Your first script

Write a script that draws a 20-period EMA, shades it by trend, and can alert when the price crosses it. Each step adds one idea.

Open the editor

In the terminal, open the chart’s menu and choose New script, or press / for the Indicators dialog and choose New script. The editor checks the script as you type and lists its problems underneath; Add to chart runs it on the chart, and Save keeps it in this browser under its name.

Name it and read the bars

Every script starts with indicator(name, options). overlay: true draws on the price pane; without it, the script gets a pane of its own below.

indicator('My EMA', { overlay: true });

const chart = bars();

bars() with no options is the chart’s own bars. It is a stream: a source of data with a handler that runs on each bar.

Calculate on every bar

Indicators such as ta.ema are created once at the top, then updated in a handler. The handler runs once for every bar of history, and again on every update of the bar still forming.

indicator('My EMA', { overlay: true });

const chart = bars();
const ema = ta.ema(20);
const line = plot.line('EMA');

chart.onBar(() => {
  line.set(ema.update(chart.close));
});

Update an indicator on every bar, before any early return: it only advances on the bars it is updated on.

Add a setting

input.* declares a setting the chart’s settings panel shows. Changing it runs the script again from the first bar.

const length = input.int('Length', 20, { min: 1 });
const ema = ta.ema(length);

Colour by trend

A plot’s set takes an optional colour for that bar. color.up and color.down are the theme’s own, so they follow the reader’s theme.

chart.onBar(() => {
  const value = ema.update(chart.close);
  line.set(value, chart.close >= value ? color.up : color.down);
});

Alert on a cross

alert(name) declares an alert, which the reader arms on the chart. fire fires it when the condition is true, at most once per bar, and only on live data.

indicator('My EMA', { overlay: true });

const length = input.int('Length', 20, { min: 1 });

const chart = bars();
const ema = ta.ema(length);
const crossUp = ta.crossover();
const line = plot.line('EMA', { width: 2 });
const crossed = alert('Close crossed above the EMA');

chart.onBar(() => {
  const value = ema.update(chart.close);
  line.set(value, chart.close >= value ? color.up : color.down);
  crossed.fire(crossUp.update(chart.close, value), `Close ${fmt.price(chart.close)} crossed the EMA`);
});

Next step

Learn how a script runs: the rules behind what you just wrote.

Start

Guides

Examples

Reference

Open the editor

Name it and read the bars

Calculate on every bar

Add a setting

Colour by trend

Alert on a cross

Next step

​Open the editor

​Name it and read the bars

​Calculate on every bar

​Add a setting

​Colour by trend

​Alert on a cross

​Next step

Open the editor

Name it and read the bars

Calculate on every bar

Add a setting

Colour by trend

Alert on a cross

Next step