Falsy - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

A falsy (sometimes written falsey) value is a value that is considered false when encountered in a Boolean context.

JavaScript uses type conversion to coerce any value to a Boolean in contexts that require it, such as conditionals and loops.

There are 8 falsy values:

`false`	The keyword false
`0`	The number zero
`-0`	The number negative zero
`0n`	BigInt, when used as a boolean, follows the same rule as a Number. `0n` is falsy.
`""`	Empty string value
null	null - the absence of any value
undefined	undefined - the primitive value
NaN	NaN - not a number

Objects are falsy if and only if they have the [[IsHTMLDDA]] internal slot. This slot only exists in document.all and cannot be set using JavaScript.

Examples

Examples of falsy values in JavaScript (which are coerced to false in Boolean contexts, and thus bypass the if block):

if (false)
if (null)
if (undefined)
if (0)
if (-0)
if (0n)
if (NaN)
if ("")

If the first object is falsy, it returns that object

false && "dog"
// ↪ false

0 && "dog"
// ↪ 0

Specification
ECMAScript (ECMA-262) The definition of '`ToBoolean` abstract operation' in that specification.

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