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A falsy 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.

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 (NaN)
if ('')
if ("")
if (``)
if (document.all)

document.all has been used for browser detection in the past and the HTML specification defines a willful violation of the ECMAScript standard here to keep compatibility with legacy code (if (document.all) { // Internet Explorer code here(except IE11) } or using document.all without checking its presence first: document.all.foo).

Sometimes written falsey, although in English usually turning a word into an adjective with a -y, any final e is dropped (noise => noisy, ice => icy, shine => shiny)

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