How to set a javascript var as undefined

Question

Given:

console.log(boo); this outputs undefined

Given:

var boo = 1;
console.log(boo); this outputs 1

Aft

Answer 1

Solution

To reliably set a variable boo to undefined, use a function with an empty return expression:

boo = (function () { return; })();

After executing this line of code, typeof(boo) evaluates to 'undefined', regardless of whether or not the undefined global property has been set to another value. For example:

undefined = 'hello';
var boo = 1;
console.log(boo); // outputs '1'
boo = (function () { return; })();
console.log(boo); // outputs 'undefined'
console.log(undefined); // outputs 'hello'

EDIT But see also @Colin's simpler solution!

Reference

This behavior is standard as far back as ECMAScript 1. The relevant specification states in part:

Syntax

return [no LineTerminator here] Expression ;

Semantics

A return statement causes a function to cease execution and return a value to the caller. If Expression is omitted, the return value is undefined.

To view the original specifications, refer to:

• ECMAScript 5.1
• JavaScript (Mozilla)

Appendix

For completeness, I have appended a brief summary of alternate approaches to this problem, along with objections to these approaches, based on the answers and comments given by other responders.

1. Assign undefined to boo

boo = undefined; // not recommended

Although it is simpler to assign undefined to boo directly, undefined is not a reserved word and could be replaced by an arbitrary value, such as a number or string.

2. Delete boo

delete boo; // not recommended

Deleting boo removes the definition of boo entirely, rather than assigning it the value undefined, and even then only works if boo is a global property.