JavaScript var Declaration

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Syntax

var identifier;
var identifier = expression;
var identifier1 = expression1, identifier2 = expression2;

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Scope Semantics

• Function Scope: A var declared within a function is inaccessible outside of that function’s lexical environment.
• Module Scope: A top-level var declared within an ES module or CommonJS module is scoped strictly to that module’s environment record, not the global environment.
• Lack of Block Scope: Unlike let and const, var ignores block boundaries (such as those created by if, for, or while statements). A var declared inside a block is scoped to the containing function, module, or global environment.

function scopeExample() {
    if (true) {
        var x = 10; 
    }
    console.log(x); // Outputs: 10 (x is scoped to scopeExample, not the block)
}

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Hoisting and Initialization

• Declaration Hoisting: The identifier is registered in the environment record before any code is executed.
• Default Initialization: Hoisted var variables are automatically initialized with the primitive value undefined. The actual assignment expression remains in its original location and is evaluated during the execution phase.

console.log(a); // Outputs: undefined (ReferenceError is not thrown)
var a = 5;
console.log(a); // Outputs: 5

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Redeclaration

var b = 1;
var b = 2; // Valid syntax, overwrites previous value
var b;     // Ignored, b remains 2

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Global Object Binding and Modules

// In a traditional browser script:
var c = 100;
console.log(window.c); // Outputs: 100

// In an ES module (<script type="module">) or Node.js module:
var d = 200;
console.log(globalThis.d); // Outputs: undefined