type error absence?

[errant raven]

function abc (d: {d:string} | null) {
    const aux = {a: 't', b: 1, c: false, ...(d && {d: 11})} // if d is null this expression ...(d && ..) does not compile, so why doesn't the typechecker complain here?
    aux
}

Why doesn't typescript complain here? When d is null the the expression reduces to ...false which is invalid syntax?

[grim edge]

@errant raven That's not invalid syntax in an object spread.

[sterile flax]

When d is null the the expression reduces to ...false
&& short-circuits to a value, not to a boolean, so it'd result in null

same for ||

[frigid dune]

It does complain on the playground 🤔
https://www.typescriptlang.org/play/?ts=5.6.2#code/GYVwdgxgLglg9mABFApgZygCgG4EMA2IKAXIgEZxz4q5gCUiA3gFACQATilCO0owHSCcBIogA+YpgF86UgNzMpzIA

[halcyon temple]

@frigid dune Here's a shortened URL of your playground link! You can remove the full link from your message.

mishall8399#0

Playground Link

Preview:ts function test(value: boolean) { return {...(value || {})} }

You can choose specific lines to embed by selecting them before copying the link.

[frigid dune]

Oh wait, it is because of boolean, null is fine

[sterile flax]

falsy values are special cased

[grim edge]

Yeah, boolean is errored by TS (though it's allowed at runtime and just doesn't add anything to the object)

[sterile flax]

x && actualspread is a common-enough pattern to warrant that, i guess

[halcyon temple]

2jt0#0

Playground Link

Preview:ts function abc(d: {d: string} | null) { const aux = { a: "t", b: 1, c: false, ...(null && {d: 11}), } // if d is null this expression ...(d && ..) does not compile, so why doesn't the typechecker complain here? aux }

You can choose specific lines to embed by selecting them before copying the link.

[sterile flax]

value || {} errors
value && {} does not error
value ? {} : true errors
value ? {} : false does not error

[grim edge]

Ah, it really is a falsy expression exception, huh.

Since ...false is also an error

[errant raven]

why is first example ok while the second gives an error?
(https://www.typescriptlang.org/play/?ts=5.6.2&ssl=9&ssc=1&pln=1&pc=1#code/GYVwdgxgLglg9mABFApgZygCgG4EMA2IKAXImCPvgJSIDeAUAJABOKUIzStAdLzgUUQAfIXQC+VMQG56Y+vVCRYCZOigBGfoRJkK1Okzzamrdpzq9umcpWGjaE6bPpA)

[halcyon temple]

@errant raven Here's a shortened URL of your playground link! You can remove the full link from your message.

2jt0#0

Playground Link

Preview:```ts
function test(value: null) {
return {...(value || {})}
}

function test1(value: null) {
value
return {...(null || {})}
}```

[errant raven]

so weird

or in other words, why doesn't the first example give the same error as the second? Why isn't typescript capable see that the param of a function will have a value on null always?

[grim edge]

I don't think the first examle should be an error?

Oh you're talking about Chris's examples

I think TS doesn't error on value && {} because it's useful and works at runtime, so why error?

[errant raven]

function test(value: null) {
  return {...(value || {})} // no error
}

function test1(value: null) {
  value
  return {...(null || {})} // error This kind of expression is always falsy.
}

this example

[grim edge]

The "this expression is always falsy" checks are brand new (TS 5.6) and intentionally cautious

[errant raven]

oh right

in older version it's the same