not condition fails (jQuery)

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Check if element exists in jQuery [duplicate] 8 answers

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You need to check the number of elements found, as jQuery returns an empty collection in the event of no elements being found (which evaluates to a truthy value in the if statement):

if ( !$(".first.last").length )

I'd suggest testing against 0 explicitly, rather than relying on a falsey value, though:

if ( $(".first.last").length === 0 )

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您可以通过检查从JQuery返回的第一个对象来简化它：

if (!$(".first.last")[0]){
    console.log("div with both first and last classes does not exists");
} else {
  console.log("it exists");
}

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there are two ways:

1. check the length:

if ( !$(".first.last").length ) { // check the length if none returns 0 == false

2. check the visibility:

if ( !$(".first.last").is(':visible') ) { // returns boolean true/false

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