Why Does `for...in` Iteration Sometimes Include Unexpected Properties, and How Can I Avoid This?

Iterating Object Properties: Unveiling the Mysteries of Propt

When iterating through the properties of an object using the for...in loop, a variable known as propt emerges. This variable represents each property key of the object. But why does it work this way?

The reason is that the Object.prototype contains additional properties that are inherited by all objects. These properties are not part of the specific object but are shared by all objects. For instance, toString and hasOwnProperty are two common examples.

To avoid inadvertently iterating over these inherited properties, it's crucial to include an additional check: Object.prototype.hasOwnProperty.call(obj, prop). This check ensures that the property being iterated over belongs specifically to the object and has not been inherited from the base class.

For clarity, here's an updated code sample:

for (var prop in obj) {
    if (Object.prototype.hasOwnProperty.call(obj, prop)) {
        // do stuff
    }
}

Additionally, while it's possible to use obj.hasOwnProperty(prop) instead, this method may malfunction if the object has an unrelated field with the same name. As such, calling hasOwnProperty through Object.prototype is more reliable.

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