Knockout extenders

Knockout Extenders

When working with TypeScript and Knockout.js, you may come across a scenario where you need to extend the functionality of observables. This is where Knockout extenders come into play. Extenders allow you to add custom behavior to observables, such as validation or formatting.

In this blog post, we will explore how to use Knockout extenders in TypeScript and provide multiple solutions to common problems you may encounter.

Solution 1: Creating a Basic Extender

To create a basic extender in TypeScript, you can use the ko.extenders function provided by Knockout.js. Here’s an example of how to create an extender that limits the length of a string:

import * as ko from 'knockout';

function maxLength(target: any, maxLength: number) {
  const result = ko.pureComputed({
    read: target,
    write: (newValue: string) => {
      if (newValue.length > maxLength) {
        target(newValue.substring(0, maxLength));
      } else {
        target(newValue);
      }
    }
  });

  return result;
}

ko.extenders.maxLength = maxLength;

// Usage
const name = ko.observable("").extend({ maxLength: 10 });

name("John Doe");
console.log(name()); // Output: "John Doe"

name("This is a very long name");
console.log(name()); // Output: "This is a"

In this example, we define a custom extender called maxLength that limits the length of the string to the specified maxLength value. The extender is then applied to the name observable using the extend function.

Solution 2: Chaining Multiple Extenders

Knockout extenders can be chained together to apply multiple behaviors to an observable. Let’s say you want to limit the length of a string and make it uppercase. Here’s how you can achieve that:

function maxLengthAndUpperCase(target: any, maxLength: number) {
  const result = ko.pureComputed({
    read: target,
    write: (newValue: string) => {
      if (newValue.length > maxLength) {
        target(newValue.substring(0, maxLength).toUpperCase());
      } else {
        target(newValue.toUpperCase());
      }
    }
  });

  return result;
}

ko.extenders.maxLengthAndUpperCase = maxLengthAndUpperCase;

// Usage
const address = ko.observable("").extend({ maxLengthAndUpperCase: 20 });

address("123 Main Street");
console.log(address()); // Output: "123 MAIN STREET"

address("This is a very long address");
console.log(address()); // Output: "THIS IS A VERY LON"

In this example, we define a custom extender called maxLengthAndUpperCase that combines the functionality of the maxLength and toUpperCase extenders. The extender is then applied to the address observable.

By chaining multiple extenders together, you can create powerful and flexible observables that meet your specific requirements.

Knockout extenders are a powerful tool for extending the functionality of observables in TypeScript. Whether you need to limit the length of a string, validate user input, or apply any other custom behavior, extenders provide a clean and reusable solution. By following the examples provided in this blog post, you’ll be able to leverage extenders effectively in your TypeScript projects.

Knockout extenders