init()

Declare indicators and initialize state. Called once before the backtest starts.

Signature

function init(ctx)

Parameters

ctx.indicator()

Declare indicators that will be pre-computed by the engine:

ctx.indicator(name, type, options)

Key insight: Indicators are computed by the engine on the host side before onBar runs. This is much faster than computing them yourself.

Display Option

Add display: true to show the indicator on the chart:

// This indicator will appear on the price chart
ctx.indicator('ema', 'EMA', { period: 21, source: 'close', display: true });

// This indicator is computed but NOT shown on chart (default)
ctx.indicator('rsi', 'RSI', { period: 14 });

Example

function init(ctx) {
  // Trend indicators - shown on chart
  ctx.indicator('fastEma', 'EMA', { period: ctx.p.fastLength, source: 'close', display: true });
  ctx.indicator('slowEma', 'EMA', { period: ctx.p.slowLength, source: 'close', display: true });
  
  // Momentum - not displayed (used for signal logic only)
  ctx.indicator('rsi', 'RSI', { period: 14 });
  ctx.indicator('macd', 'MACD', { fastPeriod: 12, slowPeriod: 26, signalPeriod: 9 });
  
  // Volatility
  ctx.indicator('atr', 'ATR', { period: 14 });
  ctx.indicator('bbands', 'BBANDS', { period: 20, stdDev: 2 });
  
  // Initialize state
  ctx.state.tradeCount = 0;
  ctx.state.lastSignalBar = -1;
  ctx.state.inPosition = false;
}

Available Indicators

Moving Averages

Trend

Momentum / Oscillators

Volatility

Volume

Source Options

The source option can be:

• 'open', 'high', 'low', 'close', 'volume'
• 'hl2' — (high + low) / 2
• 'hlc3' — (high + low + close) / 3
• 'ohlc4' — (open + high + low + close) / 4

ctx.state

Initialize mutable state that persists across bars:

function init(ctx) {
  ctx.state.consecutiveLosses = 0;
  ctx.state.peakEquity = 0;
  ctx.state.lastEntryPrice = null;
}

State is accessed in onBar via ctx.state:

function onBar(ctx, i) {
  ctx.state.consecutiveLosses++;
}

Multi-Output Indicators

Some indicators return multiple series:

function init(ctx) {
  ctx.indicator('macd', 'MACD', { fastPeriod: 12, slowPeriod: 26, signalPeriod: 9 });
  ctx.indicator('bbands', 'BBANDS', { period: 20, stdDev: 2 });
}

function onBar(ctx, i) {
  // Access MACD components
  const macdLine = ctx.ind.macd.macd[i];
  const signalLine = ctx.ind.macd.signal[i];
  const histogram = ctx.ind.macd.histogram[i];
  
  // Access Bollinger Bands
  const upper = ctx.ind.bbands.upper[i];
  const middle = ctx.ind.bbands.middle[i];
  const lower = ctx.ind.bbands.lower[i];
}

Common Mistakes

Computing indicators in onBar

// WRONG - Don't compute indicators yourself
function onBar(ctx, i) {
  const ema = calculateEma(ctx.series.close, 20); // Slow!
}

// RIGHT - Declare in init, engine computes them
function init(ctx) {
  ctx.indicator('ema', 'EMA', { period: 20 });
}

Forgetting to initialize state

// WRONG - state.count is undefined on first bar
function onBar(ctx, i) {
  ctx.state.count++; // NaN!
}

// RIGHT - Initialize in init
function init(ctx) {
  ctx.state.count = 0;
}

Related

• define() — Parameter declaration
• onBar() — Trading logic
• Available Indicators — Full indicator reference