Initialised property in class instance is "undefined" if set to zero or an empty string

Question

I have an instance of a class, in which several number and string properties are initialized to 0 or "" respectively. When accessing these properties they are "undefined". Initializing these properties to anything else, ie 0.1 or " " and it is considered defined.

Why? Are 0 and "" equivalent to undefined?

export class Car {
     id = 0
     name = ""
}

If I have an instance of Car and try to access a property it will be "undefined",

let myCar = new Car
if (myCar.id) {
    console.log('yay')
} else {
    console.log('boo')
}

It will show 'boo'.

0 and empty string are default values for those types respectively and will evaluate to false. change `if(myCar.id)` to `if(myCar.id !== undefined)` — DetectivePikachu, Aug 08 '19 at 20:29

score 2 · Accepted Answer · answered Aug 08 '19 at 20:28

In javascript, the value 0 is evaluated as a falsy statement. Same for an empty string "". If you want to test if the property is undefined you need to do so explicitely

let myCar = new Car
if (myCar.id !== undefined) {
    console.log('yay')
} else {
    console.log('boo')
}

Initialised property in class instance is "undefined" if set to zero or an empty string

1 Answers1