How do JavaScript variables go up?

Question

I know that JavaScript moves all variable declarations to the top of the function. However, I expected the same result in console.log () below. However, I get NaN in the first case and 4 per second.

Can someone explain this? I know that there are already asked questions and answers on StackOverflow, but this question has something to do with defining a function. for example, does JavaScript translate function variables to the end of variable declarations?

var doB = function() { return a+1}; var b = a+1; var a = 3; console.log(b); // NaN console.log(doB()); // 4 

Answer 1

Due to the rise, this is what your code technically looks like this:

 var doB, b, a; doB = function() { return a+1; }; b = a + 1; a = 3; console.log(b); // NaN console.log(doB()); // 4 

Since the code is executed from top to bottom, when b initialized to a + 1 , a has not yet been initialized (it is undefined ). So really, b trying to be set as undefined + 1 , which is NaN

Answer 2

After the variables are raised, your code basically acts as follows:

 var doB; var a; var b; doB = function() { return a + 1 }; b = a + 1; a = 3; console.log(b); // NaN console.log(doB()); // 4 

Since a not defined when you define b , b set to NaN as undefined + 1 === NaN .

a is included in the scope of the function, so by the time it is called, a already has a value of 3 .

Answer 3

JavaScript scope

I hope to use the most correct terminology that I know at the moment, and I also know that this is not entirely correct. I am not a javascript guru, but this question you are asking about is related to the JavaScript realm . You can find many articles on this topic, as well as some Q & A on SO too, as I found with examples.

Step by step

I will try to play line by line what your snipnet does. Please comment if you notice the wrong term somewhere, I like to study always, I can: D

line 1

 var doB = function() { return a+1}; 

means

Save the variable with the name "doB" in the current area, assign its contents to an anonymous function.

So...

a now doesn't matter.

---

line 2

 var b = a+1; 

means

Save the variable with the name b in the current area, add its contents to new Integer(1) in addition to the variable a .

So...

At this point, a is undefined because it is not declared in the current scope, none of the available scopes are available. The operation undefined + a is equal to NaN . a assigned to NaN as a value.

---

line 3

 var a = 3; 

means

Save the variable with the name a in the current area, assign its contents to new Integer(3)

So...

Now we have a variable a that stores an integer.

---

line 4

 console.log(b); 

The conclusion is NaN , because b is actually NaN .

---

line 5

 console.log( doB() ); 

means

Call the console.log function with the value returned by the doB function doB with no arguments as the first argument.

So...

doB is called and returns: a + 1

What means

new Integer(1) in addition to the variable a . a does not exist in the local function region, but exists in the region accessible inside the function. The value is taken from there, which is actually 3 . Console Exits 4 .

Answer 4

a undefined when you try to add 1 to it by assigning it b .

 var b = undefined+1; // = NaN