How to WRITE DECLARATION for function with only one array as first parameter or spread of elements

Question

Consider this function from webpack-merge

smart(...configuration | [...configuration])

It can accept one and only one Configuration[] or ...args: Configuration[] and returns Configuration.

What type does this function have?

UPDATE:

I want to make typings for webpack-merge.

This is what I have so far...

declare module 'webpack-merge' {

  import { Configuration } from 'webpack'

  type StrategyType = 'prepend' | 'append' | 'replace'
  type StrategyMap = { [feild: string]: StrategyType }
  type ConfigurationFolder = (
    conf?: Configuration[], 
    ...args: Configuration[]) => Configuration

  export const merge: ConfigurationFolder
  export const multiple: ConfigurationFolder
  export const smart: ConfigurationFolder

  export const smartStrategy: (options: StrategyMap) => ConfigurationFolder
  export const unique: ConfigurationFolder

  export default merge
}

...but it doesnt work as expected.

Here is the test:

import * as webpackMerge from 'webpack-merge'

const webpackConfig = webpackMerge.smartStrategy({ entry: 'replace' })(
    configs,
    baseConfig 
    # must fail here: only one `Configuration[]` as first param,
    # or only `Configuration, Configuration, Configuration, ....`
  )

UPDATE2

What I want:

class Configuration { }
type Func = (...configuration: Configuration[]) => Configuration;

// usage
const myFunc: Func = (...configuration: Configuration[]) => {
  return new Configuration();
}

const conf = new Configuration;
const confs = [conf];

const r1 = myFunc(confs); // right
const r2 = myFunc(conf); // right
const r3 = myFunc(conf, conf); // right

const f1 = myFunc(confs, confs); // must fail, cause can accept only one array
const f2 = myFunc(conf, confs); // must fail, cause or array, or spread
const f3 = myFunc(confs, conf); // must fail, same

Here is a playable version of what I want.

UPDATE3

spin-off

Didn't understand. What do you want to do?

Pronoy999
– Pronoy999

2017-08-27 18:42:08 +00:00
Commented Aug 27, 2017 at 18:42 — Pronoy999
– Pronoy999, Commented Aug 27, 2017 at 18:42
@PronoyMukherjee Updated

srghma
– srghma

2017-08-27 18:49:09 +00:00
Commented Aug 27, 2017 at 18:49 — srghma
– srghma, Commented Aug 27, 2017 at 18:49

Shaun Luttin · Accepted Answer · 2017-08-27 20:02:38Z

3

We can use function overloading to describe a function that...

can accept one and only one Configuration[] or ...args: Configuration[] and returns Configuration

Here is the declaration.

declare function smart(...configuration: Configuration[]): Configuration;
declare function smart(configuration: Configuration[]): Configuration;

Here is an example of its usage.

// in the module's *.d.ts declaration file...

declare class Configuration { 
  public context: any;
  public entry: any;
  public output: any;
}

declare function myFunc(...configuration: Configuration[]): Configuration;
declare function myFunc(configuration: Configuration[]): Configuration;

// in the module consumer's *.ts file...

const conf = new Configuration();
const confs = [conf];

const r1 = myFunc(confs); // right
const r2 = myFunc(conf); // right
const r3 = myFunc(conf, conf); // right

const f1 = myFunc(confs, confs); // must fail, cause can accept only one array
const f2 = myFunc(conf, confs); // must fail, cause or array, or spread
const f3 = myFunc(confs, conf); // must fail, same

edited Aug 27, 2017 at 20:02

answered Aug 27, 2017 at 18:45

Shaun Luttin

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6 Comments

Shaun Luttin Over a year ago

@bjornmelgaard It's hard to understand, because the actual signature of the merge.smart function accepts an object: github.com/survivejs/webpack-merge/blob/…

srghma Over a year ago

no, it accepts (...configuration | [...configuration]) npmjs.com/package/webpack-merge#smart-merging

Shaun Luttin Over a year ago

@bjornmelgaard Thank you for clarifying. Does the update to the answer work for you?

Shaun Luttin Over a year ago

@bjornmelgaard I think function overloading my work for your situation.

srghma Over a year ago

mmm, looks awesome, can you show how to make function body, look UPDATE3

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