TypeScript allow for optional arguments - overloading methods

Question

I have this method signature on a class:

lock(key: string, opts: any, cb?: LMClientLockCallBack): void;

if a user uses it like so:

lock('foo', null, (err,val) => {

});

they will get the right typings. However, if they omit the options argument and do this:

lock('foo', (err,val) => {

});

then tsc sees the callback function as type any, just like this:

Is there any way to allow users to avoid passing an empty object or null as the second argument, and shift the callback over?

I tried overloading the method, into two definitions:

  lock(key: string, cb: LMClientLockCallBack, n?: LMClientLockCallBack) : void;

  lock(key: string, opts: any, cb?: LMClientLockCallBack) { ... }

but it still doesn't compile, there are new problems:

and if I try this:

  lock(key: string, cb: LMClientLockCallBack) : void;

  lock(key: string, opts: any, cb?: LMClientLockCallBack) { ... }

I get this:

Surely there must be a solution to this one?

score 15 · Accepted Answer · answered Jul 06 '18 at 05:13

When overloading the method in TypeScript, the implementation doesn't count as one of overloads. You should thus create three function definitions: two ones for different overloads, as you've already done, and the third - with optional argument and real implementation, which is in your code erroneously connected to the second overload definition. So you'll have:

lock(key: string, cb: LMClientLockCallBack) : void;
lock(key: string, opts: any, cb: LMClientLockCallBack): void;

lock(key: string, opts: any, cb?: LMClientLockCallBack) { ... }

So, if the second argument is a callback - user will get the first overload typings, if it is anything else, but the third argument is a callback - the second overload typings. But in both cases they will call the same implementation, just like it would be in pure JavaScript.

`the implementation doesn't count as one of overloads.` that single statement saved my day..... — Antoniossss, Dec 31 '20 at 14:42

TypeScript allow for optional arguments - overloading methods

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