Typescript function taking one or array of objects

Question

We are using simple function declaration quite a lot where function takes either single object or array of objects of some type.

Simple declaration is:

interface ISomeInterface {
    name: string;
}

class SomeClass {
    public names: ISomeInterface[] = [];

    public addNames(names: ISomeInterface | ISomeInterface[]): void {
        names = (!Array.isArray(names)) ? [names] : names;
        this.names = this.names.concat(names);
    }    
}

But TypeScript throws "type is not assignable" error.

Is there better way of doing this? Obviously we could have two separate functions, but I think handling single vs multiple this way is quite fine.

Answer 1

You can make it easier

 addNames(names: ISomeInterface | ISomeInterface[]): void {         this.names = this.names.concat(names);  }  

From MDN

The concat() method returns a new array comprised of the array on which it is called joined with the array(s) and/or value(s) provided as arguments.

Answer 2

You could also use the rest parameter:

interface ISomeInterface {     name: string; }  class SomeClass {     public names: ISomeInterface[] = []; // create an instance if applicable.      addNames(...names: ISomeInterface[]): void {         // the names argument will always be an array         this.names = this.names.concat(names);     } } 

You can call it like:

addNames(name1); // just pass one addNames(name1, name2, name3); // pass more comma separated addNames(...[name1, name2, name3]); // pass an array. 

Please note that I removed the function keyword, because otherwise the this keyword inside the body block might lose scope depending on who's calling it.

Answer 3

I think this is what you want

interface ISomeInterface {
    name: string;
}

class SomeClass {
    public names: ISomeInterface[];

    addNames(names: ISomeInterface | ISomeInterface[]): void {
        names = (names instanceof Array) ? names : [names];
        this.names = this.names.concat(<ISomeInterface[]>names)
    }    
}

You want to use instanceOf, not isArray.

Answer 4

The official way typescript handles this is with multiple function signatures, for example:

addNames(names: ISomeInterface): void;
addNames(names: ISomeInterface[]): void;
addNames(names: any): void {
    ...
}

You can see more information in the official handbook here