Number.isInteger is no longer type guarding

[tkolanko]

Bug Report

🔎 Search Terms

isInteger
type guard

🕗 Version & Regression Information

5.0.2

NOTE: there is no 5.0.2 version on the playground so I selected nightly

• This changed between versions 4.9.5 and 5.0.2

⏯ Playground Link

5.1.0 Playground

4.9.5 Playground

💻 Code

type State = string | number

const makeState = (): State => {

    if (Math.random() > 0.5) {
        return 123;
    }
    return 'abc';

}


const value = makeState();
//    ^?

if (Number.isInteger(value)) {
    console.log(value < 10)
}

const valueIsNumber = (v: any): v is number => {
    return Number.isInteger(v);
}

if (valueIsNumber(value)) {
    console.log(value < 10)
}

🙁 Actual behavior

Number.isInteger guards are no longer inferring that a value is a number

🙂 Expected behavior

Number.isInteger should type guard

[MartinJohns]

Number.isInteger was never type guarding. In your 4.9 playground you can see in line 18 that value is still typed State, not number. But TypeScript 5.0 now forbids implicit coercions in relational operators, resulting in the error you see.

[fatcerberus]

btw it can’t be changed to v is number because type guards work in both directions (i.e. returning false implies it’s not a number). See #15048

[tkolanko]

ah ok thanks - I understand now