Why doesn't Object.getOwnPropertyNames() list all properties and methods?
Question
I am trying to extract all the properties and methods from an object which happens to be a string: var str = "Hello World!"
If I use the command Object.getOwnPropertyNames(str) I get a list of properties and methods: ["0", "1", "2", "3", "length"] . However I know there are other methods like .toUpperCase() which belong to the string object but they are not listed.
My question : why is the method .toUpperCase() not listed? What can I do to list it out with many others ( .indexOf() ...)?
Here is the code:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Display properties and methods from objects</title>
</head>
<body>
<script type="text/javascript">
var str= 'Hello World!'
var listPropertiesMethods = Object.getOwnPropertyNames(str)
console.log(listPropertiesMethods);
</script>
</body>
</html>
3 answers
solution1
6 ACCPTED 2018-01-05 13:06:20
Cause the properties you list ( indexOf , ...) are not part of the string object itself, but are rather part of its prototype:
Object.getOwnPropertyNames(
Object.getPrototypeOf("str")
)
solution2
3 2018-01-05 13:08:05
You have just look at the proto-type of your object and you get what you want:
console.log(str.__proto__);
solution3
0 2018-01-05 13:07:12
The Object.getOwnPropertyNames(obj) method returns only the properties of the obj (like length). You can use Object.getPrototypeOf(obj) to get a more complete list of methods/properties.
Ex.:
Object.getOwnPropertyNames("Test")
(5) ["0", "1", "2", "3", "length"]
Object.getPrototypeOf("Test")
String {"", formatUnicorn: ƒ, truncate: ƒ, splitOnLast: ƒ, contains: ƒ, length: 0, …}
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