Using var in javascript to define the precompilation Effect of variables, javascriptvar

Using var in javascript to define the precompilation Effect of variables, javascriptvar

Note: The javascript syntax is very flexible. The flexible cost is that it can easily lead to nonstandard coding and unexpected running results. ECMAScript 5 introduces strict mode. In the future, we 'd better use "use strict" to compile javascript code. The following test code is not based on the strict mode.

 

Alert (a); var a; // declared unassigned alert (B); // undeclared variable

The execution result is: a prints undefined and B reports an error. This statement is executed before alert (.

 

alert(c);var c = 1;alert(c);

Execution result: Print undefined first, and then print 1. In fact, the declaration is in advance, but the assignment statement still does not change the position.

 

alert(a);a = 1;

Execution result: the code returns an error. It can be seen that there is no var keyword, the variable declaration will not be in advance, and there is no "pre-compilation" process.

 

Function TestClass () {// If the var keyword is not used, the global variable val = 1; alert (val); // The global variable is stored in the window object alert (window. val) ;}; // call the TestClass constructor test = new TestClass (); // verify that it is a global variable alert (val );

Var is not used. In fact, a global variable is defined and will be placed in the window object.

 

Function TestClass () {val = 1; alert (val); // 1 alert (window. val); // undefined var val = 10; alert (val); // 10 alert (window. val); // undefined} var test = new TestClass (); alert (val); // js reports an error, the variable is defined

We can see that when using var to define a variable, the variable declaration will be made in advance, but the assignment will not be made in advance.

How to Use the defined var variable in innerHtem in Js

<Script language = "javascript">
Var j = 0;
Var vertical = "";
Var location;
Function add ()
{

J = sqd. rows. length;
Location = vertical + j;
NewRow = document. all. sqd. insertRow (-1)
Newcell = newRow. insertCell ()
Newcell. innerHTML = j
Newcell = newRow. insertCell ()
Newcell. innerHTML = "<input type = 'text' id = '" + location + "'/> "//
Newcell = newRow. insertCell ()
Newcell. innerHTML = "<input type = 'text'/>"
Document. getElementById (location). value = "B ";
Alert (location );
Alert (j );

}
</Script>

Javascript var

It cannot be omitted. In javascript, variables that are not declared using var are treated as global variables.

Therefore, when writing javascript scripts, try to add the var declaration before the variables to ensure that the variables are not used as global variables repeatedly.
Especially in functions with recursive calls, pay special attention to using var to declare cyclic variables. Otherwise, recursive calls may produce unexpected errors because the variables are modified recursively, debugging and searching are difficult.