Initial state

When a state machine starts, it starts by entering the machine’s initial state. Likewise, when a compound state is entered, its initial state is also entered. The initial state is not a separate state, but more like an indication of which state that the machine should start in by default.

A transition from a different state can target a substate of a compound state, thereby bypassing the state’s initial state. An initial state should be thought of as the default starting point of a compound state if a transition points directly to the compound state.

Notation

A state machine, or compound state’s initial state is specified by way a black circle with exactly one transition arrow to a sibling state that should be entered. The black circle and arrow have no name, it is the target state that is called the ‘initial’ state.

Black circle pointing to a state labeled A. A is the initial state.

Usage

Initial states are impossible to avoid, and follows from the definition of compound states: For a compound state, whenever it becomes active, exactly one of its direct children are also active. The “initial” state simply denotes which of the child states become active when this happens.

Picking the right initial state can help decouple the various parts of the statechart. It allows transitions to target the compound (parent) state instead of a child directly. This allows collapsing the view of a compound state to see the bigger picture.

SCXML

In Statechart XML, the initial state is denoted in one of these ways:

using the initial attribute of the <state> element:

<state id="P" initial="A">
  <state id="B"/>
  <state id="A"/>
</state>

Using the <initial> element instead of a <state> element. The <initial> element is identical to any other <state> except that it is an <initial> state.
```
<state id="P">
  <state id="B"/>
  <initial id="A"/>
</state>
```
Implicitly, by relying on document order, the first <state> element will be the initial state:
```
<state id="P">
  <state id="A"/>
  <state id="B"/>
</state>
```

XState

In XState, a compound state must have the initial property declaring the key of the initial child state node.

"P": {
  "type": "compound", // optional
  "initial" : "A",
  "states" : {
    "A": {},
    "B": {}
  }
}