Bollinger Bands

The Bollinger Bands indicator from the public library (bollinger), written against the current DSL.

What it plots

A middle line on price (the SMA of the close) with upper and lower bands standardDeviations standard deviations away.

Key decisions

Full script

meta({
  shortName: "BB",
  kind: "overlay",
  dataSource: "candles",
  category: "custom",
});

dialog({
  title: "Bollinger Bands Configuration",
  description: "Configure Bollinger Bands settings",
  width: 400,
  height: 400,
  resizable: false,
  showEnabledCheckbox: true,
  resetToDefaults: {
    enabled: true,
    confirmationMessage: "Are you sure you want to reset all Bollinger Bands settings to their default values?",
  },
});

const period = input.number({
  id: "period",
  label: "Period",
  default: 20,
  min: 1,
  step: 1,
});

const standardDeviations = input.number({
  id: "standardDeviations",
  label: "Standard Deviations",
  default: 2,
  min: 0.1,
  step: 0.1,
});

const middleColor = input.color({
  id: "middleColor",
  label: "Middle Color",
  default: "#2962FF",
  allowAlpha: true,
});

const bandColor = input.color({
  id: "bandColor",
  label: "Band Color",
  default: "#FF6D00",
  allowAlpha: true,
});

const lineStyle = input.lineStyle({
  id: "lineStyle",
  label: "Line Style",
  default: LineStyle.Solid,
});

const labelsEnabled = input.boolean({
  id: "labelsEnabled",
  label: "Show Labels",
  default: true,
});

warmup((w) => w.window(period));

layout(
  tab({ id: "general", title: "General" },
    section({ id: "parameters", title: "Parameters" }, period, standardDeviations),
    section({ id: "display", title: "Display Options" }, labelsEnabled),
  ),
  tab({ id: "appearance", title: "Appearance" },
    section({ id: "lines", title: "Lines", columns: 2 }, middleColor, bandColor, lineStyle),
  ),
);

output.line({
  id: "middle",
  lineWidth: 2,
  colorFrom: middleColor,
  lineStyleFrom: lineStyle,
  labelFrom: labelsEnabled,
  titleWhenLabeled: "BB Mid",
});

output.line({
  id: "upper",
  lineWidth: 1,
  colorFrom: bandColor,
  lineStyleFrom: lineStyle,
  labelFrom: labelsEnabled,
  titleWhenLabeled: "BB Upper",
});

output.line({
  id: "lower",
  lineWidth: 1,
  colorFrom: bandColor,
  lineStyleFrom: lineStyle,
  labelFrom: labelsEnabled,
  titleWhenLabeled: "BB Lower",
});

// onBar runs again for the forming bar on every tick. The chart rolls ctx.accum back between
// ticks; foldBar does not depend on that, so the same code is right on every runtime.
// foldBar keeps two states: `done` (as of the last closed bar) and `tip` (this bar). `step`
// builds the tip from `done` without mutating it, so a repeat of the same bar recomputes the tip
// instead of stacking on top of it. A new bar time commits the tip.
function foldBar(ctx, id, init, step) {
  const s = ctx.accum(id, () => ({ t: null, done: init, tip: init }), (s) => {
    if (s.t !== ctx.time) {
      s.done = s.tip;
      s.t = ctx.time;
    }
    s.tip = step(s.done);
    return s;
  });
  return s.tip;
}

function mean(values) {
  let sum = 0;
  for (const v of values) sum += v;
  return sum / values.length;
}

/** @param {ScriptedCtx} ctx */
export function onBar(ctx) {
  const p = ctx.params.period;
  const mult = ctx.params.standardDeviations;

  const closes = foldBar(ctx, "closes", [], (prev) => prev.concat(ctx.close).slice(-p));
  if (closes.length < p) return;

  const sma = mean(closes);
  let sq = 0;
  for (const c of closes) sq += (c - sma) * (c - sma);
  const sd = Math.sqrt(sq / p); // population standard deviation of the same window

  ctx.plot("middle", sma);
  ctx.plot("upper", sma + sd * mult);
  ctx.plot("lower", sma - sd * mult);
}

Related

Series types for bands, and onBar for the forming-bar rule behind foldBar.