Error ts1345 an expression of type void cannot be tested for truthiness

Table of contents
• An expression of type ‘void’ cannot be tested for truthiness
• An expression of type ‘void’ cannot be tested for truthiness
• Swagger-api
  /
  swagger-codegen
  Public
• Microsoft
  /
  TypeScript
  Public
• An expression of type ‘void’ cannot be tested for truthiness

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An expression of type ‘void’ cannot be tested for truthiness

localStorage.setItem('todoitems', JSON.stringify(this.todoitems)) 
  ? localStorage.setItem('todoitems', JSON.stringify(this.todoitems)) 
  : [];

if (this.todoitems !== null)
    localStorage.setItem('todoitems', JSON.stringify(this.todoitems))

An expression of type ‘void’ cannot be tested for truthiness

Copied!// 👇️ function returns type void
function sum(a: number, b: number) {
  const result = a + b;
  // 👉️ forgot to return
}

// ⛔️ Error: An expression of type 'void' cannot be tested for truthiness.ts(1345)
const result = sum(10, 10) ? 20 : 0;

// ⛔️ Error: An expression of type 'void' cannot be tested for truthiness.ts(1345)
if (sum(10, 10)) {
  console.log('success');
}

Copied!// 👇️ function now returns a value
function sum(a: number, b: number): number {
  const result = a + b;

  return result; // 👈️ explicit return
}

const result = sum(10, 10) ? 20 : 0;

if (sum(10, 10)) {
  console.log('success');
}

Copied!// 👇️ implicit return with objects
const getObj = () => ({
  name: 'Tom',
  country: 'Chile',
});

// 👇️ implicit return with primitives
const getNum = (a: number, b: number) => a + b;

Swagger-api
/
swagger-codegen
Public

…}-petstore.sh

…}-petstore.sh

…}-petstore.sh

based on swagger-api#9012

An expression of type ‘void’ cannot be tested for truthiness

localStorage.setItem('todoitems', JSON.stringify(this.todoitems)) 
  ? localStorage.setItem('todoitems', JSON.stringify(this.todoitems)) 
  : [];

if(this.todoitems !== null)
    localStorage.setItem('todoitems', JSON.stringify(this.todoitems))

Microsoft
/
TypeScript
Public

const logger = (...args) => console.log(args) || args;

const logger = (...args) => console.log(args) || args;

const logger = (...args) => (console.log(args), args);

An expression of type ‘void’ cannot be tested for truthiness

localStorage.setItem('todoitems', JSON.stringify(this.todoitems)) 
  ? localStorage.setItem('todoitems', JSON.stringify(this.todoitems)) 
  : [];

public getUser(userId: string) {
return this.http.get<{
  _id: string;
  registrationStep: number;
  userType: string;
  isAdmin: boolean;
}>(BACKEND_URL+"getUserData/" + userId);

getIsAdmin() {
this.getUser(this.getUserId()).subscribe(result => {
  return result.isAdmin;
});

    import {
  CanActivate,
  ActivatedRouteSnapshot,
  RouterStateSnapshot,
  Router
} from "@angular/router";
import { Injectable } from "@angular/core";
import { Observable } from "rxjs";

import { AuthService } from "./auth.service";

@Injectable()
export class AdminGuard implements CanActivate {
  constructor(private authService: AuthService, private router: Router) {}

  canActivate(
    route: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ): boolean | Observable<boolean> | Promise<boolean> {
    const isAdmin = this.authService.getIsAdmin();
    if (!isAdmin) {
      this.router.navigate(['auth/admin']);
    }
    return isAdmin;
  }
}

// authService
getIsAdmin(): Observable<boolean> {
    return this.getUser(this.getUserId()).pipe(
        map(user => user.isAdmin)
    );
}

canActivate(…): Observable<boolean> {
    const isAdmin$ = this.authService.getIsAdmin();
    return isAdmin$.pipe(
        tap(isAdmin => {
            if(!isAdmin) {
                this.router.navigate(['auth/admin']);
            }
        })
    );
}

Swagger-api
/
swagger-codegen
Public

parameters:
        - name: attachment1
          type: file
          in: formData
          required: false

if (attachment1 !== undefined) {
            formParams = formParams.append('attachment1', <any>attachment1) || formParams;
}

let formParams: { append(param: string, value: any): void; };

import { Observable } from 'rxjs/Observable'

import { Observable } from 'rxjs'

sed -i 's/rxjs/Observable/rxjs/' ./src/app/swagger/api/api.service.ts 
sed -i 's/formParams = formParams/formParams/' ./src/app/swagger/api/api.service.ts 
sed -i 's/ || formParams//' ./src/app/swagger/api/api.service.ts 

docker run --rm -v ${PWD}/src/app/swagger:/local swaggerapi/swagger-codegen-cli:unstable generate -i http://192.168.1.47:5070/swagger/v1/swagger.json -l typescript-angular -o /local --additional-properties ngVersion=6.0.0

    let formParams: { append(param: string, value: any): void; };
    let useForm = false;
    let convertFormParamsToString = false;
    if (useForm) {
        formParams = new FormData();
    } else {
        formParams = new HttpParams({encoder: new CustomHttpUrlEncodingCodec()});
    }
    if (name !== undefined) {
        formParams = formParams.append('name', <any>name) || formParams;
    }
    if (status !== undefined) {
        formParams = formParams.append('status', <any>status) || formParams;
    }

let formParams: { append(param: string, value: any): void; };

let formParams: { append(param: string, value: any): void | HttpParams; };

…t be tested for truthiness swagger-api#8836

…t be tested for truthiness swagger-api#8836

…t be tested for truthiness swagger-api#8836

find ./src/swagger/api/*.service.ts -type f -exec sed -i '' -e 's/rxjs//rxjs/' {} ;
find ./src/swagger/api/*.service.ts -type f -exec sed -i '' -e 's/formParams = formParams/formParams/' {} ;
find ./src/swagger/api/*.service.ts -type f -exec sed -i '' -e 's/ || formParams//' {} ;

# builder.ts, new file
export interface FormBuilder {
  append(param: string, value: any): FormBuilder;
}

export class FormDataBuilder<T extends { append(param: string, value: any): void; }> implements FormBuilder {

  append(param: string, value: any): FormDataBuilder<T> {
    this.append(param, value);
    return this;
  }
}

# generated service
let formParams: FormBuilder;
if (useForm) {
    formParams = new FormDataBuilder<FormData>();
} else {
    formParams = new HttpParams({encoder: new CustomHttpUrlEncodingCodec()});
}

        // HERE: if we use HttpParams the append function returns a new HttpParams object
        // instead if we use FormData the append function will not return anything
        let formParams: { append(param: string, value: any): void | HttpParams; };
        let useForm = false;
        let convertFormParamsToString = false;
        if (useForm) {
            formParams = new FormData();
        } else {
            formParams = new HttpParams({ encoder: new CustomHttpUrlEncodingCodec() });
        }

        if (newPassword !== undefined) {
            // here there is no more the syntax error
            formParams = formParams.append('newPassword', <any>newPassword) || formParams;
        }
        // here nothing changes
        return this.httpClient.post<any>(`${this.basePath}/accounts/reset-password`,
            convertFormParamsToString ? formParams.toString() : formParams,
            {
                withCredentials: this.configuration.withCredentials,
                headers: headers,
                observe: observe,
                reportProgress: reportProgress
            }
        );

java -jar .swagger-codegen-cli-2.4.1.jar generate -i http://localhost:5100/swagger/v1/swagger.json -l typescript-angular -o ../src/api

$file = '..srcapiapithexyz.service.ts'
$find = 'let formParams: { append(param: string, value: any): void; };'
$replace = 'let formParams: { append(param: string, value: any): void | HttpParams; };'

(Get-Content $file).replace($find, $replace) | Set-Content $file

var gen = 'java -jar src/swagger/swagger-codegen-cli-2.4.13.jar generate -i src/swagger/swagger-spec.yml -l typescript-angular -o src/app/generated';
var rxFix = 'sed -i 's/rxjs\/Observable/rxjs/' ./src/app/generated/api/*.service.ts';
var frmFix = 'sed -i "s/ || formParams//"  ./src/app/generated/api/*.service.ts';
var frmFix2 = 'sed -i 's/formParams = formParams/formParams/' ./src/app/generated/api/*.service.ts';

runCommand(gen, (error, stdout, stderr) => {
  console.log('Output: ' + stdout)
  if(error !== null){
    console.log("Error: " + error);
    return;
  }
  if(stderr !== null){
    console.log("Stderr: " + stderr);
  }
});

runCommand(rxFix, (error, stdout, stderr) => {
  console.log('Output: ' + stdout)
  if(error !== null){
    console.log("Error: " + error);
    return;
  }
  if(stderr !== null){
    console.log("Stderr: " + stderr);
  }
});

runCommand(frmFix, (error, stdout, stderr) => {
  console.log('Output: ' + stdout)
  if(error !== null){
    console.log("Error: " + error);
    return;
  }
  if(stderr !== null){
    console.log("Stderr: " + stderr);
  }
});

runCommand(frmFix2, (error, stdout, stderr) => {
  console.log('Output: ' + stdout)
  if(error !== null){
    console.log("Error: " + error);
    return;
  }
  if(stderr !== null){
    console.log("Stderr: " + stderr);
  }
});

Previous PostNext Post

Typescript Online Compiler

Write, Run & Share Typescript code online using OneCompiler’s Typescript online compiler for free. It’s one of the robust, feature-rich online compilers for Typescript language. Getting started with the OneCompiler’s Typescript editor is easy and fast. The editor shows sample boilerplate code when you choose language as Typescript and start coding.

About Typescript

Typescript(JS) is a strongly typed programming language that builds on JavaScript, developed and maintained by Microsoft.

Key Features

• Superset of Javascript
• portable
• Strong static typing
• supports OOPS
• Language extension to ECMAScript 6 with other features like Type annotations and compile-time type checking, Type inference and Type erasure, Interfaces, Enumerated types, Generics, Namespaces, Tuples
• .ts extension

Syntax help

variable declaration

Operators

Loops

1. If:

IF is used to execute a block of code based on a condition.

Syntax

if(condition){
    // code
}

2. If-Else:

Else part is used to execute the block of code when the condition fails.

Syntax

if(condition){
    // code
} else {
    // code
}

3. Switch:

Switch is used to replace nested If-Else statements.

Syntax

switch(condition){
    case 'value1' :
        //code
        break;
    case 'value2' :
        //code
        break;
    .......
    default :
        //code
        break;
}

4. For

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
//code  
} 

let arr = [1, 2, 3, 4, 5];
for (let ele of arr) {
 // code
}

for (let index in arr) {
//code
}

5. While

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
  // code 
}  

6. Do-While

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
  // code 
} while (condition); 

Arrow functions

Arrow Functions helps developers to write code in concise way, it’s introduced in ES6.
Arrow functions can be written in multiple ways. Below are couple of ways to use arrow function but it can be written in many other ways as well.

Syntax:

() => expression

Example:

const numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
const squaresOfEvenNumbers = numbers.filter(ele => ele % 2 == 0)
                                    .map(ele => ele ** 2);
console.log(squaresOfEvenNumbers);

Function Overloading

Typescript provides function overloading where multiple functions with the same name but different parameter types and return type is possible. But, the number of parameters should be the same.

function Addition(a:string, b:string):string;

function Addition(a:number, b:number): number;

function Addition(a: any, b:any): any {
    return a + b;
}
Addition("Hello ","foo"); // outputs Hello foo
Addition(2,3); //outpus 5