Angular - variable undefined

Question

I am writing a service but am encountering a problem.

The service contains a variable that is implementing an interface. When the service is first created the variable holds the value of undefined.

To explain it with less code I came up with the following example.

This is an interface.

interface objectWithSensors {
  sensor1: number[];
  sensor2: number[];
}

The service has a varaible of this type declared as:

private sensorObject: objectWithSensors;

At this point in time this object is undefined.

Is there a way to define it at this point (put an object in there that I can later modify) or should I change the interface to a class with public attributes?

You should initialize sensorObject with a value. You need to define a class that implements this interface. You then instantiate an object of this class, for example in your service's constrictor, or you use dependency injection to assign it. — CodeCaster
– CodeCaster, Commented Aug 28, 2019 at 13:00
That's how I was told to use interfaces in a conventional way as well. But from what I understand Angular uses them to make binding to properties more easy. Declaring a variable of that interface type makes it possible to do stuff like sensorObject.sensor1.push(1). In short the variable implements the interface and you do not need an entire class. — SecretIndividual
– SecretIndividual, Commented Aug 28, 2019 at 13:04
Right, but you can't define default values for members in an interface. You need to either create a class that implements the interface, which provides the default, or you need to provide the default values when instantiating the variable. — Rich
– Rich, Commented Aug 28, 2019 at 13:14

ukn · Accepted Answer · 2019-08-28 12:52:46Z

2

Use a class if you want to have a default value. Otherwise, do something like that:

sensorObject.sensor1 = [];
sensorObject.sensor2 = [];

answered Aug 28, 2019 at 12:52

ukn

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2 Comments

SecretIndividual Over a year ago

I am trying to prevent the use of Classes here. I was thinking the same about it as your second option but did not want to pollute the space I am declaring all attributes in. Guess I could also just put the instantiation in a function and call that before I need the variable.

ukn Over a year ago

You can do it as a one liner just like the other answer if you don't want a class but it gets annoying quickly if you have to build the whole object just to add default value every time you want to use it.

Will Alexander · Accepted Answer · 2019-08-28 12:52:47Z

2

You can simply instantiate the object:

private sensorObject: objectWithSensors = {
  sensor1: [],
  sensor2: []
}

answered Aug 28, 2019 at 12:52

Will Alexander

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1 Comment

SecretIndividual Over a year ago

Trying to keep the space I use for declaring attributes as clean as possible. I just realized I could still use this technique by moving the instantiation to a function and call that before I use it.

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