JavaScript: The Good Parts

for in Statement

The for in statement allows for looping through the names of all of the properties of an object. Unfortunately, it also loops through all of the members that were inherited through the prototype chain. This has the bad side effect of serving up method functions when the interest is in the data members.

The body of every for in statement should be wrapped in an if statement that does filtering. if can select for a particular type or range of values, it can exclude functions, or it can exclude properties from the prototype. For example:

for (name in object) {
    if (object.hasOwnProperty(name)) {
        ....
    }
}