validators.js

function validate(val, ...validateFunctions) {
    for (let f of validateFunctions) {
        const err = f(val);
        if (err !== val) {
            return err;
        }
    }
    return null;
}

function validateRequired(val) {
    if (!val) {
        return "required";
    }
    return val;
}

function validateIsNum(val) {
    if (val !== 0 && !val) {
        return val;
    }
    if (!Number.isInteger(val)) {
        return "shuld be num";
    }
    return val;
}

console.log(validate("test", validateRequired, validateIsNum))
console.log(validate("", validateRequired, validateIsNum))
console.log(validate(1, validateRequired, validateIsNum))