tslint / codelyzer / ng lint error: “for (… in …) statements must be filtered with an if statement”

To explain the actual problem that tslint is pointing out, a quote from
the JavaScript documentation of the for…in statement:

The loop will iterate over all enumerable properties of the object
itself and those the object inherits from its constructor’s prototype
(properties closer to the object in the prototype chain override
prototypes’ properties).

So, basically this means you’ll get properties you might not expect to get (from the object’s prototype chain).

To solve this we need to iterate only over the objects own properties. We can do this in two different ways (as suggested by @Maxxx and @Qwertiy).

First solution

for (const field of Object.keys(this.formErrors)) {
    ...
}

Here we utilize the Object.Keys() method which returns an array of a given object’s own enumerable properties, in the same order as that provided by a for…in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

Second solution

for (var field in this.formErrors) {
    if (this.formErrors.hasOwnProperty(field)) {
        ...
    }
}

In this solution we iterate all of the object’s properties including those in it’s prototype chain but use the Object.prototype.hasOwnProperty() method, which returns a boolean indicating whether the object has the specified property as own (not inherited) property, to filter the inherited properties out.