What are the Differences in Javascript Variable Declaration Syntaxes (Including Global Variables)?

Differences in Variable Declaration Syntax in Javascript (Including Global Variables)

Introduction

Javascript provides multiple syntaxes for declaring variables, including the var, let, and const keywords. These syntaxes differ in their behavior, scope, and interaction with the global object.

Differences in Syntax and Behavior

1. var a = 0;

• Declares a global variable, which becomes a property of the global object (window on browsers or potentially this on other platforms).
• The variable binding is created before any code runs, making it accessible throughout the global scope.
• The property created is enumerable.

1.1 let a = 0; (Introduced in ES2015)

• Declares a global variable within the global environment but not as a property of the global object.
• The variable binding is created before code execution, but the variable is inaccessible until the let statement is reached (Temporal Dead Zone).
• The variable is block-scoped, unlike var.

1.2 const a = 0; (Introduced in ES2015)

• Similar to let but declares a constant (immutable) variable.
• Must be assigned an initial value.
• Prevents accidental reassignment.

2. a = 0;

• Attempts to declare a global variable implicitly without using a keyword.
• Strongly discouraged and may lead to errors, especially in strict mode.

3. window.a = 0;

• Explicitly assigns a property to the global object, effectively creating a global variable.
• Can also be written as globalThis.a = 0; (introduced in ES2020) or this.a = 0; within the global scope.

4. this.a = 0;

• Similar to window.a = 0; but uses this to access the global object.
• Can be used for backwards compatibility or to avoid potential issues with minification or other optimizations.

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