typescript Object is possibly undefined in similar cases

Question

type Person = {
    account?: {
        money: number
    }
}

const person: Person = {}

// case1 - this is not error
if (person.account?.money === 0) {
    console.log("I have no money");
}

// case2 - Object is possibly 'undefined'.(2532)
if (person.account?.money > 0) {
    console.log("I have money!!");
}

Why does not typescript show error on case 1, but only case 2?
What's difference?

Matthieu Riegler · Accepted Answer · 2022-07-10 15:06:51Z

2

You can't do mathematical comparisons (<,>, <=, >=) with optional variables, it makes no sense and Typescript is reminding you to specify what to do in the situation.

> undefined < 1
  // false
> undefined > 1
  // false

This rule is enabled by the strictNullChecks option.

answered Jul 10, 2022 at 15:06

Matthieu Riegler

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4 Comments

Byeongin Yoon Over a year ago

Thanks, How about this?

if (person.account?.money > 0) {     console.log(person.account.money); => // Object is possibly 'undefined'.(2532)  why?? }

I think person.account cannot be undefined

Matthieu Riegler Over a year ago

If the compiler is complaining, a field has to be optional, maybe money is ?

Byeongin Yoon Over a year ago

I think that if person.account?.money > 0 is ture, then we can sure that person.account is never undefined. Check this link please!

Matthieu Riegler Over a year ago

remove the > 0 part and you have you fix. First check for person.account to be defined.

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