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The logical NOT (!) is a unary operator that evaluates its operand, coerces the resulting value to a boolean primitive, and returns the logical complement of that boolean. If the operand is truthy, it returns false; if the operand is falsy, it returns true. 
!expression

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Mechanics and Type Coercion

When the JavaScript engine evaluates the ! operator, it executes the following sequence:
  1. Evaluates the provided expression.
  2. Applies the internal ECMAScript ToBoolean abstract operation to the evaluated result.
  3. Inverts the resulting boolean value.
Because JavaScript employs implicit type coercion, the ! operator does not require a boolean operand. The language’s truthiness and falsiness rules allow the operator to accept any data type and force coercion to a boolean primitive.

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Falsy Evaluation

Applying ! to any falsy value strictly yields true. JavaScript defines exactly eight falsy values. The operator evaluates all of the following to true: 
!false      // true
!0          // true (Number zero)
!-0         // true (Negative zero)
!0n         // true (BigInt zero)
!""         // true (Empty string)
!null       // true (Absence of any object value)
!undefined  // true (Uninitialized variable)
!NaN        // true (Not a Number)

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Truthy Evaluation

Applying ! to any value not present in the falsy list yields false. This includes all objects, arrays, and functions, regardless of whether they are empty. 
!true           // false
!1              // false (Non-zero number)
!"hello"        // false (Non-empty string)
![]             // false (Empty array is an object, therefore truthy)
!{}             // false (Empty object is truthy)
!function(){}   // false (Functions are objects, therefore truthy)

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Operator Precedence

The ! operator has a very high precedence, sitting below grouping () and member access, but above all arithmetic, relational, and equality operators. It associates right-to-left. Because of this high precedence, it binds tightly to its immediate operand. 
// Evaluates as !(typeof x) === "object"
// !(typeof x) becomes false, so false === "object" evaluates to false
!typeof x === "object" 

// Evaluates the equality first, then inverts the boolean result
!(typeof x === "object")

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Double NOT (!!)

Stacking two logical NOT operators (!!) is syntactically valid. The first ! coerces the operand to a boolean and inverts it. The second ! inverts it back. Mechanically, this exposes the exact result of the internal ToBoolean operation without the final inversion. 
!!1         // true  (!1 is false, !false is true)
!!0         // false (!0 is true, !true is false)
!!"text"    // true
!!null      // false
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