How Do Different JavaScript Variable Declaration Syntaxes Affect Global Scope?

Differentiating Variable Declaration Syntaxes in JavaScript, Including Global Variables

In JavaScript, when declaring variables in the global scope, there are subtle but significant differences between various syntaxes. Let's explore these differences:

1. var vs. let vs. const

• var a = 0;: Creates a global variable which is also a property of the global object (accessible as window in browsers). This syntax has been deprecated due to potential unintended side effects and variable hoisting issues.
• let a = 0; (ES2015 ): Creates a global variable that is not a property of the global object. It introduces a "Temporal Dead Zone (TDZ)" where the variable is not accessible until the let statement is encountered.
• const a = 0; (ES2015 ): Creates a global constant, which is also not a property of the global object. Const bindings have additional restrictions: you must provide an initializer, and the value cannot be changed at runtime.

2. a = 0;

Strongly discouraged, this syntax declares a global variable without explicit declaration. It is considered an error in strict mode.

3. window.a = 0; vs. globalThis.a = 0;

• window.a = 0: Equivalent to a = 0 when used in global scope. It creates a global property on the window object.
• globalThis.a = 0: A more modern alternative to window.a that works in all environments (including non-browser contexts).

4. this.a = 0;

Creates a property on the global object (when used in global scope). However, this syntax is not recommended as it can be ambiguous in certain situations.

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