TypeScript - destructing array after conversion with `as` operator causes compilation error: expression expected

Question

I'm using TypeScript 2.9.2.

A 3rd party library URI.js has a static method declared as:

joinPaths(...paths: (string | URI)[]): URI;

Now I have a variable named urlPaths declared as urlPaths: string | string[], the following code is giving me error [ts] Expression expected. at the spread operator:

URI.joinPaths(typeof urlPaths === 'string' ? urlPaths as string : ...(urlPaths as string[]))

But if I extract the ternary operator expression out as a separate variable, it is fine:

const paths = typeof urlPaths === 'string' ? [urlPaths as string] : urlPaths as string[];
URI.joinPaths(...paths);

What's wrong with my syntax here?

Titian Cernicova-Dragomir · Accepted Answer · 2018-07-30 10:11:13Z

1

Spread syntax is supported on arguments to functions, so your ... should be at the outer most level:

URI.joinPaths(... (typeof urlPaths === 'string' ? [urlPaths as string] : (urlPaths as string[])));

But also note that the asserions are redundant, typescript will figure out the type without them since typeof urlPaths === 'string' is a type guard and urlPaths: string | string[]

URI.joinPaths(... (typeof urlPaths === 'string' ? [urlPaths] : urlPaths));

answered Jul 30, 2018 at 10:11

Titian Cernicova-Dragomir

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