The for...in statement allows you to loop through the names of all of the properties of an object. Unfortunately, the list of
properties includes all those properties that were inherited through the prototype chain. This has the bad side effect of serving up functions when
the interest is in data properties. Programs that don't take this into account can fail.
Therefore, the body of every for...in statement should be wrapped in an if statement that filters which properties are
acted upon. It can select for a particular type or range of values, or it can exclude functions, or it can exclude properties from the prototype.
for (name in object) {
doSomething(name); // Noncompliant
}
for (name in object) {
if (object.hasOwnProperty(name)) {
doSomething(name);
}
}
Loops used to clone objects are ignored.
for (prop in obj) {
a[prop] = obj[prop]; // Compliant by exception
}