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    A falsy value is a value that translates to false when evaluated in a Boolean context.

    JavaScript uses type coercion in Boolean contexts.

    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 (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:

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    Contributors to this page: fscholz, klez, Andrew_Pfeiffer, hbloomer
    Last updated by: Andrew_Pfeiffer,