javascripttypescriptairbnb-js-styleguide

How to perform a forEach loop on a variable defied as string | Array<string>?


I am using the airbnb typescript style guideline and I have defined a type like this:

export interface Question {
  id: number;
  type: 'text' | 'multipleOption' | 'checkBox';
  question: string;
  answer: string | Array<string>;
}

A variable that fits this interface is created and I need to perform a forEach on the answer to check some conditions, but I get a linting error saying that

Property 'forEach' does not exist on type 'string | string[]'.
  Property 'forEach' does not exist on type 'string'

I would like to perform a check that enables me to perform this foreach without modifying the guideline configuration or changing the type in the interface Question definition.

I have tried:

if (question.answer.constructor === Array) {
    question.answer.forEach(...)
}

if (Array.isArray(question.answer)) {
    question.answer.forEach(...)
}
if (typeof question.answer !== 'string') {
    question.answer.forEach(...)
}

But none of the above removes the lining error.


Solution

  • if (Array.isArray(question.answer)) {
        (question.answer as Array<string>).forEach(...)
    }
    

    also you can do this to have a uniform code

        let answers : Array<string> = [];
        if (Array.isArray(question.answer)) {
            answers = [...question.answer];
        } else if (!!question.answer){
            answers = [question.answer];
        }
    
        answers.forEach(answer => ....)