Revisions to How do I check if an object has a specific property in JavaScript?

Good point about falsy values, but bad suggestion to use `typeof`.

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edited Dec 25, 2021 at 5:12

if (typeof x.key != "undefined") {

}

if(x.hasOwnProperty("key")){
  // …
}

Becausebecause

if (x.key)

if(x.key){
  // …
}

fails if x.key resolves to falseis falsy (for example, x.key ==== "").

if (typeof x.key != "undefined") {

}

Because

if (x.key)

fails if x.key resolves to false (for example, x.key = "").

if(x.hasOwnProperty("key")){
  // …
}

because

if(x.key){
  // …
}

fails if x.key is falsy (for example, x.key === "").

Crucial formatting, etc.

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edited Nov 23, 2014 at 13:47

if (typeof x.key != "undefined") {

}

Because

if (x.key)

fails if if x.key resolves to false (egfor example, x.key = "").

if (typeof x.key != "undefined"){

}

Because

if (x.key)

fails if x.key resolves to false (eg x.key = "").

if (typeof x.key != "undefined") {

}

Because

if (x.key)

fails if x.key resolves to false (for example, x.key = "").

added 28 characters in body

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edited May 20, 2011 at 13:01

if (typeof x.key != "undefined"){

if (typeof x.key != "undefined"){

}

} Because

if (x.key)

Because iffails (x.key) fails if x.keyif x.key resolves to falsefalse (eg x.key = ""x.key = "").

if (typeof x.key != "undefined"){

}

Because if (x.key) fails if x.key resolves to false (eg x.key = "").

if (typeof x.key != "undefined"){

}

Because

if (x.key)

fails if x.key resolves to false (eg x.key = "").

Source Link

answered Sep 25, 2008 at 19:39

werehamster

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