Revision 750039 of Variable

<p>A variable is a named location for storing a {{Glossary("Value", "value")}}. That way an unpredictable value can be accessed through a predetermined name.</p>

<ul>
 <li>{{Glossary("JavaScript")}} variables can hold these types of values: number, string, object, function, or undefined. (NaN or Infinity is a special number, null is a special object)</li>
 <li>A variable name is an identifier of one or more alphanumeric characters. The underscore and dollar sign are also allowed, but the first character cannot be a digit. Example:</li>
</ul>

<pre>
var a, _, $, _0, $_, a1, a_, __, $$, __xx__; // good name
var #, 0a, %1, a&amp;, a*, 1ab;   // bad
var a.a;                      // bad

var a = {a:1};    // good.  The first 'a' is a variable name having a value of object.  
                  //        Second 'a' is not called as a variable name, but called as property name of the object.
console.log(a.a)  // good.  First 'a' is a variable name have a value of object.  Second 'a' is the property name of the object-a.
</pre>

<ul>
 <li>Depending on the context, a variable can be global or local:</li>
</ul>

<dl>
 <dt>global variable</dt>
 <dd>belongs to the global space. Global variables can be accessed by any {{Glossary("Function", "function")}}.</dd>
 <dt>local variable</dt>
 <dd>belongs to the function in which the variable is declared, and can be accessed only within that function, not from other functions or from the global space.</dd>
 <dt>global space</dt>
 <dd>the JavaScript code outside all functions</dd>
</dl>

<p>Example:</p>

<pre style="margin-left: 40px;">
var a = 1;    // 'a' will be global variable in browser, but a local variable in NodeJS (see notes to NodeJS)
b = 'a';      // 'b' will be global variable both in browser and NodeJS
function f1() {
  var a = 2;             // 'a' will be a local variable.  It hides the global variable-a.
  console.log(a);        // 2   accessing the local variable-a
  console.log(b);        // 'a' accessing the global variable-b
  var z = 'zzz';         // any variable declared within a function will never be accessed outside the function.
}
function f2() {
  var a = 3;             // local variable
  console.log(a)         // 3
  console.log(b)         // 'a'
  // Within any other function, there is no way to access the var z declared in f1()
}
f1();                    // calling function-f1
f2();                    // calling function-f2
console.log(f1.z);       // undefined.  The var-z declared also can never be accessed by code in the global space (not embedded by any function).
                         //             'f1.z' is not the coding to access the var-z in f1(). 'f1.z' has other meaning.
f1.z = 'zz??';           // 'f1.z' means a property of the function-f1.  
                         // It is OK to attach a property-z to any function like this.
console.log(f1.z);       // 'zz??'.   'f1' is a function and 'z' is its property created at previous line.   </pre>

<ul>
 <li>Assigning a value to an undeclared variable within a function will create a global variable. However, if the function has a line "use strict", it will throw an exception:</li>
</ul>

<pre style="margin-left: 40px; font-size: 14px; word-spacing: 0px;">
"use strict";  // "use strict" has no effect outside a function.
var a = "x";   // create a local variable-a.
b = "x";       // create a global variable-b.
function f1() {
    c = "x"    // create a global variable-c.
    "use strict";
    d = "x";   // throw exception
}
function f2() {
    e = "x"; // global variable-e is created.  The "use strict" in global space or f1() will not have an effect here.
}
f1();
f2();</pre>

<ul>
 <li>In the browser, the global space is called <code>window</code>, in NodeJS <code>global</code>. <code>window</code> and <code>global</code> are also global variables and have the value of an object. The object is known as <code>global-object</code>, because any global variable is a property of the <code>global-object</code>. &nbsp;See example below:</li>
</ul>

<pre style="margin-left: 40px;">
// file: a.js
b = 1;    // 'b' will be global variable.  It will also be a property of the 'global-object'.
function f1() {
  var b = 2;             // 'b' will be a local variable
  console.log(b)         // 2
  console.log(window.b)  // 1 in browser, but exception in NodeJS since 'window' is undefined.   
                         // 'window' is the variable name that has a value of an object.
                         // or, in short, 'window' is an object.  
                         // 'b' is a property of 'window'
  console.log(global.b)  // 1 in NodeJS, but exception in browser since 'global' is undefined.
                         // 'global' is an object in NodeJS.
}
f1();          
console.log(window.window === window) // true in browser, but exception in NodeJS.  
                                      // 'window.window' and 'window' are 
                                      // both pointing to the same value, 
                                      // which is the 'global-object'.

console.log(global.global === global) // true in NodeJS, but exception in browser.</pre>

<h3 id="notes_to_NodeJS.3A">notes to NodeJS:</h3>

<p style="margin-left: 40px;">File a.js has 2 lines:</p>

<pre style="margin-left: 40px;">
var a = 1; // 'a' will be a local variable in NodeJS, but a global variable in browser.
b = 'a';   // 'b' will be global variable in both NodeJS and browser.
</pre>

<pre style="margin-left: 40px;">
c:\&gt;node a.js </pre>

<p style="margin-left: 40px;">nodeJS will always embed the running *.js under a function of 5 parameters:</p>

<pre style="margin-left: 40px;">
function(exports, require, module, __dirname, __filename) {
    var a = 1;    // local variable
    b = 'a';      // global variable
}</pre>