Typescript use string or string[]

Question

I have this definition:

interface Field {
  question: string,
  answer: string | string[],
}

but theres error in my text editor if i do this:

if (typeof answer === 'string') {
  const notEmpty = answer.trim().length > 0
}

it says string[] doesn't implement trim. is my approach wrong? should i use 2 interfaces? thanks

updated:

my tsconfig.json:

{
  "compilerOptions": {
    "allowSyntheticDefaultImports": true,
    "jsx": "react-native",
    "lib": ["dom", "esnext"],
    "moduleResolution": "node",
    "noEmit": true,
    "skipLibCheck": true,
    "resolveJsonModule": true,
    "strict": true,
    "baseUrl": ".",
    "paths": {
      "*": ["*", "src/*"]
    }
  }
}

I cannot reproduce your error. Please provide a minimal reproducible example which actually demonstrates your issue. — jcalz
– jcalz, Commented Jan 28, 2021 at 15:39
@Gh05d except that per my and jcalz's comments, it doesn't complain... — Jared Smith
– Jared Smith, Commented Jan 28, 2021 at 15:40
It's probably that i is of type number and the compiler doesn't know how to narrow based on that. Without a minimal reproducible example though I'm not going to go through the effort of demonstrating this. If you want to increase your chances of getting a helpful response, I suggest you follow the guidelines in How to Ask, specifically about providing a minimal reproducible example suitable for dropping into a standalone IDE. Good luck! — jcalz
– jcalz, Commented Jan 28, 2021 at 15:51

Orelsanpls · Accepted Answer · 2021-01-28 15:50:52Z

3

You have this error because you use a variable : i to go throught your object.

playground

interface Field {
  question: string,
  answer: string | string[],
}

const answerArray: Field['answer'][] = ['tes'];

const i = 0;

if (typeof answerArray[i] === 'string') {
  const notEmpty = answerArray[i].trim().length > 0
}

Between the two line :

if (typeof answerArray[i] === 'string') {

const notEmpty = answerArray[i].trim().length > 0

TypeScript consider that answerArray[i] could result in different values.

The soluce is to store the value inside of a pointer, like :

playground

interface Field {
  question: string,
  answer: string | string[],
}

const answerArray: Field['answer'][] = ['tes'];

const i = 0;

const myAnswer = answerArray[i];

if (typeof myAnswer === 'string') {
  const notEmpty = myAnswer.trim().length > 0
}

answered Jan 28, 2021 at 15:50

Orelsanpls

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3 Comments

nanakondor Over a year ago

thank you so much,,, how is this possible ?!

Aron Over a year ago

Where did this code come from? That's not in the question...

Orelsanpls Over a year ago

@Aron I did retro engineered it based on the code provided by OP.

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