WEBVTT 00:00.870 --> 00:06.330 Nowadays, JavaScript is one of the most powerful and widely used programming language, and day by 00:06.330 --> 00:08.680 day it becomes more and more popular in the world. 00:09.390 --> 00:14.490 Therefore, there is a huge demand on JavaScript developers, and that's the main reason why I have 00:14.490 --> 00:17.940 decided to create this section throughout the course. 00:17.940 --> 00:23.640 You have met almost everything that you need to know regarding JavaScript today, and if you have understood 00:23.640 --> 00:28.500 and learned all of this stuff, well, then I guarantee that you won't have any trouble to start the 00:28.500 --> 00:31.220 job, at least as JavaScript developer. 00:31.920 --> 00:37.170 But regardless of that, I'm going to go through the commonly asked questions on JavaScript interviews 00:37.170 --> 00:39.720 and also show you relevant answers. 00:40.900 --> 00:46.660 I would like to recommend you to post the video after each question, think about it and try to remind 00:46.660 --> 00:49.640 the answer and check if you are correct or not. 00:50.800 --> 00:56.020 The first question that is commonly asked after releasing its sixth version is the following. 00:56.800 --> 01:01.270 What is the difference among the Latin cast variable declarations? 01:02.910 --> 01:08.580 All right, let's recall it once again, the first thing that I want to note is that the declaration 01:08.580 --> 01:11.190 has been using since JavaScript was created. 01:11.730 --> 01:15.540 As for the Latin declarations, they are added in essence version. 01:16.230 --> 01:20.730 At first I'm going to compare VA to both Latin constant declarations. 01:21.300 --> 01:28.590 The main distinction between them is scoping variables with var declaration or function scoped and Latin 01:28.590 --> 01:30.090 Carlstadt blocks called. 01:31.070 --> 01:34.700 OK, let's write an example, create function A. 01:37.450 --> 01:41.200 That inside code block create variable B. 01:43.000 --> 01:44.020 Which equals the 10. 01:46.250 --> 01:49.460 And run it in council, right, Costa Lockpick. 01:52.840 --> 01:57.140 Without that, we have an error saying that B is not defined. 01:57.880 --> 02:04.330 That's because Variable B is created inside scope of Function A. And we do not have access on this variable 02:04.330 --> 02:09.310 outside function at the same will happen if we use that. 02:13.500 --> 02:15.480 And also concerned. 02:18.240 --> 02:22.140 But in case of if statements, then the result will be different. 02:22.770 --> 02:27.000 Let's create if and as a condition right here. 02:27.000 --> 02:27.420 True. 02:29.240 --> 02:34.070 Then create variable inside call, the prices were equal to 10. 02:35.320 --> 02:36.910 And then run it in council. 02:40.330 --> 02:47.410 Reload and he said that we have done so, it means that one variable is created with var declaration, 02:47.770 --> 02:49.980 it's accessible from outside the block. 02:50.560 --> 02:54.790 It means that VA is not the block skulked, as you know, in block. 02:54.790 --> 02:59.400 I mean, everything that is between curly braces, that's for the letton cost. 02:59.410 --> 03:08.440 If we change VA into that, then we will get an error, which tells us that A is not defined and the 03:08.440 --> 03:11.920 exact same happens if we change that into cost. 03:14.840 --> 03:16.400 So still, we have an ever. 03:18.180 --> 03:24.330 All right, the next difference between Valette and cost variable declarations is the following if we 03:24.330 --> 03:33.840 create a variable using VAR and then run it in council, but right before declaration. 03:36.380 --> 03:43.250 Then we will get undefined, and that's because of hoisting, but when we use let or const. 03:46.220 --> 03:51.290 Instead of undefined, we get an error is not defined. 03:55.070 --> 04:00.890 All right, now let's see differences between left and cost declarations, I'm sure that you remember 04:00.890 --> 04:01.170 them. 04:01.690 --> 04:03.200 Why don't we use that declaration? 04:03.200 --> 04:05.410 We can reassign to it the value. 04:05.810 --> 04:06.690 So that's right. 04:07.040 --> 04:11.090 A course of ten, then a equals to 220. 04:12.140 --> 04:13.940 And run it in council. 04:16.670 --> 04:20.180 So you see that value of air is changed from 10 to 20. 04:20.810 --> 04:25.370 But in case of consent, it's not possible if we change all that into cost. 04:26.410 --> 04:32.740 Then we will get an error assignment to Kossuth Variable, so if we don't want to change the value of 04:32.740 --> 04:35.560 a variable, then we have to use constant declaration. 04:36.910 --> 04:42.610 Another distinction between cost and that is that if you declare that variable and not assigned a value 04:42.610 --> 04:43.030 to it. 04:45.930 --> 04:50.190 Then it will be undefined, but in case of constant variable. 04:52.980 --> 04:59.430 We cannot do it if we declare the variable using cost, then we definitely need to assign value to it. 05:01.100 --> 05:06.830 All right, let's move on to the next question, which is the following what is the difference between 05:06.830 --> 05:11.420 undefined and now undefined and primitive data types? 05:11.420 --> 05:18.020 They both mean that the variable doesn't have the value, but the difference is that undefined is automatically 05:18.020 --> 05:22.820 assigned to a variable when it's declared you don't have to do it manually. 05:22.820 --> 05:27.980 JavaScript engine does it automatically, but now is an assignment value. 05:28.670 --> 05:32.950 When you want the variable to have no value, then you have to assign to it. 05:32.960 --> 05:38.790 Now, actually, you can assign undefined MANUELITA variable, but it's not recommended. 05:39.830 --> 05:45.290 Besides that, if you check type of undefined and now you will get different results. 05:46.940 --> 05:49.130 Right, consultative type of. 05:50.940 --> 05:51.960 A defined. 05:54.500 --> 05:55.910 And do the same for. 05:57.570 --> 05:57.990 No. 05:59.160 --> 06:05.250 So in case of undefiled, we have type of define, but as for now, we have an object. 06:06.920 --> 06:13.490 Actually undefined and now is kind of confusing part of JavaScript and interviewers like to ask questions 06:13.490 --> 06:14.040 about them. 06:14.930 --> 06:15.440 All right. 06:15.470 --> 06:21.230 The next then one of the most commonly asked question is the difference between composing operators, 06:21.230 --> 06:26.690 which are double equals and triple equals, double equals comprise an operator who compares all of the 06:26.690 --> 06:27.310 values. 06:27.950 --> 06:35.320 Let's say we have variables, let A equals the 10 and that B equals to ten as well. 06:36.050 --> 06:39.500 Let's run in Castle A double equals B. 06:42.610 --> 06:48.800 So you say that we have to because those values are definitely equal, if we change double equal to 06:48.820 --> 06:54.510 triple equals, then we will get still through because the values and also types are equal. 06:55.000 --> 06:56.440 Both variables are numbers. 06:57.600 --> 07:04.620 If we change one of the values into spring and also change triple equals into double equals, then we 07:04.620 --> 07:08.190 will get to because as we said, double equals comparison. 07:08.270 --> 07:09.960 The values are not data types. 07:10.710 --> 07:16.470 And here in both cases, we have values as the opposite happens if we use triple equals. 07:18.080 --> 07:24.050 We will get false because it appears that all of the values, but also the data types and in this case, 07:24.050 --> 07:25.910 we have no string. 07:26.900 --> 07:30.190 All right, let's go ahead and see what is the next question? 07:31.280 --> 07:35.160 What is the difference between function statements and function expressions? 07:36.620 --> 07:38.000 Let's create both of them. 07:39.350 --> 07:43.310 In case of function statement, we have to write following Function A. 07:45.560 --> 07:53.420 And then Calabrese ride right, consider log high Asfour function expression's, we have to create variable 07:53.810 --> 08:00.230 and assign to it anonymous function the right variable be equal to function. 08:03.010 --> 08:04.270 And then running castle. 08:06.490 --> 08:10.630 Hello, let's invoke both of them. 08:13.990 --> 08:17.490 Then reload and we have high and hello. 08:18.070 --> 08:22.900 Both functions work, but if we call them before, they are declarations. 08:24.160 --> 08:30.820 Then for function statement, we will get the same results, but for function expression, we will get 08:30.820 --> 08:34.150 an error, which tells us that B is not a function. 08:35.270 --> 08:41.720 So this is the main and kind of insignificant difference between function statement and function expression. 08:42.560 --> 08:47.720 All right, let's move on to the next question, which is about the difference between regular functions 08:47.720 --> 08:48.920 and arole functions. 08:49.520 --> 08:53.570 Actually, erro functions are released, a new version of JavaScript in Essex. 08:54.020 --> 08:59.950 And the first and obvious difference between regular and eral functions is in syntax at birth. 08:59.960 --> 09:10.700 Let's write constants a equals to function and place your parameters X and Y and then return the sum 09:10.940 --> 09:15.250 of those parameters, then invoke the function right. 09:15.260 --> 09:21.390 Consider log A with the arguments as five and ten. 09:22.250 --> 09:24.860 So this is a regular function expression. 09:25.560 --> 09:27.860 And now let's write erro function. 09:28.400 --> 09:36.140 Right, constant P equals inserting parentheses parameters X and Y that are all function. 09:37.270 --> 09:41.560 And again, it's the sum of Paramjit X and Y. 09:43.990 --> 09:47.380 Let's invoke function B, right, cancel that log. 09:48.550 --> 09:51.790 Be with the arguments 20 and 30. 09:54.330 --> 10:02.210 Reload and we have 50 so many differences in syntax is that in case of error functions, we don't use 10:02.210 --> 10:03.170 keyword function. 10:03.470 --> 10:09.490 If we have only one statement in code block like it's here that we don't need to use Calabrese. 10:12.390 --> 10:14.100 And even returned Keywood. 10:18.040 --> 10:24.840 Also, if we have only one parameter, then we don't need to use parentheses, but in case if we do 10:24.840 --> 10:28.470 not have any parameters, then we need to place empty parentheses here. 10:28.890 --> 10:34.500 The next and more important difference between regular and error functions is about this special, this 10:34.500 --> 10:35.070 variable. 10:35.760 --> 10:37.320 Let's use another example. 10:37.330 --> 10:40.840 I'm going to select hatting element, which we have on page here. 10:41.570 --> 10:45.310 Actually, this is known example for you, but because of that, it's very important. 10:45.330 --> 10:50.730 I'm going to show you again, let's select hatting elements and attach the event. 10:50.730 --> 10:55.770 Listener writes, document that query selector. 10:58.490 --> 11:07.850 Then right here, H1 an element and attached to it, event listener with click event that place your 11:07.850 --> 11:08.780 anonymous function. 11:10.980 --> 11:16.410 And inside that anonymous function creates function expression, right, concerned a. 11:17.570 --> 11:18.650 He calls the function. 11:21.790 --> 11:30.490 And let's run in council, this variable also invoke function A. 11:33.010 --> 11:39.670 Let's reload and click on Harding, you see that in case of regular function, this variable points 11:39.670 --> 11:41.020 to global window object. 11:42.430 --> 11:45.520 But if we change a regular function into a role function. 11:47.250 --> 11:54.120 Then this what will change dereference and it will point to the elements on which event listener is 11:54.120 --> 11:54.630 attached. 11:55.830 --> 12:04.010 It happens because this variable of erro function points to the same object as its surroundings. 12:04.020 --> 12:09.360 Culp's, this variable does, but on a global level, both refer to global. 12:09.360 --> 12:10.150 We object. 12:10.710 --> 12:13.890 OK, let's go ahead and see what is the next question. 12:14.340 --> 12:16.200 Next question is about hoisting. 12:17.710 --> 12:23.260 From many developers and also in many resources online, it will make the following definition about 12:23.260 --> 12:28.900 hosting, hosting means that JavaScript engine physically moves variable declarations. 12:29.860 --> 12:36.420 But honestly, it's not quite correct, actually, JavaScript engine doesn't move the code physically, 12:37.240 --> 12:39.700 so let me explain once again what hoisting means. 12:40.730 --> 12:48.610 Before the code runs, JavaScript engine on the creation phase automatically assigns to variables undefined. 12:49.130 --> 12:54.890 So let's write the example create variable AI, which equals to hello. 12:57.110 --> 13:03.260 That run it in council, but write it before the verbal declaration. 13:05.920 --> 13:06.460 Reload. 13:07.450 --> 13:13.260 And we have undefined so as we know, JavaScript engine reads the code line by line, from top to down. 13:13.870 --> 13:20.230 At first it found Castle Dot Log A and because of that, at this point, the value to variable is assigned 13:20.230 --> 13:21.040 as undefined. 13:21.490 --> 13:22.580 We got undefined. 13:23.560 --> 13:27.670 So again, remember, that engine doesn't move code physically. 13:28.240 --> 13:32.290 It just assigns automatically undefined two variables on the creation phase. 13:33.280 --> 13:33.670 All right. 13:34.120 --> 13:37.830 Let's discuss another possible question that you may have gotten interview. 13:38.440 --> 13:44.890 So what isn't immediately invoked function expression and what it is used for, immediately invoked 13:44.890 --> 13:51.430 function expression or simply if it is function, which is invoked immediately after its declaration 13:52.510 --> 13:57.430 in order to create if we need to place parentheses and write in its anonymous function. 14:00.520 --> 14:08.800 And then after or before closing brace, we need to place another parenthesis, let's right inside it, 14:09.130 --> 14:10.030 cancel the clock. 14:10.540 --> 14:11.080 Hello? 14:14.560 --> 14:16.220 Reload and we have hello. 14:17.260 --> 14:24.610 Actually, if it is a great tool in JavaScript, they're used for to create blocks for data privacy 14:24.970 --> 14:27.890 and also we use them in order to avoid naming conflicts. 14:28.810 --> 14:33.850 Besides that, if it's not great when we write the code using module pattern, which you have already 14:33.850 --> 14:35.980 seen in our quiz application project. 14:37.570 --> 14:38.040 All right. 14:38.590 --> 14:43.210 The next question that I want to show you is actually one of the most commonly asked one. 14:43.210 --> 14:45.400 And it refers to closure's. 14:46.280 --> 14:51.710 There is really higher chance to get this question on your JavaScript interview, so let's recall what 14:51.710 --> 14:52.760 closures are. 14:52.790 --> 14:55.910 I'm going to create a very simple example, right? 14:55.910 --> 14:59.370 Function A inside. 14:59.370 --> 15:03.200 It creates constant X, which equals to five. 15:04.270 --> 15:05.710 Then return function. 15:07.370 --> 15:08.900 And in that function. 15:10.230 --> 15:14.040 Create another variable Y, which equals the 10. 15:16.470 --> 15:26.640 And then running castle to some of these variables, X plus Y, let's invoke those functions right consent. 15:28.560 --> 15:30.090 Equals to a. 15:31.320 --> 15:33.150 And that call be 15:35.820 --> 15:37.780 revoked and we have 15. 15:38.970 --> 15:46.830 So when the function is executed, then a return function still has access on variable X and happens 15:46.830 --> 15:47.880 because of the closure. 15:48.870 --> 15:54.570 So in general, closures save the variables even when the function finishes execution. 15:55.440 --> 15:55.940 All right. 15:56.370 --> 16:02.640 So I have represented some of the probably ask questions on JavaScript interviews and I have demonstrated 16:02.640 --> 16:07.910 briefly answers on some of these topics are explained in detail throughout the course. 16:08.400 --> 16:14.210 So if you still do not feel confident in those topics, then I recommend to watch videos again. 16:15.440 --> 16:21.260 Actually, I cannot guarantee 100 percent that those questions will be asked on your interview, but 16:21.260 --> 16:25.730 it's common experience and there is a higher chance to get the questions like these. 16:27.020 --> 16:30.650 Honestly, I would be glad and excited if I could help you.