How does the double exclamation (!!) work in javascript? [duplicate]

Question

I'm going through the Discover Meteor demo, and am struggling to figure out how exactly 'return !! userId;' works in this section:

Posts.allow({   insert: function(userId, doc) {   // only allow posting if you are logged in     return !! userId;   } }); 

Answer 1

! is the logical negation or "not" operator. !! is ! twice. It's a way of casting a "truthy" or "falsy" value to true or false, respectively. Given a boolean, ! will negate the value, i.e. !true yields false and vice versa. Given something other than a boolean, the value will first be converted to a boolean and then negated. For example, !undefined will first convert undefined to false and then negate it, yielding true. Applying a second ! operator (!!undefined) yields false, so in effect !!undefined converts undefined to false.

In JavaScript, the values false, null, undefined, 0, -0, NaN, and '' (empty string) are "falsy" values. All other values are "truthy."^(1):7.1.2 Here's a truth table of ! and !! applied to various values:

 value     │  !value  │  !!value ━━━━━━━━━━━┿━━━━━━━━━━┿━━━━━━━━━━━  false     │ ✔ true   │   false  true      │   false  │ ✔ true  null      │ ✔ true   │   false  undefined │ ✔ true   │   false  0         │ ✔ true   │   false  -0        │ ✔ true   │   false  1         │   false  │ ✔ true  -5        │   false  │ ✔ true  NaN       │ ✔ true   │   false  ''        │ ✔ true   │   false  'hello'   │   false  │ ✔ true