How Do I Delete a JavaScript Variable?

How to Remove a JavaScript Variable

Overview

In JavaScript, global variables can be created without explicit declaration using the var keyword. If you wish to erase a global variable's existence entirely, the approach to take depends on how it was defined.

Unsetting Variables

Variables Created with var

Global variables declared with var cannot be unset using the delete operator. This is because they are stored in the VariableEnvironment, where references are not typically deletable.

Variables Created Without var

Global variables created without using var are considered properties of the global object (usually window). To remove such variables, you can use the delete operator. This will remove the property and its associated value.

Technical Explanation

Using var

When variables are declared with var, they are added to the VariableEnvironment attached to the current scope. This environment contains references to variables, which cannot be removed.

Without Using var

When assigning a value to a global variable without using var, JavaScript searches for the reference in the LexicalEnvironment. If not found, it looks in the parent LexicalEnvironment. The top-level LexicalEnvironment is bound to the global object, so if the variable is not found in any scope, it becomes a property of the global object.

Notes

• var declarations are "hoisted" to the beginning of their scope.
• "Strict mode" affects variable lookup rules, potentially resulting in "undeclared variable" errors.

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