Function createComputed

packages/core/primitives/signals/src/computed.ts:59–93

(
  computation: () => T,
  equal?: ValueEqualityFn<T>,
)

 * Create a computed signal which derives a reactive value from an expression.
 */
export function createComputed<T>(
  computation: () => T,
  equal?: ValueEqualityFn<T>,
): ComputedGetter<T> {
  const node: ComputedNode<T> = Object.create(COMPUTED_NODE);
  node.computation = computation;

  if (equal !== undefined) {
    node.equal = equal;
  }

  const computed = () => {
    // Check if the value needs updating before returning it.
    producerUpdateValueVersion(node);

    // Record that someone looked at this signal.
    producerAccessed(node);

    if (node.value === ERRORED) {
      throw node.error;
    }

    return node.value;
  };

  (computed as ComputedGetter<T>)[SIGNAL] = node;
  if (typeof ngDevMode !== 'undefined' && ngDevMode) {
    computed.toString = () =>
      `[Computed${node.debugName ? ' (' + node.debugName + ')' : ''}: ${String(node.value)}]`;
  }

  runPostProducerCreatedFn(node);

  return computed as unknown as ComputedGetter<T>;
}

/**
 * A dedicated symbol used before a computed value has been calculated for the first time.