TypeScript
i18next has embedded type definitions. If you want to enhance IDE Experience and prevent errors (such as type coercion), you should follow the instructions below in order to get the t function fully-type safe (keys and return type).
If your project spans multiple i18next instances with different translation resources, you probably can't use type-safe translations.
Here you'll find a simple guide on how to best use TypeScript for i18next.
Discover how to unleash the full potential of i18next in your TypeScript applications by mastering type-safe translations, ensuring accurate localization and eliminating runtime errors.
Create a declaration file
TypeScript definitions for i18next can be extended by using Type Augmentation and Merging Interfaces. So the first step is creating a declaration file (i18next.d.ts
), for example:
// import the original type declarations
import "i18next";
// import all namespaces (for the default language, only)
import ns1 from "locales/en/ns1.json";
import ns2 from "locales/en/ns2.json";
declare module "i18next" {
// Extend CustomTypeOptions
interface CustomTypeOptions {
// custom namespace type, if you changed it
defaultNS: "ns1";
// custom resources type
resources: {
ns1: typeof ns1;
ns2: typeof ns2;
};
// other
}
}
Or, if you want to include all namespaces at once, you can use our preferred approach:
i18n.ts
export const defaultNS = "ns1";
export const resources = {
en: {
ns1,
ns2,
},
} as const;
i18n.use(initReactI18next).init({
lng: "en",
ns: ["ns1", "ns2"],
defaultNS,
resources,
});
i18next.d.ts
import { resources, defaultNS } from "./i18n";
declare module "i18next" {
interface CustomTypeOptions {
defaultNS: typeof defaultNS;
resources: typeof resources["en"];
}
}
We recommend creating a @types
directory under src
or above it and placing all your type declarations there. E.g.: @types/i18next.d.ts
Some examples
various examples (from simple i18next only to react-i18next prod ready)
Custom Type Options
We provide a few options that can improve TypeScript for i18next
. All options come with default values, and if you want to change them, you just need to add them under CustomTypeOptions
interface in your i18next type declaration file (i18next.d.ts
).
defaultNS
'translation'
Default namespace. This is more practical in React applications, so when you call useTranslation()
hooks without passing the namespace, it will infer the types for the translation
namespace.
resources
object
Resources to initialize with. This is the most important option that is used to infer the appropriate keys and return types.
fallbackNS
false
Fallback namespace. string or array of namespaces to lookup key if not found in given namespace. See NS fallback docs.
keySeparator
'.'
Char to separate keys.
nsSeparator
':'
Char to split namespace from key
pluralSeparator
'_'
Char to split namespace from key
contextSeparator
'_'
Char to split context from key
returnNull
true
Allows null values as valid translation.
returnObjects
false
Allows objects as valid translation result
allowObjectInHTMLChildren
false
Flag that allows HTML elements to receive objects. This is only useful for React applications where you pass objects to HTML elements so they can be replaced to their respective interpolation values (mostly with Trans component)
interpolationPrefix
'{{'
Prefix for interpolation
interpolationSuffix
'}}'
Suffix for interpolation
strictKeyChecks
false
Flag that enables strict key checking even if a defaultValue
has been provided. This ensures all calls of t
function don't accidentally use implicitly missing keys.
Troubleshooting
Intellisense not working
Try to update the used TypeScript version (>= v5 is recommended).
Out of memory (OOM) errors
Running typechecking with key validation might result in OOM errors. This can be facilitated by additional factors like:
large codebase with a lot of namespace with hundreds of keys
running typechecking alongside other tools like
ESLint
combined withtypescript-eslint
If you report a OOM error, please provide an easy way to reproduce the issue using:
online sandbox
example repository
Not working interpolation values
t
function infers interpolation values, but it'll only work if the translation files (resources) are placed in a ts file and using as const
(like this) or an interface in a d.ts file (can be generated like this), JSON files don't support as const
to convert objects to be type literals (yet).
Type 'HTMLAttributes<T>' is not assignable to type...
Type 'HTMLAttributes<T>' is not assignable to type...
This happens when skipLibCheck
is disabled. Setting skipLibCheck
in tsconfig to true
will remove this issue.
Type error - template literal
If you face this issue:
Argument of type 'string' is not assignable to parameter of type ...
When using the following approach (template literal with an expression):
// with i18next
i18next.t(`${expression}.title`);
// with react-i18next
const { t } = useTranslation();
t(`${expression}.title`);
Or:
// with react-i18next
const { t } = useTranslation(`${ns}Default`);
TypeScript will lose the literal value, and it will infer the key
as string, which will cause to throw the error above. In this case, you will need to assert the template string as const
, like this:
// with i18next
i18next.t(`${expression}.title` as const);
// with react-i18next
const { t } = useTranslation();
t(`${expression}.title` as const);
For now, this is the only possible workaround. This is a TypeScript limitation that will be address at some point in the future.
Type error - excessively deep and possibly infinite
If you face this issue whenever calling the t
function:
TS2589: Type instantiation is excessively deep and possibly infinite.
That probably means you did not set up your type declaration correctly, so review your configuration or check here for some similar cases that may help you. If needed, you can always open an issue on Github to get some help from us.
Tagged Template Literal (react-i18next
only)
react-i18next
only)If you are using the tagged template literal syntax for the t
function, like this:
t`key1.key2`;
The keys
and return
type inference will not work, because TemplateStringsArray does not accept generic types yet. You can use Tagged Template Literal syntax, but it will accept any string as argument.
Customize t
function return when returnObjects
is set to true
but CustomTypeOptions.resources
is not used (>= v23)
t
function return when returnObjects
is set to true
but CustomTypeOptions.resources
is not used (>= v23)When no resources
are defined inside CustomTypeOptions
and returnObject
options is set to true
t
function returns a $SpecialObject
type:
type $SpecialObject = object | Array<string | object>;
Due to his anatomy it can be easily casted to a better defined type as you can see from the following examples:
Example with object
const tResult = t('myTypeKey', { returnObjects: true }) as { title: string, text: string };
expectTypeOf(tResult).toEqualTypeOf<{ title: string; text: string }>();
Example with array
const tResult = t('myTypeKey', { returnObjects: true }) as Array<string>;
expectTypeOf(tResult).toEqualTypeOf<Array<string>>();
Example without casting using type parameters
type MyCustomReturn = { title:string; text: string };
const tResult = t<string, { returnObjects: true }, MyCustomReturn>('myKey', { returnObjects: true });
expectTypeOf(tResult).toEqualTypeOf<MyCustomReturn>();
Troubleshooting (< 23)
Slow compilation time
In order to fully type the t
function, we recursively map all nested keys from your primary locale files or objects. Depending on the number of keys your project have, the compilation time could be noticeably affected. If this is negatively influencing your productivity, this feature might not be the best choice for you. If needed, you can always open an issue on Github to get some help from us.
Argument of type 'DefaultTFuncReturn' is not assignable to parameter of type xyz
t
function can return null
, this behaviour is set by default, if you want to change it, set returnNull
type to false
.
// i18next.d.ts
import 'i18next';
declare module 'i18next' {
interface CustomTypeOptions {
returnNull: false;
...
}
}
I also recommend updating your i18next configuration to behave accordantly:
i18next.init({
returnNull: false,
// ...
});
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