This answer is out of date; see @ggradnig's answer.

The best available workaround:

const furnitureObj = { chair: 1, table: 1, lamp: 1 };
type Furniture = keyof typeof furnitureObj;
const furniture = Object.keys(furnitureObj) as Furniture[];

Ideally we could do this:

const furniture = ['chair', 'table', 'lamp'];
type Furniture = typeof furniture[number];

Unfortunately, today furniture is inferred as string[], which means Furniture is now also a string.

We can enforce the typing as a literal with a manual annotation, but it brings back the duplication:

const furniture = ["chair", "table", "lamp"] as ["chair", "table", "lamp"];
type Furniture = typeof furniture[number];

TypeScript issue #10195 tracks the ability to hint to TypeScript that the list should be inferred as a static tuple and not string[], so maybe in the future this will be possible.

I get roleIds from backend, if a user has only 1 role the roleIds become string and not array so we have to convert it to array by [ ] in angular TokenService class:

get roleIds(): string[] {

    let roles: string[] = JSON.parse(JSON.stringify(this.decodedToken['RoleIds']));

    let roleIds: string[] = [];
    if (typeof roles == "string") { // if user has "only 1 role" the roles is a string
      roleIds = [roles]; // convert to array
    } else { // // if user has "more than 1 role" roles is string[] array (array typeof is object)
      roleIds = roles;
    }

    return roleIds;
  }

The interface that i implement:

export class Candidate {
id: number;
name: string;
skills: string|string[]; }

In the component i try to upload my data through a service. The code in the component.ts:

import { Component, OnInit } from '@angular/core';
import { CandidatesService } from '../candidates.service';
import { Validators } from '@angular/forms';
import { Observable } from 'rxjs/Observable';

import { Candidate } from '../candidate';

@Component({
  selector: 'app-candidates-adding',
  templateUrl: './candidates-adding.component.html',
  styleUrls: ['./candidates-adding.component.css']
})
export class CandidatesAddingComponent implements OnInit {
  candidate: Candidate = new Candidate();

  constructor(private candidatesService: CandidatesService) { }

  ngOnInit() {

  }

  saveCandidate(): void {
    if(this.candidate.skills.indexOf(', ') >= 0) {
      this.candidate.skills = this.candidate.skills.split(', '); 
    }
    this.candidatesService.addCandidate(this.candidate).subscribe();
  }
}

And in the template:

<input [(ngModel)]="candidate.name" placeholder="name" minlength="4">
<input [(ngModel)]="candidate.skills" placeholder="name" minlength="1">
<button (click)="saveCandidate()">Adauga candidat</button>

For example:

  const themeMode = ['light', 'dark'] as const;
  type MainTheme = typeof themeMode[number];
  const values = [...themeMode];

  // iterate on themeMode as const assertion
  values.map((theme) => console.log(theme));

Use the join method from the Array type.

a.value = [a, b, c, d, e, f];
var stringValueYouWant = a.join();

The join method will return a string that is the concatenation of all the array elements. It will use the first parameter you pass as a separator - if you don't use one, it will use the default separator, which is the comma.