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• What an exclamation mark means in JS/TS
• Resources

What an exclamation mark means in JS/TS

!var: Returns false if its single operand can be converted to true; otherwise, returns true.

That means if var is either null, false or undefined, the result of !var would be true. And if var is something else, the result of !var is false.

Here is an example.

When the variable is not null, false or undefined, it does not execute the code in the condition.

var user = { name: 'K-Sato', age: 23 }

if (!user) {
  console.log('Not executed')
}

But if the variable is null, false or undefined, the code in the condition gets executed.

var user = null

if (!user) {
  console.log('Executed')
}

Resources

• Logical operators - JavaScript | MDN