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; } 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.