用于检查对象中属性的存在的“typeof”与“in”

Question

I have an object called foo . I want to check if it has the property bar .

Should I use this:

if (typeof foo.bar !== "undefined")

Or this:

if ("bar" in foo)

Is there any difference?

Answer 1

"typeof" does not care if the property exists or not and it will return undefined even if the property exists but has a value of "undefined"

While, "in" will return true if the property exists and has a value of "undefined"

For an example, the following would return either true or false depending on which you use:

let person = {
  name: 'John',
  age: undefined
}

console.log('age' in person)
// true
console.log(typeof person.age !=="undefined") 
//false

Answer 2

Your question is like "which one is the 'best' for checking the presence of a property in an object", therefore the latter ( "bar" in foo ) is what you're looking for.

The result of if("bar" in foo) is "only do this if there's a 'bar' property in foo" which what you're asking for.

You could also go for typeof foo.bar != "undefined" but it would be useless overkill.