Imperative API

There are a few API layers that React Intl provides and is built on. When using React Intl you'll be interacting with its API (documented here) and its React components.

Why Imperative API?

While our components provide a seamless integration with React, the imperative API are recommended (sometimes required) in several use cases:

  • Setting text attributes such as title, aria-label and the like where a React component cannot be used (e.g `)
  • Formatting text/datetime... in non-React environment such as Node, Server API, Redux store, testing...
  • High performance scenarios where the number of React components rendered becomes the bottleneck (e.g Finance stock portfolio rendering, virtual tables with a lot of cells...)

The intl object

The core of react-intl is the intl object (of type IntlShape), which is the instance to store a cache of all Intl.* APIs, configurations, compiled messages and such. The lifecycle of the intl object is typically tied to the locale & the list of messages that it contains, which means when you switch locale, this object should be recreated.

tip

The intl object should be reused as much as possible for performance.

There are a few ways to get access to the intl object:

  • useIntl hook: Once you've declared your IntlProvider, you can get access to the intl object via calling this hook in your functional React component
  • injectIntl HOC: In class-based React components, you can wrap them with the injectIntl HOC and intl should be available as a prop.
  • createIntl: In a non-React environment (Node, vue, angular, testing... you name it), you can directly create a intl object by calling this function with the same configuration as the IntlProvider.

useIntl hook

If a component can be expressed in a form of function component, using useIntl hook can be handy. This useIntl hook does not expect any option as its argument when being called. Typically, here is how you would like to use:

import React from 'react'
import {useIntl, FormattedDate} from 'react-intl'
const FunctionComponent: React.FC<{date: number | Date}> = ({date}) => {
const intl = useIntl()
return (
<span title={intl.formatDate(date)}>
<FormattedDate value={date} />
</span>
)
}
export default FunctionComponent

To keep the API surface clean and simple, we only provide useIntl hook in the package. If preferable, user can wrap this built-in hook to make customized hook like useFormatMessage easily. Please visit React's official website for more general introduction on React hooks.

injectIntl HOC

type WrappedComponentProps<IntlPropName extends string = 'intl'> = {
[k in IntlPropName]: IntlShape
}
type WithIntlProps<P> = Omit<P, keyof WrappedComponentProps> & {
forwardedRef?: React.Ref<any>
}
function injectIntl<
IntlPropName extends string = 'intl',
P extends WrappedComponentProps<IntlPropName> = WrappedComponentProps<any>
>(
WrappedComponent: React.ComponentType<P>,
options?: Opts<IntlPropName>
): React.ComponentType<WithIntlProps<P>> & {
WrappedComponent: typeof WrappedComponent
}

This function is exported by the react-intl package and is a High-Order Component (HOC) factory. It will wrap the passed-in React component with another React component which provides the imperative formatting API into the wrapped component via its props. (This is similar to the connect-to-stores pattern found in many Flux implementations.)

By default, the formatting API will be provided to the wrapped component via props.intl, but this can be overridden when specifying options.intlPropName. The value of the prop will be of type IntlShape, defined in the next section.

import React, {PropTypes} from 'react'
import {injectIntl, FormattedDate} from 'react-intl'
interface Props {
date: Date | number
}
const FunctionalComponent: React.FC<Props> = props => {
const {
date,
intl, // Injected by `injectIntl`
} = props
return (
<span title={intl.formatDate(date)}>
<FormattedDate value={date} />
</span>
)
}
export default injectIntl(FunctionalComponent)

createIntl

This allows you to create an IntlShape object without using Provider. This allows you to format things outside of React lifecycle while reusing the same intl object. For example:

import {createIntl, createIntlCache, RawIntlProvider} from 'react-intl'
// This is optional but highly recommended
// since it prevents memory leak
const cache = createIntlCache()
const intl = createIntl({
locale: 'fr-FR',
messages: {}
}, cache)
// Call imperatively
intl.formatNumber(20)
// Pass it to IntlProvider
<RawIntlProvider value={intl}>{foo}</RawIntlProvider>

IntlShape

interface IntlConfig {
locale: string
timeZone?: string
formats: CustomFormats
textComponent?: React.ComponentType | keyof React.ReactHTML
messages: Record<string, string> | Record<string, MessageFormatElement[]>
defaultLocale: string
defaultFormats: CustomFormats
onError(err: string): void
}
interface IntlFormatters {
formatDate(value: number | Date, opts: FormatDateOptions): string
formatTime(value: number | Date, opts: FormatDateOptions): string
formatRelativeTime(
value: number,
unit: Unit,
opts: FormatRelativeOptions
): string
formatNumber(value: number, opts: FormatNumberOptions): string
formatPlural(value: number, opts: FormatPluralOptions): string
formatMessage(descriptor: MessageDescriptor, values: any): string
}
type IntlShape = IntlConfig & IntlFormatters

This interface is exported by the react-intl package that can be used in conjunction with the injectIntl HOC factory function.

The definition above shows what the props.intl object will look like that's injected to your component via injectIntl. It's made up of two parts:

  • IntlConfig: The intl metadata passed as props into the parent <IntlProvider>.
  • IntlFormatters: The imperative formatting API described below.

locale, formats, and messages

The user's current locale and what the app should be rendered in. While defaultLocale and defaultFormats are for fallbacks or during development and represent the app's default. Notice how there is no defaultMessages, that's because each Message Descriptor provides a defaultMessage.

defaultLocale and defaultFormats

Default locale & formats for when a message is not translated (missing from messages). defaultLocale should be the locale that defaultMessages are declared in so that a sentence is coherent in a single locale. Without defaultLocale and/or if it's set incorrectly, you might run into scenario where a sentence is in English but embeded date/time is in Spanish.

textComponent

Provides a way to configure the default wrapper for React Intl's <Formatted*> components. If not specified, <React.Fragment> is used. Before V3, span was used instead; check the migration guide for more info.

onError

Allows the user to provide a custom error handler. By default, error messages are logged using console.error if NODE_ENV is not set to production.

wrapRichTextChunksInFragment

When formatting rich text message, the output we produced is of type Array<string | React.ReactElement>, which will trigger key error. This wraps the output in a single React.Fragment to suppress that.

defaultRichTextElements

A map of tag to rich text formatting function. This is meant to provide a centralized way to format common tags such as <b>, <p>... or enforcing certain Design System in the codebase (e.g standardized <a> or <button>...). See https://github.com/formatjs/formatjs/issues/1752 for more context.

formatDate

function formatDate(
value: number | Date,
options?: Intl.DateTimeFormatOptions & {format?: string}
): string

This function will return a formatted date string. It expects a value which can be parsed as a date (i.e., isFinite(new Date(value))), and accepts options that conform to DateTimeFormatOptions.

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Result
9/20/2020

formatTime

function formatTime(
value: number | Date,
options?: Intl.DateTimeFormatOptions & {format?: string}
): string

This function will return a formatted date string, but it differs from formatDate by having the following default options:

{
hour: 'numeric',
minute: 'numeric',
}

It expects a value which can be parsed as a date (i.e., isFinite(new Date(value))), and accepts options that conform to DateTimeFormatOptions.

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SyntaxError: Unexpected token (1:49)
1 : return (intl.formatTime(Date.now()) // "4:03 PM")
                                                     ^

formatRelativeTime

browser support

This requires Intl.RelativeTimeFormat which has limited browser support. Please use our polyfill if you plan to support them.

type Unit =
| 'second'
| 'minute'
| 'hour'
| 'day'
| 'week'
| 'month'
| 'quarter'
| 'year'
type RelativeTimeFormatOptions = {
numeric?: 'always' | 'auto'
style?: 'long' | 'short' | 'narrow'
}
function formatRelativeTime(
value: number,
unit: Unit,
options?: Intl.RelativeTimeFormatOptions & {
format?: string
}
): string

This function will return a formatted relative time string (e.g., "1 hour ago"). It expects a value which is a number, a unit and options that conform to Intl.RelativeTimeFormatOptions.

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in 0 seconds
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Result
24 hr. ago

formatNumber

This function uses Intl.NumberFormat options.

function formatNumber(
value: number,
options?: Intl.NumberFormatOptions & {format?: string}
): string

This function will return a formatted number string. It expects a value which can be parsed as a number, and accepts options that conform to NumberFormatOptions.

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Result
$1,000.00

Formatting Number using unit

Currently this is part of ES2020 NumberFormat. We've provided a polyfill here and react-intl types allow users to pass in a sanctioned unit:

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1,000kB
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1,000 degrees Fahrenheit

formatPlural

type PluralFormatOptions = {
type?: 'cardinal' | 'ordinal' = 'cardinal'
}
function formatPlural(
value: number,
options?: Intl.PluralFormatOptions
): 'zero' | 'one' | 'two' | 'few' | 'many' | 'other'

This function will return a plural category string: "zero", "one", "two", "few", "many", or "other". It expects a value which can be parsed as a number, and accepts options that conform to PluralFormatOptions.

This is a low-level utility whose output could be provided to a switch statement to select a particular string to display.

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one
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other
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other
multiple language support

This function should only be used in apps that only need to support one language. If your app supports multiple languages use formatMessage instead.

formatList

browser support

This requires Intl.ListFormat which has limited browser support. Please use our polyfill if you plan to support them.

type ListFormatOptions = {
type?: 'disjunction' | 'conjunction' | 'unit'
style?: 'long' | 'short' | 'narrow'
}
function formatList(
elements: (string | React.ReactNode)[],
options?: Intl.ListFormatOptions
): string | React.ReactNode[]

This function allows you to join list of things together in an i18n-safe way. For example, when the locale is en:

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Me, myself, and I
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5 hours, 3 minutes

formatDisplayName

browser support

This requires Intl.DisplayNames which has limited browser support. Please use our polyfill if you plan to support them.

type FormatDisplayNameOptions = {
style?: 'narrow' | 'short' | 'long'
type?: 'language' | 'region' | 'script' | 'currency'
fallback?: 'code' | 'none'
}
function formatDisplayName(
value: string | number | object,
options?: FormatDisplayNameOptions
): string | undefined

Usage examples:

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Simplified Chinese (Singapore)
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Devanagari
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Chinese Yuan
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United Nations

formatMessage

Message Syntax

String/Message formatting is a paramount feature of React Intl and it builds on ICU Message Formatting by using the ICU Message Syntax. This message syntax allows for simple to complex messages to be defined, translated, and then formatted at runtime.

Simple Message:

Hello, {name}

Complex Message:

Hello, {name}, you have {itemCount, plural,
=0 {no items}
one {# item}
other {# items}
}.

See: The Message Syntax Guide.

Message Descriptor

React Intl has a Message Descriptor concept which is used to define your app's default messages/strings and is passed into formatMessage. The Message Descriptors work very well for providing the data necessary for having the strings/messages translated, and they contain the following properties:

  • id: A unique, stable identifier for the message
  • description: Context for the translator about how it's used in the UI
  • defaultMessage: The default message (probably in English)
type MessageDescriptor = {
id: string
defaultMessage?: string
description?: string | object
}
Extracting Message Descriptor

You can extract inline-declared messages from source files using our CLI.

Message Formatting Fallbacks

The message formatting APIs go the extra mile to provide fallbacks for the common situations where formatting fails; at the very least a non-empty string should always be returned. Here's the message formatting fallback algorithm:

  1. Lookup and format the translated message at id, passed to <IntlProvider>.
  2. Fallback to formatting the defaultMessage.
  3. Fallback to source of translated message at id.
  4. Fallback to source of defaultMessage.
  5. Fallback to the literal message id.

Above, "source" refers to using the template as is, without any substitutions made.

Usage

type MessageFormatPrimitiveValue = string | number | boolean | null | undefined
function formatMessage(
descriptor: MessageDescriptor,
values?: Record<string, MessageFormatPrimitiveValue>
): string
function formatMessage(
descriptor: MessageDescriptor,
values?: Record<
string,
MessageFormatPrimitiveValue | React.ReactElement | FormatXMLElementFn
>
): string | React.ReactNodeArray

This function will return a formatted message string. It expects a MessageDescriptor with at least an id property, and accepts a shallow values object which are used to fill placeholders in the message.

If a translated message with the id has been passed to the <IntlProvider> via its messages prop it will be formatted, otherwise it will fallback to formatting defaultMessage. See: Message Formatting Fallbacks for more details.

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Hello, Eric!

with ReactElement

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Hello, Eric!

with rich text formatting

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Hello, Eric!

The message we defined using defineMessages to support extraction via babel-plugin-react-intl, but it doesn't have to be if you're not using the Babel plugin.

simple message

Messages can be simple strings without placeholders, and that's the most common type of message.

defineMessages/defineMessage

interface MessageDescriptor {
id?: string
description?: string | object
defaultMessage?: string
}
function defineMessages(
messageDescriptors: Record<string, MessageDescriptor>
): Record<string, MessageDescriptor>
function defineMessage(messageDescriptor: MessageDescriptor): MessageDescriptor

These functions is exported by the react-intl package and is simply a hook for our CLI & babel/TS plugin to use when compiling default messages defined in JavaScript source files. This function simply returns the Message Descriptor map object that's passed-in.

import {defineMessages, defineMessage} from 'react-intl'
const messages = defineMessages({
greeting: {
id: 'app.home.greeting',
description: 'Message to greet the user.',
defaultMessage: 'Hello, {name}!',
},
})
const msg = defineMessage({
id: 'single',
defaultMessage: 'single message',
description: 'header',
})