"Type 'string | string[]' is not assignable to type 'string'

Question

I am using node + typescript and integrated swagger for API Calls. I have a following request in swagger

http://localhost:3033/employees/search/?username=test

Here I want to find the records having username = test.

So I parse the query string using URL module, like below and I am getting the username in my console.

var params = URL.parse(req.url, true).query;
console.log(params); //gives me [Object: null prototype] { username: 'test' } in my console

But whenever I try to assign params.username to a constant like below

const username:string =params.username;

It gives me the following error

Type 'string | string[]' is not assignable to type 'string'. Type 'string[]' is not assignable to type 'string'.

My full code is as below

import * as URL from 'url';

public getUserByName = async (req: Request, res: Response, next: NextFunction) => {
var params = URL.parse(req.url, true).query;
const username:string =params.username;

try {
  const findOneUserData: Employee = await this.userService.findUserByName(username);
  res.status(200).json({ data: findOneUserData, message: 'findOne' });
} catch (error) {
  next(error);
}

}

Answer 1

You can tell the compiler to cast the param as a string with

const username = params.username as string

Answer 2

The problem in your code is, that params.username can be either a string or a string[].

The following scenario returns a simple string

const {query} = url.parse('example.com?test=1', true);
// query.test === '1'

while the example below returns an array because the same query-parameter key is used twice

const {query} = url.parse('example.com?test=1&test=2', true);
//query.test === ['1', '2']

So you could handle the array case explicitly e.g.

const username = Array.isArray(params.username) ? params.username[0] : params.username

Answer 3

Okay ,so I got the mistake that I made. The first one was what @ford04 has mentioned in the comment. I was directly assigning Object to the string. I changed it in my question. The second was converted the parsed query-string to string by toString() function.

public getUserByName = async(req: Request, res: Response, next: NextFunction) => {
  var params = URL.parse(req.url, true).query;
  console.log(params.username);
  const username: string = params.username.toString(); //this is the key

  try {
    const findOneUserData: Employee = await this.userService.findUserByName(username);
    res.status(200).json({
      data: findOneUserData,
      message: 'findOne'
    });
  } catch (error) {
    next(error);
  }
}