Testing for a String to just be defined in Yup

README.md

// See https://runkit.com/snekse/yup-tests-string-defined/1.0.0
// and https://runkit.com/snekse/yup-tests-string-defined/2.0.0
yup.string().min(0).strict(true).nullable(true).notOneOf([undefined]);

// If we want to allow nulls, just add `.nullable(true)`
const schema = yup.string().min(0).strict(true).nullable(true).notOneOf([undefined]);

console.log(schema.isValidSync(""));

v1.0.0.js

const yup = require("yup");

const okString = 'foo';
const shouldBeOkString = '';
let nullString = null;
let unString;
let emptyArr;

const schema = yup.string().min(0).strict(true).notOneOf([undefined]);

const okStringResult = 
    schema.label('okString').isValidSync(okString); 
const shouldBeOkStringResult =
    schema.label('shouldBeOkString').isValidSync(shouldBeOkString);
const nullStringResult = 
    schema.label('nullString').isValidSync(nullString);
const unStringResult = 
    schema.label('unString').isValidSync(unString);
const emptyArrResult = 
    schema.label('emptyArr').isValidSync(emptyArr);


console.log(`okStringResult: ${okStringResult}`); 
console.log(`shouldBeOkStringResult: ${shouldBeOkStringResult}`); 
console.log(`nullStringResult: ${nullStringResult}`);
console.log(`unStringResult: ${unStringResult}`);
console.log(`emptyArrResult: ${emptyArrResult}`);

const finalResult = 
    okStringResult && 
    shouldBeOkStringResult && 
    ! nullStringResult && 
    ! unStringResult && 
    ! emptyArrResult;
    
console.log(`finalResult passes: ${finalResult}`);

v2.0.0.js

const yup = require("yup");

const okString = 'foo';
const shouldBeOkString = '';
let nullString = null;
let unString;
let emptyArr;

function isDefinedString(val) {
  return this.min(0).strict(true).nullable(false).notOneOf([undefined]);    
}

yup.addMethod(yup.string, 'isDefined',  isDefinedString)

const schema = yup.string().isDefined();

const okStringResult = 
    schema.label('okString').isValidSync(okString); 
const shouldBeOkStringResult =
    schema.label('shouldBeOkString').isValidSync(shouldBeOkString);
const nullStringResult = 
    schema.label('nullString').isValidSync(nullString);
const unStringResult = 
    schema.label('unString').isValidSync(unString);
const emptyArrResult = 
    schema.label('emptyArr').isValidSync(emptyArr);


console.log(`okStringResult: ${okStringResult}`); 
console.log(`shouldBeOkStringResult: ${shouldBeOkStringResult}`); 
console.log(`nullStringResult: ${nullStringResult}`);
console.log(`unStringResult: ${unStringResult}`);
console.log(`emptyArrResult: ${emptyArrResult}`);

const finalResult = 
    okStringResult && 
    shouldBeOkStringResult && 
    ! nullStringResult && 
    ! unStringResult && 
    ! emptyArrResult;
    
console.log(`finalResult passes: ${finalResult}`);

If there's a better way to do this, please leave a comment.