var

var name1;
var name1 = value1;
var name1 = value1, name2 = value2;
var name1, name2 = value2;
var name1 = value1, name2, /* …, */ nameN = valueN;

nameN

The name of the variable to declare. Each must be a legal JavaScript identifier or a destructuring binding pattern.

valueN Optional

Initial value of the variable. It can be any legal expression. Default value is undefined.

The scope of a variable declared with var is one of the following curly-brace-enclosed syntaxes that most closely contains the var statement:

• Function body
• Static initialization block

Or if none of the above applies:

• The current module, for code running in module mode
• The global scope, for code running in script mode.

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, `y` is scoped to `bar`
}

foo();

Importantly, other block constructs, including block statements, try...catch, switch, headers of one of the for statements, do not create scopes for var, and variables declared with var inside such a block can continue to be referenced outside the block.

for (var a of [1, 2, 3]);
console.log(a); // 3

In a script, 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. 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.

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

The list that follows the var keyword is called a binding list and is separated by commas, where the commas are not comma operators and the = signs are not assignment operators. Initializers of later variables can refer to earlier variables in the list and get the initialized value.

var declarations, wherever they occur in a script, are processed before any code within the script 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, static initialization block, or script source in which it occurs.