How to use the core-js/fn/array/flatten function in core-js

validateForm(display = true) {
        const { form } = this.props;

        let invalid = false;
        let { validationErrors, form: stateForm } = this.state;

        flatten(form).forEach((input) => {
            if (!input.validationRules) {
                return;
            }

            let [ valid, validationError ] =
                this.validateInput(input.name, stateForm[input.name], input.validationRules);

            validationErrors = {
                ...validationErrors,
                ...validationError
            };

            if (!valid) {
                invalid = true;
            }
        });

if (!entry) {
                        orderedFields.splice(j, 1, field);
                    } else if (Array.isArray(entry)) {
                        orderedFields[j].push(field);
                    } else {
                        orderedFields[j] = [entry, field];
                    }

                    continue;
                }

                orderedFields.push(field);
            }

            return flatten(orderedFields);
        }

        return fields;
    }