Typescript: What is the difference between if(variable), if(!variable) and if(!!variable)?

Question

In typescript, what is the difference between
if (x)

if (!x)

if (!!x)

Could this have different behaviors depends on the type of variable? (string, number, object, etc...).
What I mean with the sentence above is:
const myString = 'hello';
const myObject = new MyClass(10, 'Luke');
const ten = 10;

if(myString) 
if(!myString)
if(!!myString)

if(myObject)
if(!myObject)
if(!!myObject)

if(ten)
if(!ten)
if(!!ten)

In this example, when the code enters in one if instead of another?

Nikita Madeev · Answer

Typescript has nothing to do with it, it's javascript.
if...else

The if statement executes a statement if a specified condition is
truthy. If the condition is falsy, another statement can be executed.

! - logical not operator, can convert value to inverted boolean
!! - convert value to boolean (analog Boolean)

falsy values: number 0, BigInt 0n, null, undefined, boolean false, number NaN, string ''


const myString = 'hello';
const myObject = { test: 123 };
const ten = 10;

console.log(myString);
console.log(!myString);
console.log(!!myString, Boolean(myString));

console.log(myObject);
console.log(!myObject);
console.log(!!myObject, Boolean(myObject));

console.log(ten);
console.log(!ten);
console.log(!!ten, Boolean(ten));

Typescript: What is the difference between if(variable), if(!variable) and if(!!variable)?

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