> ## Documentation Index
> Fetch the complete documentation index at: https://docs.darvas.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Bar context

> ctx provides OHLCV data, bar metadata, and index information for the current bar inside onBar.

## Overview

`ctx` is a global object available everywhere in your script. Inside `onBar` it reflects the current bar being processed. All OHLCV accessors are **functions** that accept an optional `offset` argument - the number of bars to look back from the current bar.

```js theme={null}
onBar(() => {
  const close  = ctx.close();    // current bar close
  const prev   = ctx.close(1);   // previous bar close
  const change = close - prev;
});
```

## Bar metadata

<ParamField path="ctx.length" type="number">
  Total number of bars in the current window.
</ParamField>

<ParamField path="ctx.bar" type="string">
  Current timeframe string, e.g. `"1H"`, `"4H"`, `"1D"`.
</ParamField>

<ParamField path="ctx.instId" type="string">
  Current instrument, e.g. `"BTC-USDT-SWAP"`.
</ParamField>

<ParamField path="ctx.i()" type="number">
  Current bar index. 0 = oldest bar in window, `ctx.length - 1` = newest.
</ParamField>

<ParamField path="ctx.barIndex()" type="number">
  Alias for `ctx.i()`.
</ParamField>

<ParamField path="ctx.isLast()" type="boolean">
  `true` when processing the most recent (realtime) bar.
</ParamField>

## OHLCV accessors

All accessors accept an optional `offset` - bars back from the current bar. `offset = 0` (default) is the current bar; `offset = 1` is the previous bar.

<ParamField path="ctx.open(offset?)" type="number | null">
  Open price.
</ParamField>

<ParamField path="ctx.high(offset?)" type="number | null">
  High price.
</ParamField>

<ParamField path="ctx.low(offset?)" type="number | null">
  Low price.
</ParamField>

<ParamField path="ctx.close(offset?)" type="number | null">
  Close price.
</ParamField>

<ParamField path="ctx.volume(offset?)" type="number | null">
  Volume.
</ParamField>

<ParamField path="ctx.time(offset?)" type="number | null">
  Bar open time as unix seconds (UTC).
</ParamField>

## Composite price accessors

<ParamField path="ctx.hl2(offset?)" type="number | null">
  `(high + low) / 2`
</ParamField>

<ParamField path="ctx.hlc3(offset?)" type="number | null">
  `(high + low + close) / 3` - typical price, commonly used with `ta.cci`.
</ParamField>

<ParamField path="ctx.ohlc4(offset?)" type="number | null">
  `(open + high + low + close) / 4`
</ParamField>

## Passing accessors as sources

The OHLCV accessors are **function references** - they match the `(offset?) => number | null` signature that `ta.*` functions expect as their `source` argument:

```js theme={null}
// Correct: pass ctx.close as a function reference
const rsi = ta.rsi(ctx.close, 14);

// Correct: input.source returns the same shape
const src = input.source("Source", "close");
const rsi2 = ta.rsi(src, 14);

// Wrong: calling ctx.close() gives you a scalar, not a source
// const rsi3 = ta.rsi(ctx.close(), 14); // TypeError
```

## Example: multi-bar lookback

```js theme={null}
onBar(() => {
  const c0 = ctx.close();   // current close
  const c3 = ctx.close(3);  // close 3 bars ago

  // Detect 3-bar momentum
  const momentum = c0 - c3;
  plot("3-bar Momentum", momentum);

  // Log bar info on last bar
  if (ctx.isLast()) {
    console.log(ctx.instId, ctx.bar, "index:", ctx.i());
  }
});
```

## Related pages

<CardGroup cols={2}>
  <Card title="Inputs" href="/essentials/inputs">
    input.source returns the same function shape.
  </Card>

  <Card title="Moving averages" href="/ta/moving-averages">
    ta.\* functions that accept a source parameter.
  </Card>

  <Card title="NaN handling" href="/essentials/nan-handling">
    What to do when ctx.close returns null.
  </Card>
</CardGroup>