Shorter way to validate boolean from string "true" / "false" / overwrite a schema

[rottmann]

e.g. you call a rest-api endpoint with //some/endpoint?isVisible=true
The server receives a string isVisible = 'true'

How you validate it in a shorter way?

const isVisibleSchema = z.union([ z.literal('true'), z.literal('false') ]).optional().transform( (val) => val === 'true') // overkill

const humanReadableWouldBe = z.boolean().optional() // not work, cause it is a string

Is there a way to extend / overwrite z.boolean?

[FlorianWendelborn]

I’d use z.preprocess for this. Just do:

import { z } from 'zod'

const stringToBoolean = (value: unknown) => {
    switch (value) {
        case "true":
        case "TRUE":
        case "1":
            return true
            
        case "false":
        case "FALSE":
        case "0":
            return false

        default:
            return undefined
    }
}

const schema = z.preprocess(stringToBoolean, z.boolean()).optional()

And then just put your utility functions like stringToBoolean into a dedicated file. E.g. utilities/zod-preprocess.ts

If that’s still too much, you could even put a

const booleanStringSchema = z.preprocess(stringToBoolean, z.boolean())

into e.g. utilities/schemas and then just reference it whenever you need it:

const environmentVariablesSchema = z.object({
    NO_COLOR: booleanStringSchema.optional(),
    USE_TLS: booleanStringSchema.default(false),
})

[rottmann]

Coherent diskussion: #804

@FlorianWendelborn Thx, had some similar solution.

[ryanhaticus]

Please see coercion: https://github.com/colinhacks/zod#coercion-for-primitives

[FlorianWendelborn]

@ryanhaticus coercion is likely not a good idea here since it's literally just a Boolean cast. Here's one example where it likely produces the opposite of what you'd expect in the API proposed by OP

I'd personally still recommend using a preprocess utility similar to the one I posed above as it doesn't have these unexpected behaviors, so no need to re-open this ticket