JavaScript: Global Variable

Global variable is a variable whose scope is whole program. That is, it has meaning anywhere in the software.

Global variable can be created by:

• Variable name assigned but not declared. example: x = 3;. This is called implied global.
• Variable declared with var and it's not inside a function.

[see var Name Scope]

Every global variable name is also a global property name, and vice versa.

First, here's clarification of terms:

• This is name declaration: var x;
• This is variable assignment: x = 3;
• This is name declaration and assingment: var x = 3;

Behavior:

• declared and assigned → good.
• declared and unassigned → has default value of undefined.
• undeclared and assigned → implied global variable. (not exactly the same as declared global variable. Their configurable attribute's value is different.)
• undeclared and unassigned (and accessed) → ReferenceError.

Global Variable = Property of Global Object

Global variable is just property of the Global Object .

In web browser, the Global Object is window. [see Browser Window Object]

var xx = 3;
console.log(globalThis.xx === 3);
console.log(Reflect.has(globalThis, "xx"));

/* [
all true in browser
all false in deno
] */

[see Object Overview]

[see Check Property Existence]

Undeclared Variable

When a variable is assigned but not declared, it is a global variable. This is sometimes called implied global.

yy = 3;
console.log(window.yy === 3); // true
console.log(Reflect.has(window, "yy") === true); // true

Another example. This time inside a function.

function f() {
  zz = 4;
}
f();
console.log(zz === 4); // true
console.log(Reflect.has(window, "zz")); // true

Difference Between Implied Global and True Global Variable

• Undeclared global variable's property attribute configurable has value of true.
• Globar variable declared with var, its property attribute configurable has value of false.

[see Property Attributes]

// difference between declared and undeclared variable

var x = 1;
console.log(Object.getOwnPropertyDescriptor(window, "x"));
// Object {value: 1, writable: true, enumerable: true, configurable: false}

y = 1; // undeclared
console.log(Object.getOwnPropertyDescriptor(window, "y"));
// Object {value: 1, writable: true, enumerable: true, configurable: true}

(Note: as of 2014-07-10, in Firefox, if you run code in JavaScript console, undeclared var's configurable property is the same as declared global variable (presumably for programer convenience). But in Google Chrome's JavaScript console, it behaves by spec.)

Accessing Undeclared and Unassigned Variable → ReferenceError

• Accessing a undeclared variable that haven't been assigned is a ReferenceError.
• Accessing a declared variable that haven't been assigned gets a undefined value.

x;
// ReferenceError: x is not defined

// unassigned var has value of undefined
var y;
console.log(y === undefined);