*2.

We might think that tip = 100 because it is a global variable when declaring with var keyword. However, it is actually undefined because we also have var tip = 10 INSIDE the function. As the variable tip is hoisted with default value undefined, the final result would be D. We know that undefined returns NaN when we try to divide to 2 or multiple with 2.

If we do not re-declare var tip = 10; at the end of the function, we will definately get B.

JS is fun, right?

6. What's the output?

const js = { language: 'loosely type', label: 'difficult' };

const edu = { ...js, level: 'PhD' };

const newbie = edu;

delete edu.language;

console.log(Object.keys(newbie).length);
  • A: 2;
  • B: 3;
  • C: 4;
  • D: 5;
Answer

Answer: A

This challenge revises the ES6's feature regarding spread operator ... Spread operator is quite useful for retrieving parameter in function, to unite or combine object and array in JavaScript. PHP also has this feature.

In the variable edu, we use ...js (spread operator here) to combine both objects into one. It works in the same way with array.

Then we declare another variable named newbie. IMPORTANT note: By declaring the variable like that, both variables point to the SAME POSITION in the memory. We may have known something like $a = &$b in PHP, which let both varibles work in the same way. We might have known about pass by reference in the case.

Then we have 2 as edu.language is deleted. Both objects now have only two elements.

Now is time to think about coping an object in JS either shallow or deep one.

7. What's the output?

var candidate = {
  name: 'Vuong',
  age: 30,
};

var job = {
  frontend: 'Vuejs or Reactjs',
  backend: 'PHP and Laravel',
  city: 'Auckland',
};

class Combine {
  static get() {
    return Object.assign(candidate, job);
  }

  static count() {
    return Object.keys(this.get()).length;
  }
}

console.log(Combine.count());
  • A: 5;
  • B: 6;
  • C: 7;
  • D: 8;
Answer

Answer: A

The buit-in method Object.assign(candidate, job) merges the two objects candidate and job into one object. Then the method Object.keys counts the number of key in the object.

Note that two methods get() and count() are defined as static, so they need to be called statically using Class.staticmethod() syntax. Then the final object get 5 elements.

8. What's the output?

var x = 1;

(() => {
  x += 1;
  ++x;
})();
((y) => {
  x += y;
  x = x % y;
})(2);
(() => (x += x))();
(() => (x *= x))();

console.log(x);
  • A: 4;
  • B: 50;
  • C: 2;
  • D: 10;
Answer

Answer: A

Initially x is declared with the value 1. In the first IIFE function, there are two operations. First x becomes 2 and then 3.

In the second IIFE function, x = x + y then the current value is 5. In the second operation, it returns only 1 as it undergoes 5%2.

In the third and fouth IIFE functions, we get 2 x = x + x and then 4 x = x * x. It is more than simple.

9. What's the output?

$var = 10;

$f = function($let) use ($var) {
    return ++$let + $var;
};

$var = 15;
echo $f(10);
var x = 10;

const f = (l) => ++l + x;

x = 15;
console.log(f(10));
  • A: 26 and 26;
  • B: 21 and 21;
  • C: 21 and 26;
  • D: 26 and 21;
Answer

Answer: C

This question illustrates the diffences between PHP and JavaScript when handling closure. In the first snippet, we declare a closure with the keyword use. Closure in PHP is simply an anonymous function and the data is passed to the function using the keyword use. Otherwise, it is called as lambda when we do not use the keyword use. You can check the result of the snippet here https://3v4l.org/PSeMY. PHP closure only accepts the value of the variable BEFORE the closure is defined, no matter where it is called. As such, $var is 10 rather than 15.

On the contrary, JavaScript treats the variable a bit different when it is passed to anonymous function. We do not have to use the keyword use here to pass variable to the closure. The variable x in the second snippet is updated before the closure is called, then we get 26.

Note that in PHP 7.4, we have arrow function and we then do not have to use the keyword use to pass the variable to function. Another way to call a global ariable inside a function in PHP is to use the keyword global or employ the built-in GLOBAL variable $GLOBALS.

10. What's the output?

let x = {};
let y = {};
let z = x;

console.log(x == y);
console.log(x === y);
console.log(x == z);
console.log(x === z);
  • A: true true true true;
  • B: false false false false;
  • C: true true false false;
  • D: false false true true;
Answer

Answer: D

Technically, x and y have the same value. Both are empty objects. However, we do not use the value to compare objects.

z is x are two objects referring to the same memory position. In JavaScript, array and object are passed by reference. x and z therefore return true when being compared.

11. What's the output?

console.log('hello');

setTimeout(() => console.log('world'), 0);

console.log('hi');
  • A: "hello" -> "world" -> "hi"
  • B: "hello" -> "hi" -> "world"
  • C: "hi" -> "world" -> "hello"
  • D: "hi" -> "hello" -> "world"
Answer

Answer: B

Given that the function setTimeout() will be kept in the task queue before jumping back to stack, "hello" and "hi" will be printed first, then A is incorrect. That is also the case of the answers C and D.

No matter how many seconds you set to the setTimeout() function, it will run after synchronous code. So we will get "hello" first as it is put into the call stack first. Though the setTimeout() is then being put into the call stack, it will subsequently offload to web API (or Node API) and then being called when other synchronous codes are cleared. It means we then get "hi" and finally "world".

So B is the correct answer.

Credit: @kaitoubg (voz) for your suggestion regarding the timeout throttled by which I have decided to alter the question slightly. It will ensure that readers will not get confused as the previous code might bring out different results when tested on other browsers or environments. The main point of the question is about the discrepancy between the synchronous code and asynchronous code when using setTimeout..

12. What's the output?

String.prototype.lengthy = () => {
  console.log('hello');
};

let x = { name: 'Vuong' };

delete x;

x.name.lengthy();
  • A: "Vuong";
  • B: "hello";
  • C: "undefined"
  • D: "ReferenceError"
Answer

Answer: B

String.prototype.someThing = function () {} is the common way to define a new built-in method for String. We can do the same thing with Array, Object or FunctionName where FunctionName is the function designed by ourself.

That is not challenging to realise that "string".lengthy() always returns hello. Yet, the tricky part lies in the delete object where we might think that this expression will entirely delete the object. That is not the case as delete is used to delete the property of the object only. It does not delete the object. Then we get hello rather than ReferenceError.

Note that if we declare object without let, const or var, we then have a global object. delete objectName then return true. Otherwise, it always returns false.

13. What's the output?

let x = {};

x.__proto__.hi = 10;

Object.prototype.hi = ++x.hi;

console.log(x.hi + Object.keys(x).length);
  • A: 10
  • B: 11
  • C: 12
  • D: NaN
Answer

Answer: C

First we have an empty object x, then we add another property hi for x with x.__proto__.hi. Note this is equivalent to Object.prototype.hi = 10 and we are adding to the father object Object the property hi. It means every single object will inherit this propety. The property hi becomes a shared one. Say now we declare a new object such as let y = {}, y now has a propery hi inherited from the father Object. Put it simply x.__proto__ === Object.prototype returns true.

Then we overwrite the property hi with a new value 11. Last we have 11 + 1 = 12. x has one property and x.hi returns 11.

Updated (July 27th 2021). If you write Object.prototype.hi = 11; instead of Object.prototype.hi = ++x.hi; as written in the code above, then Object.keys(x) will return an empty array as Object.keys(object) only returns the property of the object itself, not the inherited ones. It means the final result will be 11 rather than 12. For some reason, the code ``Object.prototype.hi = ++x.hi;will create a property for the objectxitself and thenObject.keys(x)gives us the array["hi"]`.

Yet, if you run console.log(x.hasOwnProperty("hi")) it still returns false. By the way, when you deliberately add a property for x such as x.test = "testing", then console.log(x.hasOwnProperty("test")) returns true.

14. What's the output?

const array = (a) => {
  let length = a.length;
  delete a[length - 1];
  return a.length;
};

console.log(array([1, 2, 3, 4]));

const object = (obj) => {
  let key = Object.keys(obj);
  let length = key.length;
  delete obj[key[length - 1]];

  return Object.keys(obj).length;
};

console.log(object({ 1: 2, 2: 3, 3: 4, 4: 5 }));

const setPropNull = (obj) => {
  let key = Object.keys(obj);
  let length = key.length;
  obj[key[length - 1]] = null;

  return Object.keys(obj).length;
};

console.log(setPropNull({ 1: 2, 2: 3, 3: 4, 4: 5 }));
  • A: 333
  • B: 444
  • C: 434
  • D: 343
Answer

Answer: C

This question examines how the delete operator works in JavaScript. In short, it does nothing when we write delete someObject or delete someArray. It nonetheless completely deletes and removes a property of an object when writing something like delete someObject.someProperty. In the case of array, when we write delete someArray[keyNumber], it only removes the value of the index, keep the index intact and the new value is now set to undefined. For that reason, in the code first snippet, we get (the length) 4 elements as in the original array but only 3 properties left in the object passed when the function object() is called, as in the second snippet.

The third snippet gives us 4 as declaring an object's propery to either null or undefined does not completely remove the property. The key is intact. So the length of the object is immutable.

For those who are familiar with PHP, we have unset($someArray[index]) that remove the array element, both key and value. When print_r the array, we might not see the key and value that have been unset. However, when we push (using array_push($someArray, $someValue)) a new element in that array, we might see that the previous key is still kept, but no value and not being displayed. That is something you should be aware of. Have a look at https://3v4l.org/7C3Nf

15. What's the output?

var a = [1, 2, 3];
var b = [1, 2, 3];

var c = [1, 2, 3];
var d = c;

var e = [1, 2, 3];
var f = e.slice();

console.log(a === b);
console.log(c === d);
console.log(e === f);
  • A: true true true
  • B: false false true
  • C: true true false
  • D: false true false
Answer

Answer: D

a and b returns false because they point to different memory location even though the values are the same. If you are coming from PHP world, then it will return true obviously when we compare either value or value + type. Check it out: https://3v4l.org/IjaOs.

In JavaScript, value is passed by reference in case of array and object. Hence in the second case, d is the copy of c but they both point to the same memory position. Everything changes in c will result in the change in d. In PHP, we might have $a = &$b;, working in the similar way.

The third one gives us a hint to copy an array in JavaScript using slice() method. Now we have f, which is the copy of e but they point to different memory locations, thus they have different "life". We get false accordingly when they are being compared.

16. What's the output?

var languages = {
  name: ['elixir', 'golang', 'js', 'php', { name: 'feature' }],
  feature: 'awesome',
};

let flag = languages.hasOwnProperty(Object.values(languages)[0][4].name);

(() => {
  if (flag !== false) {
    console.log(Object.getOwnPropertyNames(languages)[0].length << Object.keys(languages)[0].length);
  } else {
    console.log(Object.getOwnPropertyNames(languages)[1].length << Object.keys(languages)[1].length);
  }
})();
  • A: 8
  • B: NaN
  • C: 64
  • D: 12
Answer

Answer: 64

The code snippet is quite tricky as it has a couple of different built-in methods handling object in JavaScript. For example, both Object.keys and Object.getOwnPropertyNames are used even thought they are quite similar except that the latter can return non-enumerable properties. You might want to have a look at this thoroughly written reference https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyNames

Object.values and Object.keys return the property value and property name of the object, respectively. That is nothing new. object.hasOwnProperty('propertyName') returns a boolean confirming whether a property exists or not.

We have flag true because Object.values(languages)[0][4].name returns feature, which is also the name of the property.

Then we have 4 << 4 in the if-else flow that returns the bitwise value, equivalent to 4*2^4 ~ 4*16 ~ 64.

17. What's the output?

var player = {
  name: 'Ronaldo',
  age: 34,
  getAge: function () {
    return ++this.age - this.name.length;
  },
};

function score(greeting, year) {
  console.log(greeting + ' ' + this.name + `! You were born in  ${year - this.getAge()}`);
}

window.window.window.score.call(window.window.window.player, 'Kiora', 2019);

score.apply(player, ['Kiora', 2009]);

const helloRonaldo = window.score.bind(window.player, 'Kiora', 2029);

helloRonaldo();
  • A: "Kiora Ronaldo! You were born in 1985", "Kiora Ronaldo! You were born in 1985", "Kiora Ronaldo! You were born in 1985"
  • B: "Kiora Ronaldo! You were born in 1991", "Kiora Ronaldo! You were born in 1991", "Kiora Ronaldo! You were born in 1999"
  • C: "Kiora Ronaldo! You were born in 1991", NaN, "Kiora Ronaldo! You were born in 1980"
  • D: "Kiora Ronaldo! You were born in 1991", "Kiora Ronaldo! You were born in 1980", "Kiora Ronaldo! You were born in 1999"
Answer

Answer: D

We can use call(), apply() and bind() to apply a function to any object. At first sight, it seems that three functions do the same thing. Yet there are some situations where they are differently employed to handle respective contexts or solve particular problems.

Of the three, only bind() can be executed after binding. We can create a variable to store the result as helloRonaldo() in the code snippet above. apply() and call() will bind and execute the function at the same time. apply() hints us a ~ array where we need to pass an array as parameter. call() hints us c or comma where we pass parameters with a comma. You might want to have a look at this post https://stackoverflow.com/questions/15455009/javascript-call-apply-vs-bind

Note that window.window.window.score or window.score or simply score do the same thing. It points to the score() function in the global scope.

The correct anwser is D. The score() and getAge() functions are nothing special. The only tricky part is that this.age is incremented each time you call the funtion getAge();

18. What's the output?

var ronaldo = { age: 34 };

var messi = { age: 32 };

function score(year, tr, t) {
  if (typeof tr === 'function' && typeof t === 'function') {
    console.log(`You score ${tr(year, t(this.age))} times`);
  }
}

const transform = (x, y) => x - y;

const title = (x) => ++x + x++;

const helloRonaldo = score.bind(ronaldo, 2029, transform, title);

helloRonaldo();

const helloMessi = score.bind(messi, 2029, transform, title);

helloMessi();
  • A: "You score 1989 times" and "You score 1963 times"
  • B: "You score 1959 times" and "You score 1989 times"
  • C: "You score 1989 times" and "You score 1953 times"
  • D: "You score 1959 times" and "You score 1963 times"
Answer

Answer: D

bind() allows us to bind a function declared with any object. Here we bind score() and both ronaldo and messi.

In score() we pass three parameters year, tr and t in which both tr and t are function. They handle simple things as defined afterwards.

When we bind score() with ronaldo and messi, we pass three parameters as declared in the score() function wherein transform and title are functions.

19. What's the output?

var person = {};

Object.defineProperties(person, {
  name: {
    value: 'Vuong',
    enumerable: true,
  },
  job: {
    value: 'developer',
    enumerable: true,
  },
  studying: {
    value: 'PhD',
    enumerable: true,
  },
  money: {
    value: 'NZD',
    enumerable: false,
  },
});

class Evaluate {
  static checkFlag(obj) {
    return Object.getOwnPropertyNames(obj) > Object.keys(obj) ? Object.getOwnPropertyNames(obj) : Object.keys(obj);
  }
}

const flag = Evaluate.checkFlag(person);

console.log(flag.length);
  • A: 1
  • B: 2
  • C: 3
  • D: 4
Answer

Answer: D

Object.keys(obj) is almost identical to Object.getOwnPropertyNames(obj) except the fact that the latter returns any type of object's property regardless of enumerable. By default enumerable is true when creating object. Using Object.defineProperties or Object.defineProperty we can manually set this option to false.

As such the object person will get 3 usingObject.keys(obj)but 4 with Object.getOwnPropertyNames(obj). In short Object.keys(obj) only returns the property setting the enumerable as true.

20. What's the output?

const id = 10;

const getID = (...id) => {
  id(id);

  function id(id) {
    console.log(typeof id);
  }
};

getID(id);
  • A: ReferenceError
  • B: 10
  • C: undefined
  • D: 'function'
Answer

Answer: D

When declaring a function inside another function, we are working with Closure in JavaScript. Note that if a function is declared as normal (rather than function expression), it is hoisted. We might see several id in the code snippet above but in fact, some of them does nothing.

The result of the code depending on the operator typeof id, which is function. So id in this operation is the id() function.

21. What's the output?

var book1 = {
  name: 'Name of the rose',
  getName: function () {
    console.log(this.name);
  },
};

var book2 = {
  name: { value: 'Harry Potter' },
};

var bookCollection = Object.create(book1, book2);

bookCollection.getName();
  • A: 'Harry Potter'
  • B: 'Name of the rose'
  • C: ReferenceError
  • D: Object object
Answer

Answer: A

Object.create allows us to create an object which is based on another object. If we do not pass the second parameter - book2 in this case - the name property of the object bookCollection will be Name of the rose inherited from the book1. It means we can provide additional properties when declaring object with Object.create.

bookCollection has its own property name and another one inherited from book1. In this case its own property name will show up as it has higher priority. That is why we get 'Harry Potter'.

22. What's the output?

(() => {
  const a = Object.create({});

  const b = Object.create(null);

  let f1 = a.hasOwnProperty('toString');

  let f2 = 'toString' in b;

  let result = f1 === false && f2 === false ? console.log((typeof a.toString()).length) : console.log(b.toString());
})();
  • A: ReferenceError
  • B: undefined
  • C: 0
  • D: 6
Answer

Answer: D

The two objects a and b are created using Object.create() operator. There is a bit of difference between them as a inherits from Object prototype but b is totally empty when we pass the null paramater. Yet hasOwnProperty('toString') always returns false neither a nor b given that toString() is not defined inside these objects. The method however is still available as it is inherited from Object prototype.

Both f1 and f2 return false. Note that we use object.hasOwnProperty('key') and ('key' in object) to check the availability of a key in an object. There is a bit difference between the two as the latter also returns the key inherited. You might want to have a look here: https://stackoverflow.com/questions/455338/how-do-i-check-if-an-object-has-a-key-in-javascript

Then typeof a.toString() returns string, which gives us 6 with the .length property.

If the syntax is odd to you, you might look for 'self-invoking function' and 'arrow function' in JavaScript.

23. What's the output?

let promise = new Promise((rs, rj) => {
  setTimeout(() => rs(4), 0);

  Promise.resolve(console.log(3));

  console.log(2);
});

promise
  .then((rs) => {
    console.log(rs ? rs ** rs : rs);
    return rs;
  })
  .then((rs) => console.log(rs == 256 ? rs : rs * rs));
  • A: 3, 2, 256, 256
  • B: 3, 2, 256, 16
  • C: 256, 16, 3, 2
  • D: 16, 256, 3, 2
Answer

Answer: B

We first declare a promise-based code with let and then call it. Given that setTimeout() is an asynchronous action, it will run last even the time is set to 0 in setTimeout(() => rs(4), 0);. Although Promise.resolve(console.log(3)) also returns a promise but it is a Microtasks, then it has a higher priority than Tasks as set by setTimeout(). You might want to have a look at this post https://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules/.

In .then() we chain the result so that we have 4^4 in the first then() and 4*4 in the second then(). Note that return rs returns the original value.

24. What's the output?

async function f() {
  let promise = new Promise((resolve, reject) => {
    setTimeout(() => resolve('done!'), 0);
  });

  setTimeout(() => console.log('world'), 0);

  console.log(await promise);

  console.log('hello');
}

f(setTimeout(() => console.log('kiora'), 0));
  • A: ReferenceError
  • B: done, hello, world
  • C: hello, done, world
  • D: kiora, done, hello, world
Answer

Answer: D

Though we do not declare any paramater for the function f(), we pass setTimeout(()=>console.log("kiora"),0) when call it. We therefore get 'kiora' first.

Given that the variable promise returns a solved promise and it is called with the keyword await, JavaScript will 'pause' at this line console.log(await promise); till the result is resolved. That is why we get "done" at the next result.

Why we do not get "world" or "hello" at the second ? As JavaScript "pauses" at the line with await keyword, we cannot get "hello" as usual (note that whenever we call setTimeout(), this function will run last because it is an asynchronous task operator), whereas setTimeout(()=> console.log("world"), 0); should always run last.

Here we might see a bit of difference when employing await keyword before asynchronous operator (in this case, we use setTimeout() as an example) or when call the function/operator without it.

25. What's the output?

function name() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve('New Zealand');
    }, 10);
  });
}

function fruit() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve('Kiwi');
    }, 20);
  });
}

(async function countryandfruit() {
  const getName = await name();
  const getFruit = await fruit();

  console.log(`Kiora: ${getName} ${getFruit}`);
})();

(async function fruitandcountry() {
  const [getName, getFruit] = await Promise.all([name(), fruit()]);

  console.log(`Hello: ${getName} ${getFruit}`);
})();
  • A: Null
  • B: Kiora
  • C: "Hello: New Zealand Kiwi" -> "Kiora: New Zealand Kiwi"
  • D: "Kiora: New Zealand Kiwi" -> "Hello: New Zealand Kiwi"
Answer

Answer: C

Both countryandfruit and fruitandcountry are self invoking functions. Both are declared with the keyword async, it means the code inside will run step by step. It helps us control the flow of data much more concise as compared to Promise-based operator or callback way.

The first function returns "Kiora: New Zealand Kiwi" and the second one ouputs "Hello: New Zealand Kiwi". We might think that the order will be the same but actually the order of the result is reversed because the function with await keyword will run step by step rather than in in parallel as Promise.all. It means fruitandcountry will run faster than countryandfruit.

You might want to have a look at the difference between the two at https://alligator.io/js/async-functions/

26. What's the output?

class MySort {
  constructor(object) {
    this.object = object;
  }

  getSort() {
    return Object.entries(this.object)[0][1].sort()[Object.values(this.object).length];
  }
}

const object = {
  month: ['July', 'September', 'January', 'December'],
};

const sortMe = new MySort(object);

console.log(sortMe.getSort());
  • A: July
  • B: September
  • C: January
  • D: December
Answer

Answer: C

Object.entries returns an array consisting of both key and value from an object while Object.values retuns an array of the values of object and Object.keys gives us an array of keys of the object. As such, Object.entries(object) in the code snippet above gives us a nested array with just one element in which the values are put in another nested array like that [["month", ["July", "September", "January", "December"]]].

For that reason, Object.entries(this.object)[0][1].sort() will actually sort the value array and return a new order as "December" -> "January" -> "July" -> "September". Hence, when we get the element with the index given by [Object.values(this.object).length] we get January because [Object.values(this.object).length] give us 1 (the length of the array given by Object.values);

27. What's the output?

const flag = [] !== !!!!![];

let f = () => {};

console.log((typeof f()).length + flag.toString().length);
  • A: NaN
  • B: 12
  • C: 13
  • D: 14
Answer

Answer: C

Comparing two arrays or two objects in JavaScript always return false because both are passed by reference, unlike primitive types such as string, number or boolean. That is why comparing [] and [] using either == or === returns false. The weird part is the !==!!!!! which is equivalent to !==, nothing special. So the flag is true.

In the expression function f(), we use arrow function here but and {} is a part of the function rather than an object. In case you want to return an object, you have to write as let f = () => ({}) or simply using normal way to define function. With the keyword return, we can easily catch the content of the function when using normal way to define function.

Thus, the typeof f() returns undefined rathern object. We then get the length 9 and the flag (true) becomes 'true' (a string, by using toString() function), which returns 3 with the property length. We finally get 13.

28. What's the output?

(function (a, b, c) {
  arguments[2] = (typeof arguments).length;
  c > 10 ? console.log(c) : console.log(++c);
})(1, 2, 3);
  • A: 4
  • B: 5
  • C: 6
  • D: 7
Answer

Answer: D

We have a self-invoking function with three parameters declared. Note that arguments inside a function returns an object consisting of the parameters of the function.

The key part here is that when we assign a value to that array (it is array-like, as mentioned above) (or any element), the function will use that value rather than the value from the parameter we pass to it when calling the function. Hence, c will be (typeof arguments).length; (6) rather than 3.

As c has a new value of 6, it is definitely less than 10, so we get the final result console.log(++c), which returns 7.

Note that arguments is not available on arrow functions. See more detailed here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments

From ES6 onwards, it is recommended to use ...restParameter given that it is a true array. It means you can manipulate the parameter with native JavaScript functions such as map, reduce or filter.

For PHP developer, we have func_get_args() in PHP that does the same thing, but it will not override the value passed. Check it by yourself at https://3v4l.org/dMfhW

29. What's the output?

class Calculator {
  constructor(a, b) {
    this.a = a;
    this.b = b;
  }
  static getFlag() {
    return new Array(this.a).length == new Array(this.b).toString().length;
  }

  getValue() {
    return Calculator.getFlag() ? typeof this.a : typeof new Number(this.b);
  }
}

const me = new Calculator(5, 5);

console.log(me.getValue());
  • A: NaN
  • B: "string"
  • C: "object"
  • D: "number"
Answer

Answer: C

We have a class named Calculator. When declaring a new instance of the object, we pass two parameters a and b. These two parameters have the same value but new Array(this.a).length is totally different from new Array(this.b).toString().length because the latter returns a string ",,,," meaning the length 4 while the former returns the length of an array and we therefore get 5.

For that reason getFlag() returns false. In getValue() we get typeof new Number(this.b); which returns object. That is a bit different from typeof b, which returns number.

30. What's the output?

var name = 'Auckland';

const nz = {
  name: 'Kiwi',

  callMe: function () {
    return this.name;
  },
};

let me = nz.callMe;

let she = nz.callMe.bind(nz);

let result = me() === nz.callMe() ? she() : `${me()} ${she()}`;

console.log(result);
  • A: undefined
  • B: "Auckland"
  • C: "Kiwi"
  • D: "Auckland Kiwi"
Answer

Answer: D

The key point in this question involves the keyword this in JavaScript. We have a simple object that contains one method and one string property name.

First, it is important to write down is that let me = nz.callMe; and then call me() is totally different from directly calling nz.callMe(). If we assign a variable to a method delared inside an object, this in that method will behave differently (when we call the variable as a method and when dirrectly call that method). In particular, in the first case, this is the window object while in the second one, this inside the function still points to property name in the object nz. It means me() returns "Auckland" while nz.callMe returns "Kiwi".

Then result will return false and we get the final output value ${me()} ${she()}. Why she() is different from me()? You might easily guess that she still bind to the object nz rather than window object as in me().

--- Còn tiếp ---

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