An error is raised when attempting to refactor [].concat.apply([], [x]) to [].concat(x)

Question

An error is raised when attempting to refactor [].concat.apply([], [x]) to [].concat(x)

When attempting to refactor

Array.prototype.concat.apply([], [x])

to [].concat(x), I encountered the following error message:

No overload matches this call.
  Overload 1 of 2, '(...items: ConcatArray<never>[]): never[]', gave the following error.
    Argument of type 'Moment | [Moment, Moment]' is not assignable to parameter of type 'ConcatArray<never>'.
      Type 'Moment' is missing the following properties from type 'ConcatArray<never>': length, join, slice
  Overload 2 of 2, '(...items: ConcatArray<never>[]): never[]', gave the following error.
    Argument of type 'Moment | [Moment, Moment]' is not assignable to parameter of type 'ConcatArray<never>'.
      Type 'Moment' is not assignable to type 'ConcatArray<never>'.ts(2769)

I need help understanding what's causing this problem and how best to refactor the code.

javascript typescript

Answer 1

Answer №1

One reason for this issue is that the [] is defined as never[]. When you utilize concat in the initial scenario with Array.prototype being any[], the concatenation occurs between any[] and (typeof x)[].

The argument is of type 'Moment | [Moment, Moment]'

If you aim to standardize it to Moment[], you can proceed as follows:

let y: Moment[];

y = ([] as any[]).concat(x);

// or an alternative method

y = "length" in x ? x : [x];

// or explore other possibilities during your refactoring process.

const [
  from = x,
  to = x
] = x as any;

You could also encapsulate this logic within a function:

function foo<T>(arg: T|T[]): T[] {
  return Array.isArray(arg) ? arg : [arg];
}

Answer 2

One reason for this issue is that the [] is defined as never[]. When you utilize concat in the initial scenario with Array.prototype being any[], the concatenation occurs between any[] and (typeof x)[].

The argument is of type 'Moment | [Moment, Moment]'

If you aim to standardize it to Moment[], you can proceed as follows:

let y: Moment[];

y = ([] as any[]).concat(x);

// or an alternative method

y = "length" in x ? x : [x];

// or explore other possibilities during your refactoring process.

const [
  from = x,
  to = x
] = x as any;

You could also encapsulate this logic within a function:

function foo<T>(arg: T|T[]): T[] {
  return Array.isArray(arg) ? arg : [arg];
}

An error is raised when attempting to refactor [].concat.apply([], [x]) to [].concat(x)

Answer №1

Similar questions

Flashing white screen when transitioning between pages on phonegap iOS system

Tips for loading and updating data simultaneously in VUEJS from a single input

Attempt to create a truncated text that spans two lines, with the truncation occurring at the beginning of the text

Utilize various CSS styles for text wrapping

A guide on scheduling automatic data insertion in mongoose.js

"The issue of Django showing a 'select a valid choice' error when trying to populate a select field

Angular 2: Musing on the potential of Hot Module Replacement and the power of @ngrx/store

Load an XML file from the local server asynchronously on the Chrome web browser

Tips for manipulating rows in HTML using jQuery when the id attribute is added

I am looking to present a nested array within an array in a tabular format

Resolving Challenges with AngularJS Form Validation and Submission

Sending request results to the client's browser in Node.js - A step-by-step guide

Exploring the differences between importing all utilities as a whole using `import * as util from "./util"` and importing a specific function only with `import {someFunction

What is the best way to adjust the screen to display the toggle element when it is opened?

Jasmine: utilizing unit test to spy on the invocation of a nested function

Tips for enhancing the transition effect of animated gifs on a webpage using JavaScript

Access to the requested URL has been restricted. Flask is unable to process the method

What is the best way to implement a comprehensive switch case in Typescript using string enums that are referencing other string enums?

Error: Unable to access the currentTime property as it is not defined

Challenges of implementing dark mode with a checkbox and local storage