[Help/Question] Function with optional parameters, ZodError: "too_small"

[kristremblay]

Perhaps I'm misunderstanding how this works, but I have made the following function:

const formatCurrency = z
  .function()
  .args(
    z.number(),
    z.string().optional(),
    z.string().optional(),
  )
  .returns(z.string())
  .implement((value: number, currency: string = 'CAD', locale: string = 'en-CA'): string => {
    return new Intl.NumberFormat(locale, {
      style: 'currency',
      currency,
      minimumFractionDigits: 0,
      maximumFractionDigits: 0
    }).format(value);
  });

It works when I call formatCurrency with three arguments, but if I call it with only the value arguments I am presented with this error:

ZodError: [
  {
    "code": "invalid_arguments",
    "argumentsError": {
      "issues": [
        {
          "code": "too_small",
          "minimum": 3,
          "inclusive": true,
          "type": "array",
          "path": [],
          "message": "Should have at least 3 items"
        }
      ],
      "name": "ZodError"
    },
    "path": [],
    "message": "Invalid function arguments"
  }
]

What is the correct way to handle optional arguments?

[JacobWeisenburger]

Explanation

optional in zod doesn't mean optional the way you might think. All it means is that the value can be undefined. undefined doesn't always mean non-existent. It is a little confusing, yes.

Most of the time, optional works the same as how you would expect, but in the case of function args or tuples, saying that it's optional just means that it needs to exist, but it can be undefined.

This is essentially what you are doing:

const args = z.tuple( [
    z.number(),
    z.string().optional(),
    z.string().optional(),
] )

console.log( args.safeParse( [ 42, 'CAD', 'en-CA' ] ).success ) // true
console.log( args.safeParse( [ 42, undefined, undefined ] ).success ) // true
console.log( args.safeParse( [ 42 ] ).success ) // false

This is pretty confusing. But if zod were to change this, it might break backwards compatibility. I'm not trying to say that zod shouldn't change this, rather that I'm not sure zod will.

---

Solution

I would suggest changing your args into an object for this instead. I find using an object is much easier to deal with optional args, but it also has a few other benefits. Such as, it's easier to understand which arg is what when you come back to this code later. Also, it allows for you to put the args in any order.

const formatCurrency = z
    .function()
    .args(
        z.object( {
            value: z.number(),
            currency: z.string().optional(),
            locale: z.string().optional(),
        } )
    )
    .returns( z.string() )
    .implement( ( { value, currency = 'CAD', locale = 'en-CA' } ): string => {
        return new Intl.NumberFormat( locale, {
            style: 'currency',
            currency,
            minimumFractionDigits: 0,
            maximumFractionDigits: 0
        } ).format( value )
    } )

console.log( formatCurrency( { value: 42, currency: 'CAD', locale: 'en-CA' } ) ) // $42
console.log( formatCurrency( { currency: 'CAD', locale: 'en-CA', value: 42 } ) ) // $42
console.log( formatCurrency( { value: 42 } ) ) // $42

[kristremblay]

Thank you for your reply. I should have come back to comment after @colinhacks replied to me on Twitter (re: this not being doable as I had attempted). I ended up doing exactly this for the same reasons you mentioned, so thank for you both taking the time to reply and for making me feel vindicated 😄

Cheers!