References

Beginner-friendly references for web development, with live, editable examples.

The JavaScript Logical Not ! operator

Operator JavaScript All modern browsers Updated
Quick answer

The logical NOT operator ! inverts a value to its opposite boolean — !true is false, and !0 is true. It first coerces the value to a boolean, then flips it. The double form !!value is a common idiom that converts any value to a real true/false.

Overview

! flips a value's truthiness. Put it before any value and you get the opposite boolean: !true is false, !"" is true (empty string is falsy), !user is true when user is missing. It evaluates the value's truthiness first, then negates it, so it always returns a real boolean.

The everyday use is negating a condition: if (!isValid) reads as "if not valid," and if (!arr.length) means "if the array is empty." It pairs with && and || to build up conditions, and binds tighter than both.

Doubling it — !!value — is a well-known idiom for converting to a boolean. The first ! negates (and coerces), the second negates back, leaving a clean true or false that matches the value's truthiness. !!"hello" is true, !!0 is false. It does the same thing as Boolean(value), just more tersely.

Syntax

!value

!true       // false
!0          // true   (0 is falsy)
!""         // true   (empty string is falsy)
!!"hello"   // true   (convert to boolean)
if (!isReady) { ... }  // negate a condition

Example

Live example 
<pre id="out" style="font:15px ui-monospace,monospace"></pre>
<script>
  const values = [0, '', 'hi', [], null];

  const lines = values.map(v =>
    JSON.stringify(v) + ' -> !!v = ' + (!!v)
  );

  document.getElementById('out').textContent = lines.join('\n');
</script>

Best practices

  • Use ! to negate a condition: if (!valid), if (!arr.length).
  • Use !!value (or Boolean(value)) to coerce any value to a real boolean.
  • Remember it coerces first — it works on any value, not just booleans.
  • It binds tighter than && and ||, so !a && b is (!a) && b.

Frequently asked questions

What does the ! operator do?
It inverts a value to its opposite boolean, coercing the value to truthy/falsy first. !true is false.
What does !! do in JavaScript?
It converts any value to a real boolean. The first ! negates, the second negates back, giving true for truthy values and false for falsy ones — the same as Boolean(value).
How do I check if an array is empty?
Use if (!arr.length) — an empty array has length 0, which is falsy, so !0 is true.
Is !!value the same as Boolean(value)?
Yes — both convert a value to its boolean truthiness. !!value is just a shorter, common idiom.
&& ||