What are the Differences in Javascript Variable Declaration Syntaxes and Their Implications?

Clarifying Differences in Variable Declaration Syntaxes in Javascript

In Javascript's global scope, declaring variables with different syntaxes can indeed lead to subtle variations.

Regarding your options:

1. var a = 0;

Declaring a variable with "var" creates a global variable that also exists as a property of the global object (e.g., "window" in browsers or "globalThis" in ES2020). This property cannot be removed using "delete" as it is an identifier binding.

2. a = 0;

Caution: This syntax is strongly discouraged. Without the "var" or "let" keyword, Javascript considers this an implicit global variable, but it becomes an error in strict mode. As such, it is not a recommended practice.

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

This syntax explicitly assigns a property to the global object. Unlike "var," these properties are removable with "delete."

4. this.a = 0;

Assigning to "this" creates a global property, but it is not recommended as it can lead to ambiguity in event handlers.

Additional ES2015 Syntax:

1.1 let a = 0;

"let" declares a global variable that is not a property of the global object. The identifier binding is created at the beginning of the enclosing block but becomes accessible only when the code execution reaches the "let" declaration.

1.2 const a = 0;

Similar to "let," "const" declares a global constant that is not a property of the global object. However, the constant's value cannot be changed.

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