Delete JavaScript function (declared with let) from chrome developer console

Question

I defined the following function in a chrome developer console-

 let splitNumber=function(num){ 
                  let arr=[];
                  arr=num.toString().split(""); 
                 return arr};

Then I tried to delete this function from the console scope using this command -

 delete window.splitNumber
  > true

Now if I try to use the same variable 'splitNumber' again in the console, I get this error -

Uncaught SyntaxError: Identifier 'splitNumber' has already been declared

Can someone please help me understand what I am doing wrong here? Please let me know if the question is not clear.

let doesn't work like var in this case :p – Jaromanda X Nov 09 '17 at 05:13 — Jaromanda X, Nov 09 '17 at 05:13

Chava Geldzahler · Accepted Answer · 2019-12-15T06:43:17.300

A property declared with `let` cannot be deleted

var, let, and const create non-configurable properties which cannot be deleted with the delete operator. Since you declared the function with let, it cannot be deleted.

Any property declared with let or const cannot be deleted from the scope within which they were defined.

Why does `delete` return true?

Generally, when a property is non-configurable and cannot be removed, delete will return false. So you may be wondering why your delete statement returns true.

Well, delete returns true if the property does not exist after deletion. This means it will return true either if the delete was successful or if the property never existed in the first place. Unlike var, let does not create a property on the global object. So delete window.splitNumber returns true because splitNumber does not exist as a property of the global window object.

If you try delete splitNumber, it will return false indicating that splitNumber exists in the global scope and cannot be deleted.

Thanks. I was not aware about it. Is there any MDN doc on this? — Shibasis Sengupta, Nov 09 '17 at 05:21
Yes, I linked to it in the answer - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/delete — Chava Geldzahler, Nov 09 '17 at 05:24

score 2 · Answer 2 · answered Nov 09 '17 at 05:27

When you declare a variable using let or const it wont be added to window object.

Your attempt to delete splitNumber on window object as no effect as it won't exists on window.

And the splitNumber still exits and it throws error when you try to redefine it.

score 0 · Answer 3 · answered Nov 09 '17 at 05:21

Redeclaring the same variable within the same function or block scope raises a SyntaxError.

if (x) {
  let foo;
  let foo; // SyntaxError thrown.
}

In ECMAScript 2015, let bindings are not subject to Variable Hoisting, which means that let declarations do not move to the top of the current execution context. Referencing the variable in the block before the initialization results in a ReferenceError (contrary to a variable declared with var, which will just have the undefined value). The variable is in a "temporal dead zone" from the start of the block until the initialization is processed.

Read the docs for more.

Delete JavaScript function (declared with let) from chrome developer console

3 Answers3

A property declared with let cannot be deleted

Why does delete return true?

A property declared with `let` cannot be deleted

Why does `delete` return true?