this.client = c;
this.clientFulladdress= `${this.client.Address || ""}, ${this.client.Town || "" }, ${this.client.County || ""}, ${this.client.PostCode || ""}`;
This is the way you can achieve the required.
If you wish not to use this.client multiple time you can also do as follows
this.client = c;
let {Address, Town, Country, PostCode} = this.client;
this.clientFulladdress = `${Address || ""}, ${Town || ""}, ${Country || ""}, ${PostCode || ""}`;
As per comment other way to do the same
this.client = c;
let {Address, Town, Country, PostCode} = this.client;
this.clientFulladdress = `${Address || ""}${Town ? ", " + Town : ""}${Country ? ", " + Country : ""}${PostCode ? ", " + PostCode : ""}`;