Truthy/falsy values

JavaScriptJunior~1 min read

Definition

In JavaScript, any value can be coerced to a logical type (boolean).

If after coercion it becomes true -> it is a truthy value.
If it becomes false -> it is a falsy value.

Easy to check:

javascript

Boolean(value) // returns true or false
!!value        // short equivalent

List of all falsy values (8 total)

Value	Description
`false`	false
`0`	the number zero
`-0`	negative zero
`0n`	BigInt zero
`""`	empty string
`null`	"nothing"
`undefined`	undefined
`NaN`	result of invalid numeric operations

Everything else in JavaScript is truthy.

Falsy examples

javascript

Boolean(false);      // false
Boolean(0);          // false
Boolean('');         // false
Boolean(null);       // false
Boolean(undefined);  // false
Boolean(NaN);        // false

Truthy examples

javascript

Boolean(true);        // true
Boolean(1);           // true
Boolean('0');         // true (string, not empty)
Boolean('false');     // true (also a string)
Boolean([]);          // true (an empty array is still truthy)
Boolean({});          // true (empty object)
Boolean(function(){});// true (function)

Example in an `if` condition

javascript

if ('hello') {
  console.log('Runs');
}

if (0) {
  console.log('Does not run');
}

Because 'hello' -> truthy, and 0 -> falsy.

Example with the `&&` operator

javascript

console.log(true && 'JS'); // "JS" (both truthy)
console.log(false && 'JS'); // false (falsy stops it)

Example with the `||` operator

javascript

console.log('' || 'default'); // "default"
console.log('Hi' || 'default'); // "Hi"

|| returns the first truthy value.

Example with double negation `!!`

javascript

!!'text'     // true
!!0          // false
!![]         // true
!!null       // false

Important: truthy is not "logically true" in the mathematical sense

JS does not "compare", it coerces values to boolean.

javascript

'false' == true   // false
!!'false'         // true

'false' is a string (truthy), but when compared to true, different logic applies (types are coerced first).

Summary

Type	Examples	Boolean value
Falsy	`false`, `0`, `-0`, `0n`, `""`, `null`, `undefined`, `NaN`	`false`
Truthy	`'0'`, `' '`, `'false'`, `[]`, `{}`, `function(){}`	`true`

In short

Truthy - "behaves like true" Falsy - "behaves like false"

Check:

javascript

if (value) {
  // runs if value is truthy
} else {
  // if value is falsy
}

Short Answer

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