A falsy value is a value that translates to false when evaluated in a Boolean context.
JavaScript uses type coercion in Boolean contexts.
Examples
Examples of falsy values in JavaScript (which translate to false and thus bypass the if block):
if (false)
if (null)
if (undefined)
if (0)
if (NaN)
if ('')
if ("")
if (document.all) [1]
[1] 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 } 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)