What is the meaning of exclamation mark in JavaScript

In JavaScript, the exclamation point "!" refers to the logical NOT operator, which is a Boolean inversion operation. It can be placed directly before the operand. The syntax is "! Operand"; "!" operation operator will convert the operand value to a Boolean value, then negate it and return it.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

In JavaScript, the exclamation mark "!" refers to the logical NOT operator.

Operator	Name	Example
!	Logical NOT	!x means if x is not true, then it is true

logical NOT operation! is a Boolean negation operation (NOT). As a unary operator, it is placed directly before the operand, converts the value of the operand to a Boolean value, then inverts and returns it.

Example 1

The following lists some logical NOT operation return values ​​of special operands.

console.log( ! {} );  //如果操作数是对象，则返回false
console.log( ! 0 );  //如果操作数是0，则返回true
console.log( ! (n = 5));  //如果操作数是非零的任何数字，则返回false
console.log( ! null );  //如果操作数是null，则返回true
console.log( ! NaN );  //如果操作数是NaN，则返回true
console.log( ! Infinity );  //如果操作数是Infinity，则返回false
console.log( ! ( - Infinity ));  //如果操作数是-Infinity，则返回false
console.log( ! undefined );  //如果操作数是undefined，则返回true

Example 2

If you perform two logical NOT operations on the operand, it is equivalent to converting the operand into a Boolean value.

console.log( ! 0 );  //返回true
console.log( ! ! 0 );  //返回false

Note: The return value of logical NOT operation must be a Boolean value.

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