How to determine whether a field exists in the passed JSON data in JS_javascript skills

How to determine whether a certain field exists in the JSON data passed in?

1.obj["key"] != undefined

This is defective. If the key is defined and the value is very often undefined, then there will be a problem with this sentence.

2.!("key" in obj)
3.obj.hasOwnProperty("key")

These two methods are better and recommended.

Original answer:

Actually, 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), usehasOwnProperty:

obj.hasOwnProperty("key") // true