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Hasan Zohdy
Hasan Zohdy

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78-Nodejs Course 2023: Resources: Custom Output handlers

We already defined out output cast types in our previous chapter, now let's see how we can define custom output handlers.


Sometimes we want to customize the final output of certain value, for example using uploadsUrl to generate a full url for the uploaded file as we store only in the database the relative path of uploads directory thus we need to return full url for the uploaded file.

Custom output handler

This is where we can define our custom output handlers, let's take an example to make everything clear, let's update our user resource class.

// src/app/users/resources/user-resource.ts
import Resource from 'core/resources/resource';
import { uploadsUrl } from 'core/utils/urls';

export default class UserResource extends Resource {
   * Output shape
  protected output: any = {
    id: 'number',
    name: 'string',
    email: 'string',
    age: 'number',
    avatar: uploadsUrl,
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Of course this will not work as we didn't transform the call of uploadsUrl to be a function call, so let's do that in our base resource class

// src/core/resources/resource.ts
import { get } from "@mongez/reinforcements";

// this will be used to skip the output property if it is missing from the given resource
const missingKey = Symbol("missing");

export default class Resource {
   * Constructor
  public constructor(protected resource: any = {}) {

   * Output shape
  protected output: any = {};

   * {@inheritDoc}
  public toJSON() {
    // final output
    const data: Record<string, any> = {};

    // loop through the output property
    for (const key in this.output) {
      // get the value type
      const valueType = this.output[key];
      // get the value, and also make sure to skip the output property if it is missing from the given resource
      let value = get(this.resource, key, missingKey);

      // skip the output property if it is missing from the given resource
      if (value === missingKey) {

      if (typeof valueType === "string") {
        // cast the value
        value = this.cast(value, valueType);
      } else if (typeof valueType === "function") {
        // call the custom output handler
        value = valueType(value);

      // just for now sett the output value to the data
      data[key] = value;

    return data;

   * Builtin casts
  protected cast(value: any, type: string) {
    switch (type) {
      case "number":
        return Number(value);
      case "float":
      case "double":
        return parseFloat(value);
      case "int":
      case "integer":
        return parseInt(value);
      case "string":
        return String(value);
      case "boolean":
        return Boolean(value);
        return value;
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We added another if statement to check the value type, if it is a function then we call it and pass the value to it, and then we return the result.

Now when we check the response, we'll see something like this.

"image": ""

As we only have in the resource object, users/my-image.jpg and we want to return the full url.

🎨 Conclusion

In this chapter, we added another feature which is using custom output handler to mutate the final output value as we want, this will give us much flexibility to customize the final output.

In our next chapter, we'll see how to use nested resources, a resource inside another resource!, stay tuned.

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