Why is TypeScript reporting that "'string' is not assignable to type 'RequestMode'"?

Question

I am trying to use the Fetch API in TypeScript, but for some reason I am getting

Type 'string' is not assignable to type 'RequestMode'.

Here is my code

export class MainService{
  constructor(){}
  request(requestObj){
    if (!requestObj.url) {
      return 'Please provide the end point url'
    }
    else {
     let requestParam = {
        method: requestObj.method ? requestObj.method : 'GET',
        headers: new Headers(),
        mode: 'cors'
      };
      fetch(endPointHost+requestObj.url,requestParam).then(function(resp){
                                        //^^^ error thrown here
        resp.json();
      })
    }
     console.log(" Reached request");
  }

}

The error reported is

TS2345: Argument of type '{ method: any; headers: Headers; mode: string; }' is not assignable to parameter of type 'RequestInit'. Types of property 'mode' are incompatible. Type 'string' is not assignable to type 'RequestMode'.

requestObj.url === undefined && requestObj.url === '' is a buf a variable cannot be undefined and '' at the same time. try if (!requestObj.url) { ... } instead — notrota
– notrota, Commented Dec 17, 2017 at 7:31
@Ryan thanks Sir it worked, i am trying to find out the reason why it worked — brk
– brk, Commented Dec 17, 2017 at 7:44
@GuyWhoKnowsStuff I completely agree with you, that was my mistake, have make the change — brk
– brk, Commented Dec 17, 2017 at 7:45

ianhowe76 · Accepted Answer · 2019-07-02 22:34:36Z

18

This worked for me: mode: 'cors' as RequestMode,

answered Jul 2, 2019 at 22:34

ianhowe76

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Comments

Frank Bessou · Accepted Answer · 2019-10-03 07:53:59Z

9

The problem here is that requestParam could potentially be modified in an way that it does not respect RequestInit interface later. You can either declare that requestParam implements RequestInit at creation:

let requestParam: RequestInit = {
    method: requestObj.method ? requestObj.method : 'GET',
    headers: new Headers(),
    mode: 'cors'
};

or put request parameters directly inside the fetch call:

fetch(endPointHost+requestObj.url, {
    method: requestObj.method ? requestObj.method : 'GET',
    headers: new Headers(),
    mode: 'cors'
}).then(function(resp){
    resp.json();
})

answered Oct 3, 2019 at 7:53

Frank Bessou

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