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TL;DR

• delete does not remove variables.
  (It's only for removing a property from an object.)

• The correct way to "unset" is to simply set the variable to null. _(source)
  (This enables JavaScript's automatic processes to remove the variable from memory.)

Example:

x = null;

More info:

Use of the delete operator on a variable is deprecated since 2012, when all browsers implemented (automatic) mark-and-sweep garbage-collection. The process works by automatically determining when objects/variables become "unreachable" (deciding whether or not the code still requires them).

With JavaScript, in all modern browsers:

• Garbage collection is performed automatically. We cannot force or prevent it.
• Objects are retained in memory while they are reachable.
• Being referenced is not the same as being reachable: a pack of interlinked objects can become unreachable as a whole. _(source)

The delete operator is only used to remove a property from an object; it does not remove variables.

Unlike what common belief suggests (perhaps due to other programming languages like delete in C++), the delete operator has nothing to do with directly freeing memory. Memory management is done indirectly via breaking references. _(source)

When using strict mode ('use strict';, as opposed to regular/"sloppy mode") an attempt to delete a variable will throw an error and is not allowed. Normal variables in JavaScript can't be deleted using the delete operator _(source) (or any other way, as of 2021).

...alas, the only solution:

Freeing the contents of a variable

To free the contents of a variable, you can simply set it to null:

var x;

// ...

x = null;    // (x can now be garbage collected)

^(source)

Further Reading:

• Memory Management (MDN Docs)
• Garbage Collection (Örebro University)
• The Very Basics of Garbage Collection (javascript.info)
• Understanding JavaScript Memory Management using Garbage Collection
• Eradicating Memory Leaks In Javascript

I am bit confused. If all you want is for a variables value to not pass to another script then there isn't any need to delete the variable from the scope.

Simply nullify the variable and then explicit check if it is or is not null. Why go through the trouble of deleting the variable from the scope? What purpose does this serve that nullifying can not?

foo = null;
if(foo === null) or if(foo !== null)