1
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If we declare a variable using there.

2
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The declaration part will be hoisted to the scope.

3
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Top if we console log the variable prior to its declaration we will not get any error messages.

4
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We will get undefined which is the default Variable value.

5
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If we declare a variable using let the declaration part will not be hoisted but a temporal dead zone

6
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will be created between the letter statement and the scope top inside this zone.

7
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The variable cannot be used.

8
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Next we create a function and name it.

9
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Test

10
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then we invoke it online.

11
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3 The test function is defined online for but invoked on line 3.

12
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What kind of result will we get we can see the function has been invoked successfully.

13
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This is because the declaration of a function will also be hoisted to the scope top.

14
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But unlike the declaration of a variable the function name and the function body will be hoisted to

15
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the scope together.

16
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This is why we can invoke a function in front of its declaration.

17
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Next we declare a global variable using the same name as the function

18
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then at the top of the scope we console log test.

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What kind of result will we get this time we can see we got the function body.

20
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The conclusion is the hoisting of a function is higher than that of a variable.

21
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Next behind the function and variable declaration we console log test again

22
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we can see this time we got the variable value not the function body.

23
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The conclusion is the declaration of a variable overrides that of a function.

24
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If in the same scope we name a function and a variable the same at the end the name will represent the

25
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variable not the function hoisting however works in the opposite way.

26
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A function hoisting will override a variable hoisting

27
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next.

28
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Let's try a more complicated scenario.

29
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We create a function outer we name its argument.

30
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Test inside the outer function we declare a variable using there and name it.

31
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Test then we create a child function and also name it test

32
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we console log test at the top of the function scope

33
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then at the end of the function scope we console log test again.

34
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What kind of result will we get this time.

35
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We invoke the outer function and set its argument value to hello

36
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we can see the first console log gives us the child function.

37
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The second one gives us the value of the test variable not the argument value we pass to the outer function.

38
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This proves the argument has the lowest priority inside a function scope.

39
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Actually the argument name is treated as a variable name in the function scope defining an argument

40
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actually means declaring a variable inside the function scope.

41
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We can prove this inside the outer function we declare test using let unlike there let does not allow

42
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us to repeatedly declare the same variable if we run the program.

43
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We will get an error message.

44
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We can see the error is caused by line 13 where we declare test using let it told us the identifier

45
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test has already been declared.

46
00:03:42,890 --> 00:03:44,750
We will get the same error message.

47
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If we repeatedly declare the same variable using let this proves the name test has already been declared

48
00:03:50,810 --> 00:03:53,100
as a variable in the function scope.

49
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The conclusion is defining an argument will create a variable under the argument name in the function

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scope.