String length validation predicate
It seems that the ok fields plus the problem fields equal all the fields.

In that case, you should consider deriving problem fields from all fields and ok fields with the Underscore difference method.

So in short:

function stringLengthValidation( mergedModelAndFormFields ) {
    let fieldsToValidate = mergedModelAndFormFields.okFields

    // extracted function
    function checkStringLength() {
        return _.pickBy( fieldsToValidate, ( value ) =&gt; {
            return validator.isLength( value[ 'content' ], {
                'min': value[ 'validation' ].minLength,
                'max': value[ 'validation' ].maxLength
            } )
        } )
    }

    let okFields = checkStringLength()

    return {
        'okFields'     : okFields,
        'problemFields': _.difference( fieldsToValidate, okFields )
    }
}


Actually, I would probably not even encapsulate what is in essence 1 statement into a separate function, it seems subjectively overkill to me:

function stringLengthValidation( mergedModelAndFormFields ) {

    let fieldsToValidate = mergedModelAndFormFields.okFields   
    let okFields = _.pickBy( fieldsToValidate, ( value ) =&gt; {
        return validator.isLength( value[ 'content' ], {
            'min': value[ 'validation' ].minLength,
            'max': value[ 'validation' ].maxLength
        } )
    });

    return {
        'okFields'     : okFields,
        'problemFields': _.difference( fieldsToValidate, okFields )
    }
}