Observable Framework 1.5.1 GitHub️ 1.8k

Comma-separated values

To load a comma-separated values (CSV) file, use FileAttachment.csv. The csv, tsv, and dsv method implementations are based on RFC 4180.

const gistemp = FileAttachment("gistemp.csv").csv({typed: true});

The value of gistemp above is a promise to an array of objects. In other code blocks, the promise is resolved implicitly and hence you can refer to it as an array of objects.

gistemp

The column names are listed in the columns property:

gistemp.columns

You can also load a tab-separated values (TSV) file using FileAttachment.tsv:

const capitals = FileAttachment("us-state-capitals.tsv").tsv({typed: true});
Inputs.table(capitals)

For a different delimiter, use FileAttachment.dsv. For example, for semicolon separated values:

const capitals = FileAttachment("us-state-capitals.csv").dsv({delimiter: ";", typed: true});

Type coercion

A common pitfall with CSV is that it is untyped: numbers, dates, booleans, and every other possible value are represented as text and there is no universal way to automatically determine the correct type.

For example, a map might identify U.S. states by FIPS code — Alabama as "01", Michigan as "26", and so on — and these identifiers should be treated as strings when looking up the values in your dataset. But in other cases — say if 1 and 26 represent temperature in degrees Celsius — you should convert these values to numbers before doing math or passing them to Observable Plot. If you don’t type your data, you may inadvertently concatenate when you meant to add, or get an ordinal rather than quantitative scale. You should type data as early as possible — when you load it — to prevent unexpected behavior later.

Dates can be particularly challenging in CSV as there are myriad ways to encode dates as text. We recommend ISO 8601 and UTC.

To coerce types automatically, set the typed option to true with file.csv or file.tsv. This uses d3.autoType to infer types and then coerce them.

If your file is not compatible with d3.autoType, you may get unexpected or invalid results; you should inspect the returned data and if needed use {typed: false} (the default) and coerce the types yourself. Here is an example of coercing types manually using a custom date format:

import {utcParse} from "npm:d3-time-format";

const parseDate = utcParse("%Y-%m-%d");
const coerceRow = (d) => ({Date: parseDate(d.Date), Anomaly: Number(d.Anomaly)});
const gistemp = FileAttachment("gistemp.csv").csv().then((D) => D.map(coerceRow));

Above, any date value that does not match the expected format will be cast as an Invalid Date, and any anomaly value that is not a number (as when the file says N/A to represent missing data) will be cast as NaN.

The file.csv and file.tsv methods assume that the first line of the file is a header indicating the (distinct) name of each column. Each subsequent line is considered as a row and converted to an object with the column names as keys.

If your file does not have such a header line, set the array option to true to get back an array of arrays instead:

FileAttachment("gistemp.csv").csv({array: true, typed: true})