How to use a postProcess with i18next?

In the i18next internationalization library, postProcess is a powerful post-processing mechanism that enables additional processing of translated strings after translation. This mechanism is specifically designed for handling plurals, formatting, or even implementing custom logic.

Steps to Use postProcess

1. Define Post Processors: During i18next initialization or configuration, define one or more post processors, which can be built-in (e.g., sprintf for data formatting) or custom.

2. Apply Post Processing: When calling the translation function (e.g., t()), specify the post processor to use by passing the postProcess option.

Example

Suppose we need to handle number formatting in the application; we can use the built-in sprintf post processor to achieve this.

First, ensure that sprintf-js is installed, as the sprintf post processor depends on this library:

bash
npm install sprintf-js

Then, initialize i18next and add sprintf as a post processor:

javascript
import i18next from 'i18next';
import sprintf from 'i18next-sprintf-postprocessor';

i18next.use(sprintf).init({
  lng: 'en',
  resources: {
    en: {
      translation: {
        key: 'The price is %s USD'
      }
    }
  }
});

Now, when translating and formatting data, use the t function and specify the postProcess option:

javascript
const price = 25;
const translatedText = i18next.t('key', { postProcess: 'sprintf', sprintf: [price] });
console.log(translatedText); // Output: "The price is 25 USD"

Custom Post Processors

We can also create custom post processors. For example, suppose we want to add a specific suffix to all translated strings:

javascript
function addSuffix(value, key, options) {
  return `${value} - custom suffix`;
}

i18next.init({
  lng: 'en',
  resources: {
    en: {
      translation: {
        greeting: 'Hello, world'
      }
    }
  },
  postProcess: ['addSuffix'],
  postProcessPassResolved: false, // Pass the original value
  interpolation: {
    skipOnVariables: false
  }
});

// Register the custom post processor
i18next.services.pluralResolver.addPostProcessor({
  name: 'addSuffix',
  process: addSuffix
});

const greeting = i18next.t('greeting');
console.log(greeting); // Output: "Hello, world - custom suffix"

In this example, we define an addSuffix post processor that appends a suffix to the original translated string and registers it during i18next initialization.

In this way, postProcess provides flexibility to customize and extend the translation processing workflow according to project requirements.