问题
I am aware that the for in loop can help iterate through properties of objects, prototypes and collections.
The fact is, I need to iterate over String.prototype, and though console.log(String.prototype) displays the complete prototype, when I do
for (var prop in String.prototype) {
console.log(prop);
}
to display the name of the elements in the prototype, it displays nothing, as if it were empty.
Do the JavaScript engines hide the basic prototypes methods, or am I doing something wrong?
回答1:
The specification says:
If the value of an attribute is not explicitly specified by this specification for a named property, the default value defined in Table 7 is used.
Table 7 — Default Attribute Values
...
[[Enumerable]] false
So it is not enumerable (as with all built-in properties).
回答2:
Like others have said, all properties in String.prototype are non-enumerable. To get a list of all the properties, including non-enumerable, use Object.getOwnPropertyNames() (newer browsers only)
回答3:
Native methods aren't visible through an for(prop in obj) iteration.
It's possible to find properties when you loop through a built-in object. In this case, the page has extended the prototype with a custom method. Frameworks (such as jQuery) often modify built-in objects in this way.
来源：https://stackoverflow.com/questions/7770869/iterate-over-string-prototype