Description

var declarations, wherever they occur, are processed before any code is executed. This is called hoisting and is discussed further below.

The scope of a variable declared with var is its current execution context and closures thereof, which is either the enclosing function and functions declared within it, or, for variables declared outside any function, global. Duplicate variable declarations using var will not trigger an error, even in strict mode, and the variable will not lose its value, unless another assignment is performed. 

'use strict';
function foo() {
  var x = 1;
  function bar() {
    var y = 2;
    console.log(x); // 1 (function `bar` closes over `x`)
    console.log(y); // 2 (`y` is in scope)
  }
  bar();
  console.log(x); // 1 (`x` is in scope)
  console.log(y); // ReferenceError in strict mode, `y` is scoped to `bar`
}

foo();

Variables declared using var are created before any code is executed in a process known as hoisting. Their initial value is undefined. 

'use strict';
console.log(x);                // undefined (note: not ReferenceError)
console.log('still going...'); // still going...
var x = 1;
console.log(x);                // 1
console.log('still going...'); // still going...

In the global context, a variable declared using var is added as a non-configurable property of the global object. This means its property descriptor cannot be changed and it cannot be deleted using delete. The corresponding name is also added to a list on the internal [[VarNames]] slot on the global environment record (which forms part of the global lexical environment). The list of names in [[VarNames]] enables the runtime to distinguish between global variables and straightforward properties on the global object.

The property created on the global object for global variables, is set to be non-configurable because the identifier is to be treated as a variable, rather than a straightforward property of the global object. JavaScript has automatic memory management, and it would make no sense to be able to use the delete operator on a global variable. 

'use strict';
var x = 1;
Object.hasOwn(globalThis, 'x'); // true
delete globalThis.x; // TypeError in strict mode. Fails silently otherwise.
delete x;  // SyntaxError in strict mode. Fails silently otherwise.

Note that in both NodeJS CommonJS modules and native ECMAScript modules, top-level variable declarations are scoped to the module, and are not, therefore added as properties to the global object.


Unqualified identifier assignments

The global object sits at the top of the scope chain. When attempting to resolve a name to a value, the scope chain is searched. This means that properties on the global object are conveniently visible from every scope, without having to qualify the names with globalThis. or window. or global..

Because the global object has a String property (Object.hasOwn(globalThis, 'String')), you can use the following code:

function foo() {
  String('s') // Note the function `String` is implicitly visible
}

So the global object will ultimately be searched for unqualified identifiers. You don't have to type globalThis.String, you can just type the unqualified String. The corollary, in non-strict mode, is that assignment to unqualified identifiers will, if there is no variable of the same name declared in the scope chain, assume you want to create a property with that name on the global object. 

foo = 'f' // In non-strict mode, assumes you want to create a property named `foo` on the global object
Object.hasOwn(globalThis, 'foo') // true

In strict mode, assignment to an unqualified identifier in strict mode will result in a ReferenceError, to avoid the accidental creation of properties on the global object.

Note that the implication of the above, is that, contrary to popular misinformation, JavaScript does not have implicit or undeclared variables, it merely has a syntax that looks like it does.


var hoisting

Because var declarations are processed before any code is executed, declaring a variable anywhere in the code is equivalent to declaring it at the top. This also means that a variable can appear to be used before it's declared. This behavior is called "hoisting", as it appears that the variable declaration is moved to the top of the function or global code. 

bla = 2;
var bla;

This is implicitly understood as:

var bla;
bla = 2;

For that reason, it is recommended to always declare variables at the top of their scope (the top of global code and the top of function code) so it's clear which variables are function scoped (local) and which are resolved on the scope chain.

It's important to point out that only a variable's declaration is hoisted, not its initialization. The initialization happens only when the assignment statement is reached. Until then the variable remains undefined (but declared): 

function do_something() {
  console.log(bar); // undefined
  var bar = 111;
  console.log(bar); // 111
}

This is implicitly understood as:

function do_something() {
  var bar;
  console.log(bar); // undefined
  bar = 111;
  console.log(bar); // 111
}