Required field in schema gets type 'string | undefined'

I’m trying to validate an input and assign it to a variable of a type that I’ve defined. All fields are required in the validator. However, when I try to assign the validated input I get this error:

Type 'ObjectFromSchema<{ prop1: { type: "string"; required: true; }; }, never>' is not assignable to type 'Body'.
  Types of property 'prop1' are incompatible.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.ts(2322)

Since the attribute prop1 is required, why is it showing as possibly undefined?

Demonstration code (the error is at the line which says const validBody: Body = body;):

import validator from 'is-my-json-valid';

type Body = { prop1: string };

function validateBody(body: any): Body {
    const bodyValidator = validator({
        type: 'object',
        required: true,
        properties: {
            prop1: {
                type: 'string',
                required: true as true
            }
        }
    });

    if (!bodyValidator(body)) {
        throw new Error('Invalid format: ' + bodyValidator.errors);
    }

    const validBody: Body = body;
    return validBody;
}

This is with is-my-json-valid version 2.20.5 and Typescript version 4.3.5, with "strict": true

[jwasnoggin]

Amazing, that’s much better.

Definitely should update the readme!

[jwasnoggin]

Got it to work using required: ['Label' as const]. I guess that’s a reasonable workaround, if not immediately obvious.

Note this also works for objects with multiple properties, eg

{
    type: 'object',
    properties: {
        name: { type: 'string' },
        value: { type: 'number' }
    },
    required: ['name' as const, 'value' as const]
}