Q:

String length validation predicate

I partially refactored the code so okFields uses the extracted function checkStringLength.
Now I also want problemFields() to use the same refactored function, but there is a difference between how okFields and problemFields use the function: you see that the problemFields has a little ! in front of validator, and I don't know how to have this vary from the same extracted function. How would you avoid the duplicate code?
function stringLengthValidation( mergedModelAndFormFields ) {
    let fieldsToValidate = mergedModelAndFormFields.okFields

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

    let okFields = checkStringLength()

    // I want to replace this with checkStringLength()
    let problemFields = _.pickBy( fieldsToValidate, ( value ) => {
        // see the '!' here that is the variation
        return !validator.isLength( value[ 'content' ], {
            'min': value[ 'validation' ].minLength,
            'max': value[ 'validation' ].maxLength
        } )
    } )

    return {
        'okFields'     : okFields,
        'problemFields': problemFields
    }
}

A:

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 ) => {
            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 ) => {
        return validator.isLength( value[ 'content' ], {
            'min': value[ 'validation' ].minLength,
            'max': value[ 'validation' ].maxLength
        } )
    });

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

