form-validation.js

import validator from 'validator';

class FormValidator{
    constructor(validations) {
        // validations is an array of validation rules specific to a form
        this.validations = validations;
    }
    validate(state){
        // start out assuming valid
        let validation = this.valid();
        // for each validation rule
        this.validations.forEach( rule=>{
            // if the field hasn't already been marked invalid by an earlier rule
            if (!validation[rule.field].isInvalid){
                // determine the field value, the method to invoke and optional args from 
                // the rule definition
                const field_value = state[rule.field] + "".toString();
                const args = rule.args || [];
                const validation_method = 
                    typeof rule.method === 'string' ?
                    validator[rule.method] : 
                    rule.method
                // call the validation_method with the current field value as the first
                // argument, any additional arguments, and the whole state as a final
                // argument.  If the result doesn't match the rule.validWhen property,
                // then modify the validation object for the field and set the isValid
                // field to false
                if(validation_method(field_value, ...args, state) !== rule.validWhen) {
                    validation[rule.field] = { isInvalid: true, message: rule.message }
                    validation.isValid = false;
                }
            }
        });
        return validation;
    }
    valid() {
        const validation = {}
    
        this.validations.map(rule => (
          validation[rule.field] = { isInvalid: false, message: '' }
        ));
    
        return { isValid: true, ...validation };
    }
}
export default FormValidator;