How do I check if a particular key exists in a JavaScript object or array?
If a key doesn't exist, and I try to access it, will it return false? Or throw an error?
转载于:https://stackoverflow.com/questions/1098040/checking-if-a-key-exists-in-a-javascript-object
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Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?
var obj = { key: undefined };
obj["key"] != undefined // false, but the key exists!
You should instead use the in operator:
"key" in obj // true, regardless of the actual value
If you want to check if a key doesn't exist, remember to use parenthesis:
!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj // ERROR! Equivalent to "false in obj"
Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:
obj.hasOwnProperty("key") // true
For performance comparison between the methods that are in, hasOwnProperty and key is undefined, see this benchmark
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