No narrowing on assert.strictEqual()

[mrtnlrsn]

Bug Report

assert.strictEqual(a, b) and assert.equal(a, b) does not result in typescript narrowing like assert(a === b)

🔎 Search Terms

assert, assert.equal, equal, narrowing

🕗 Version & Regression Information

Always (I assume)

Original feature implemented in #8010

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about equal

⏯ Playground Link

https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBAbzgZxlYBjeBDZdfICmscAvnAGZQQhwDkBxMdAUCzAJ5iFwCCcAXkQs4ouJ24AuetlZi4AO2kKAriABGxFqTYSeAIUHD5e6XXVyxyaanQKA5traEAHpBIUVCrMAgLKAIwAFC5w0vwAPnD6AJTGYoywIQB0eoICQgx0MSJiGH7IEAA2hMlFEPYpCnFwAPS1cADyANJOLK7u8J7eML7+FABMIWF8cFGx8aKJMMm2mDAAogCOKthFKXoANDLZuaL5CoUlZRVVcfVwAGK8AJIAMtIACtTcsBz0CnRwACYQhHgKCDwVzAVBwPziLg8OiRaJ0ZJwZ4QV6cD5fX7-RRAuAgsEQtJ0fTwpxAA

💻 Code

import { strict as assert } from 'assert'

type A = {
    type: 'a'
    n: number
}

type B = {
    type: 'b'
    s: string
}

export function f1(x : A | B) {
    assert(x.type === 'a')
    console.log(x.n)  // OK
}

export function f2(x : A | B) {
    assert.strictEqual(x.type, 'a')
    console.log(x.n) // FAIL: Property 'n' does not exist on type 'A | B'. Property 'n' does not exist on type 'B'.
}

🙁 Actual behavior

No typescript narrowing in function f2()

🙂 Expected behavior

Typescript narrowing in function f2() since it is equavalent to f1()

[MartinJohns]

This is no bug in TypeScript. The type annotation of strictEqual does not narrow x, because it's unaware of x. I'm not aware of any way to type this in TypeScript.

The assert example works because TypeScript sees the whole expression x.type === 'a' for narrowing purposes, but this is not the case for strictEqual. Here the asserts function is completely unaware of x, it only sees x.type.

[mrtnlrsn]

@MartinJohns Thanks for your reply.

OK, then lets call it a feature request.

This is definitely the logical behaviour that would be expected by most users, even though there may be technical reasons it does not work like that.

[fatcerberus]

Yeah, the only way to type an assertion function is either asserts x (asserts a whole expression) or asserts x is T (a type guard). Implementing this suggestion would require the ability to say asserts x === y, which is currently not possible.