Confusing error message: Operator '===' cannot be applied to types '1' and '0'.

Chris Dias (@chrisdias)

TypeScript Version: 3.0.0-dev.20180623

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Code

function foo(x: [number]) {
    return x.length === 0;
}

Bug:
The error message is: Operator '===' cannot be applied to types '1' and '0'.

I found this error very confusing. It took me a while to understand that the error meant that the type on the left hand side was 1.

In this case, I think we should instead report that the condition always evaluates to false.

Related Issues:

true == false - cannot be applied to types 'false' and 'true'. #11178

/cc Chris Dias (@chrisdias)

We should have a sub-elaboration when both operands are literal types (or unions of literal types) that says something like

The types of the values indicate this comparison is always 'false'

and is

The types of the operands appear to be unrelated

in the general case

Daniel Rosenwasser (@DanielRosenwasser)

Daniel Rosenwasser (@DanielRosenwasser) let's workshop it for a few hours 🎈

The new error is misleading. You can't compare unrelated interfaces, but it's still possible and reasonable that one object could implement both of them:

const a = { x: 0, y: 0 };
const b: { x: number } = a;
const c: { y: number } = a;
console.log(b == c);

but now it claims that "The types of these values indicate that this condition will always be 'false'."

Daniel Rosenwasser (@DanielRosenwasser)

Daniel Rosenwasser (@DanielRosenwasser) - jeffreymorlan makes a good point here, I was expecting that we'd implement the "when both operands are literal types (or unions of literal types)" constraint for this message.

great, saved my time

The current error message is:

This condition will always return 'false' since the types '1' and '0' have no overlap.

Still a bit confusing if you aren't used to literal types and where 1 comes from, but I think we can close this now

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Confusing error message: Operator '===' cannot be applied to types '1' and '0'. #25245

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Confusing error message: Operator '===' cannot be applied to types '1' and '0'. #25245

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RyanCavanaugh commented on Jun 26, 2018

RyanCavanaugh commented on Jun 26, 2018

jeffreymorlan commented on Jun 28, 2018

RyanCavanaugh commented on Jun 28, 2018

GenweiWu commented on Dec 24, 2018

mjbvz commented on Oct 13, 2021

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