coerce.boolean incorrect

[lacherogwu]

import { z } from 'zod';

const schema = z.object({
	something: z.coerce.boolean(),
});

console.log(schema.parse({ something: 'false' }));

expected:

{ something: false }

got:

{ something: true }

[vtgn]

Hello !

No, this behavior is normal.
You convert the string "false" into a boolean, and in Javascript, any non empty string is converted into the true boolean, only the empty string "" is converted into the false boolean.

You can verify it by executing this code:

console.log(Boolean("false")); // displays true

You can do something like that:

const booleanStringSchema = z.string().transform(value => {
  if (value === "true") {
    return true;
  } else if (value === "false") {
    return false;
  } else {
    throw new Error("The string must be 'true' or 'false'");
  }
});

Regards.

[lacherogwu]

I believe it's incorrect. Coercion should convert the string into the desired type. For example, coercing a boolean string 'false' should result in false, which is the expected behavior.

[vtgn]

No, you are wrong.
This is normal that coerce method uses the standard javascript conversions rules.

Go read the documentation: https://zod.dev/?id=coercion-for-primitives
Particularly this part:

This is exactly what I explained to you.

If you find these rules illogical, go blame javascript specification writers. ;)
But if you want different conversion rules, you'll have to implement them yourself like in my previous example.

Regards.

[jameslporter]

I agree that the js spec is weird but do think one would assume coerce of string value true|false to work. Pretty common to use booleans in forms which transmit as strings but we want them to be proper booleans for the database. Transfrom didn't work for me in this use case but preprocess did like so:

const booleanStringSchema = z.preprocess(value => { if(typeof value == 'boolean') return value if (value === "true") { return true; } else if (value === "false") { return false; } else { throw new Error("The string must be 'true' or 'false'"); } },z.boolean());

The first conditional is needed because the types are checked on their way out of the database and have the boolean type.

[vtgn]

I agree that the js spec is weird but do think one would assume coerce of string value true|false to work. Pretty common to use booleans in forms which transmit as strings but we want them to be proper booleans for the database. Transfrom didn't work for me in this use case but preprocess did like so:

const booleanStringSchema = z.preprocess(value => { if(typeof value == 'boolean') return value if (value === "true") { return true; } else if (value === "false") { return false; } else { throw new Error("The string must be 'true' or 'false'"); } },z.boolean());

The first conditional is needed because the types are checked on their way out of the database and have the boolean type.

Personally, I don't find these coercions rules weird.
Bash, Python, Perl use the same rules.
The goal is to be able to convert any primary type value into a boolean value.
For numbers, 0 is false, other values are true: it is probably a choice made to be similar to the boolean values in binary (0=false, 1=true).
For strings, it is most obvious to apply a coercion rule based on the string length which applies the same rule as the numbers. Imagine if you choose that "true" string is true, and other strings values are false: it is less obvious that "false", "blabla" and empty strings have the same boolean value. Same thing if "false" is false and other values are all "true". And what about the strings in another languages? "vrai"? "faux"? Much more easy and logical to do a coercion's rule based on the strings length.

[jameslporter]

@vtgn got me thinking and I was reviewing my giant diff with other changes this am. Decided only returning a value on boolean type or string value of "true" or "false". Anything else should throw the error like this.

const booleanStringSchema = z.preprocess(value => { if(typeof value == 'boolean') return value if (value === "true") { return true; } else if (value === "false") { return false; } else { throw new Error("The string must be 'true' or 'false'"); } },z.boolean());

[jameslporter]

Also worth mentioning existing db values may be undefined. For my use case I just default it to true by adding || value === undefined to the value equal string true line. More important is an error here is an app error. You'll likely still want a validator with a nice user error message that ideally they will never encounter.