A question of JavaScript about variable scope

Shinejaie Original, reproduced please indicate the source.

Last night in an Exchange group saw a netizen raised a question to ask questions for help. Just as I saw it, I thought about it and thought it would be helpful to understand the scope of JavaScript, so I tried to sort out my own problem-solving ideas and hope to help others.

First look at the following questions:

1     vararr = [1, 2, 3];2      for(vari = 0, J; j = arr[i++];) {3 Console.log (j);4     }5 6Console.log ('---------');7 Console.log (i);8Console.log ('---------');9 Console.log (j);TenConsole.log ('---------');

Before solving the problem, let's review the knowledge of the variable domain in JavaScript.

Global Variables (GLOBALS)

Global variables are variables that can be accessed anywhere, in two cases

• Declare outside the function, whether or not to use the var keyword
• Declare in function, do not use the var keyword, the statement of course must be executed to

Locals (local )Local variables can only be accessed within the declared function

• Declare in function, use the var keyword

Two points to note the place

Look at the code first:

1 // output undefined 2 3  for (var i = 0; i < 1; i++) {}; 4 5 // Output 1

• JavaScript does not exist in the statement scope , the variables defined within the statement will spread to the outside of the statement, in the example I declared in the for statement, but can be accessed outside the For statement
• I can be accessed before the For statement, except that it is not yet assigned

Start our problem solving.

i++ is added after I use:

First execution, J=arr[0], after I=1,console.log (j) Output 1

Second execution, J=arr[1], after I=2,ocnsole.log (j) Output 2

Third execution, J=arr[2], after I=3,ocnsole.log (j) Output 3

Fourth time (not conforming to condition), j=arr[3] is undefined, then I=4,ocnsole.log (j) has no output, exits for loop

After the For statement is executed, the output 4,console.log (j) Output is Console.log (i) from the above analysis undefined

The final output is:

123---------4---------undefined---------

According to the above analysis and results, we must have made clear, and then we began to extrapolate it.

To change a question by a question

Topic:

1     vararr = [1, 2, 3];2      for(vari = 0, J; j = arr[++i];) {3 Console.log (j);4     }5 6Console.log ('---------');7 Console.log (i);8Console.log ('---------');9 Console.log (j);TenConsole.log ('---------');

Answer:

1 2 2 3 3 --------- 4 3 5 --------- 6 undefined 7 ---------

To change the question by two

Topic:

1     functionxxx () {2         vararr = [1, 2, 3];3          for(vari = 0, J; j = arr[i++];) {4 Console.log (j);5         }6     }7 xxx ();8 9Console.log ('---------');Ten Console.log (i); OneConsole.log ('---------'); A Console.log (j); -Console.log ('---------');

Answer:

123--------- Error: uncaught referenceerror:i is not defined

A question of JavaScript about variable scope