EMA cross

Fires when a fast EMA crosses above a slow EMA. It reads the library Moving Average indicator twice instead of computing EMAs itself.

What it defines

An event ema_cross_up on the bar where the fast EMA moves from at or below the slow EMA to above it.

Key decisions

Piece	Why it matters
Two `indicators.declare` calls on slug `ma`	Same indicator, different `as` alias and settings.
`ctx.indicators.<as>.ma`	Reads are always nested by the dependency's output key: `ma` for the Moving Average.
`.at(1)` for the previous value	The MA plots every bar, so `.at(1)` is one bar back. No `ctx.accum`, so no `warmup` to declare.
Guard `== null`	The MA has no value until its period has filled. Skip those bars.
No `oncePer`	A cross is already one bar. Every cross is its own event.

Full script

meta({
  name: "EMA cross up",
  kind: "flow",
  slug: "ema-cross-up",
  shortName: "EmaXUp",
});

indicators.declare({
  slug: "ma",
  as: "fast",
  version: 1,
  settings: { period: 9, maType: "EMA" },
});

indicators.declare({
  slug: "ma",
  as: "slow",
  version: 1,
  settings: { period: 21, maType: "EMA" },
});

events.declare({
  id: "ema_cross_up",
  intent: "signal",
  payload: {
    fast: { type: "number", description: "Fast EMA on the cross bar" },
    slow: { type: "number", description: "Slow EMA on the cross bar" },
    close: { type: "number", description: "Close on the cross bar" },
  },
  paint: "pin",
});

outputs.declare({ id: "fast_line", kind: "line", name: "Fast", color: "#26a69a" });
outputs.declare({ id: "slow_line", kind: "line", name: "Slow", color: "#ef5350" });

export function onBar(ctx) {
  const fast = ctx.indicators.fast.ma.value;
  const slow = ctx.indicators.slow.ma.value;
  if (fast == null || slow == null) return;

  ctx.plot("fast_line", fast);
  ctx.plot("slow_line", slow);

  const prevFast = ctx.indicators.fast.ma.at(1);
  const prevSlow = ctx.indicators.slow.ma.at(1);
  if (prevFast == null || prevSlow == null) return;

  if (prevFast <= prevSlow && fast > slow) {
    ctx.emit(
      "ema_cross_up",
      { fast, slow, close: ctx.close },
      {
        label: "EMA cross ↑",
        subtitle: `${fast.toFixed(2)} > ${slow.toFixed(2)}`,
        price: ctx.close,
      }
    );
  }
}

version pins the Moving Average version the definition reads. Set it to the version the indicator library shows: pinning a version that does not exist fails lint with indicators/version-not-found. See Declaring indicator dependencies.

Next, Session OR break keeps its own state across bars.

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