"Cannot read property 'name' of undefined" when it should be defined

Question

interface IField { readonly name : string; readonly value : string; readonly inline ?: boolean; } class Field implements IField { readonly name : string; readonly value : string; readonly inline ?: boolean; constructor(data : IField) { this.name = data.name; this.value = data.value; this.inline = data.inline; } }

When trying to initialize a new variable from the Field class inside another class like so,

class Embed implements IEmbed { ... constructor(data : IEmbed) { ... this.fields = data.fields.map((field) => new Field(field)); ... } }

My program crashes with the error,

 this.name = data.name; ^ TypeError: Cannot read property 'name' of undefined

If I log the value of data inside the constructor(...) method for Field, it prints out the object without any issue (changed values for privacy),

{ value: '...message...', name: '...name...', inline: false }

But it still crashes. The value given to the Embed constructor is a parsed JSON object from a websocket, and thus the same is true for the Field constructor.

The interface for Embed, although I just pass the JSON data from the websocket to the constructor's data parameter for it. — Aidan
– Aidan, Commented Sep 26, 2021 at 15:13
We know it's an interface but it's definition is missing. As for the error, if a field is null, the copy constructor will fall like yours does. — Wiktor Zychla
– Wiktor Zychla, Commented Sep 26, 2021 at 15:25

Aidan · Accepted Answer · 2021-09-26 15:50:39Z

Ended up using the solution from this question.

class Field implements IField { readonly name : string; readonly value : string; readonly inline ?: boolean; constructor(data : Partial<Field> = {}) { Object.assign(this, data); } }

I replaced the data : any portion with data : Partial<Field> = {}, providing a default for the field. Thanks to Wiktor for steering me in the right direction.

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