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Updates the current context of the machine. 
import { assign } from 'xstate';

​
Signature

function assign<
  TContext extends MachineContext,
  TExpressionEvent extends AnyEventObject,
  TParams extends ParameterizedObject['params'] | undefined,
  TEvent extends EventObject,
  TActor extends ProvidedActor
>(
  assignment:
    | Assigner<LowInfer<TContext>, TExpressionEvent, TParams, TEvent, TActor>
    | PropertyAssigner<LowInfer<TContext>, TExpressionEvent, TParams, TEvent, TActor>
): ActionFunction<
  TContext,
  TExpressionEvent,
  TEvent,
  TParams,
  TActor,
  never,
  never,
  never,
  never
>

​
Parameters

​
assignment
Assigner | PropertyAssigner
required
An object that represents the partial context to update, or a function that returns an object that represents the partial context to update.When provided as a function, it receives AssignArgs containing:
  • context - the current state context
  • event - the event that triggered this action
  • spawn - a function to spawn child actors
  • self - reference to the current actor
  • system - reference to the actor system
When provided as an object, each property can be either:
  • A static value to assign
  • A function that receives AssignArgs and returns the value to assign

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Returns

​
ActionFunction
ActionFunction
An action function that updates the machine’s context when executed.

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Examples

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Object assignment

import { createMachine, assign } from 'xstate';

const countMachine = createMachine({
  context: {
    count: 0,
    message: ''
  },
  on: {
    inc: {
      actions: assign({
        count: ({ context }) => context.count + 1
      })
    },
    updateMessage: {
      actions: assign({
        message: ({ event }) => event.message
      })
    }
  }
});

​
Function assignment

const machine = createMachine({
  context: {
    count: 0,
    message: ''
  },
  on: {
    inc: {
      actions: assign(({ context, event }) => {
        return {
          count: context.count + 1,
          message: event.message.trim()
        };
      })
    }
  }
});

​
Using spawn in assign

import { createMachine, assign } from 'xstate';
import { childMachine } from './childMachine';

const machine = createMachine({
  context: {
    childRef: null
  },
  entry: assign({
    childRef: ({ spawn }) => spawn(childMachine, { id: 'child' })
  })
});
Custom actions should not call assign() directly, as it is not imperative. Use it only in machine configurations.
If context is undefined in the machine config, assign will throw an error at runtime.