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How do I check if an object has a specific property in JavaScript?

I found the following snippet in Twitter's JavaScript files. Why do they need to call the hasOwnProperty function to see dict has the key property? The for loop is running for each 'key' in 'dict' which means 'dict' has 'key'. Am I missing a point?

function forEach(dict, f) {

    for (key in dict) {

        if (dict.hasOwnProperty(key))

            f(key, dict[key]);

    }

}