A series of function that help convert the FormData object into a usable model.

formDataToObject.ts

// deno-lint-ignore-file no-explicit-any

/**
 * Checks if the array value being assigned to the property is for simple value
 * types, e.g. `String`, `Number`, `Boolean`, etc. or for complex types like
 * `Object`.
 *
 * @param name The name of the property on the model object.
 *
 * @returns `True` if the property is a primitive array, otherwise `false`.
 */
const isPrimitiveArray = (name: string) => name.indexOf("[]") > 0;

/**
 * Checks if the array value being assigned to the property is for complex
 * types like `Object`, or for simple value types, e.g. `String`, `Number`,
 * `Boolean`, etc.
 *
 * @param name The name of the property on the model object.
 *
 * @returns `True` if the property is a complex array, otherwise `false`.
 */
function isComplexArray(name: string) {
    const lastIndex = name.length - 1;
    const firstBraceIndex = name.indexOf("[");

    // Checks if there's actually a property name present. Must be at least one
    // character in length.
    if (firstBraceIndex < 1) return false;

    // Checks if the first brace is the last character of the field name.
    if (firstBraceIndex === lastIndex) return false;

    // Checks if there are exactly two additional character indices after the
    // first brace index.
    if (lastIndex - firstBraceIndex !== 2) return false;

    const indexNumber = name[firstBraceIndex + 1];

    // Ensures the index number in the field name is actually a number.
    if (isNaN(+indexNumber)) return false;

    const lastBraceIndex = name.indexOf("]");

    // Checks if the last brace index is actually the last character.
    if (lastBraceIndex !== lastIndex) return false;

    return true;
}

/**
 * Gets the property name of the array for the model.
 *
 * @param name The name of the property on the model object.
 *
 * @returns The field name minus the braces and index number, if present.
 */
function getArrayPropertyName(name: string) {
    const arrayBracketIndex = name.indexOf("[");

    return name.substring(0, arrayBracketIndex);
}

/**
 * Gets the index number for mapping sub-properties of a complex array.
 *
 * @param name The name of the property on the model object.
 *
 * @returns The index number of the object in the array.
 */
function getComplexArrayIndex(name: string) {
    const arrayBracketIndex = name.indexOf("[");

    return +name[arrayBracketIndex + 1];
}

/**
 * Converts the passed value to a primitive value of `Number`, `Boolean`, or
 * `String`.
 *
 * @param value The value to convert.
 *
 * @returns The value as either a number, boolean, or the passed value.
 */
function convertValue(value: any): number | boolean | string {
    if (!isNaN(value)) return +value;

    if (value === "true") return true;

    return value;
}

/**
 * Builds the properties on the passed model object based on the list of names
 * and value. The method may attempt to recursively build properties of complex
 * types.
 *
 * @param model The model to build the properties for.
 * @param names The field name, split into an array at the period.
 * @param value The value to assign to the constructed property.
 */
function buildProperty(model: any, names: string[], value: unknown) {
    const firstPropertyName = names[0];

    if (isPrimitiveArray(firstPropertyName)) {
        const arrayPropertyName = getArrayPropertyName(firstPropertyName);

        // If the array exists, push the value.
        if (Array.isArray(model[arrayPropertyName])) {
            model[arrayPropertyName].push(value);
        }
        // Else, create the new array on the property with the passed value.
        else {
            model[arrayPropertyName] = [value]
        }

        return;
    }

    if (isComplexArray(firstPropertyName)) {
        const arrayPropertyName = getArrayPropertyName(firstPropertyName);
        const indexNumber = getComplexArrayIndex(firstPropertyName);

        // If the array does not yet exist on the property, assign an empty
        // array
        if (!Array.isArray(model[arrayPropertyName])) {
            model[arrayPropertyName] = [];
        }

        // If the object being assigned values to does not yet exist in the
        // array, push an empty object to assign values to.
        if (model[arrayPropertyName].length === indexNumber) {
            model[arrayPropertyName].push({});
        }

        // Recursively build the complex object using the same method and only
        // passing in the necessary names and value.
        buildProperty(model[arrayPropertyName][indexNumber], names.slice(1), value);

        return;
    }

    // If the property is a primitive type, assign the value and return.
    if (names.length === 1) {
        model[firstPropertyName] = value;

        return;
    }

    // If the object has not yet been created for the property, assign an empty
    // object to assign values to.
    if (!model[firstPropertyName]) model[firstPropertyName] = {};

    // Recursively build the complex object using the same method and only
    // passing in the necessary names and value.
    buildProperty(model[firstPropertyName], names.slice(1), value);
}

/**
 * Converts the `FormData` to an object. The generic type is for convenience
 * when using the returned value.
 *
 * @param formData The form field data used to build the object.
 *
 * @returns The newly constructed object model.
 */
export function formToModel<TModel>(formData: FormData): TModel {
    const model: any = {};

    formData.forEach((value, key) => {
        const names = key.split(".");

        const actualValue = convertValue(value.valueOf());

        buildProperty(model, names, actualValue);
    });

    return model;
}