Option `cast`

The cast option define multiple function to transform values based on their type. It is an object where the keys represent the type and their associated values are the transform functions.

The following types are supported:

bigint
Custom function to transform BigInt values.
boolean
Custom function to transform boolean values.
date
Custom function to transform Date values.
number
Custom function to transform Number values.
object
Custom function to transform Object literals.
string
Custom function to transform String values.

The functions will be executed with 2 arguments:

value (any)
The field value being casted.
context (object)
A context object.

The accepted return values are:

null, undefined
The field will be empty.
string
The string value of the field.
object
An object containing the value property of the field as a well as options applied at the field level in case of a need to overwrite global options.

Context

The context object is passed as the second argument of the user provided function. It contains the following properties:

column (number|string)
The column name if the columns options is defined or the column name was discovered or the field position.
header (boolean)
A boolean indicating if the provided value is a part of the header.
index (number)
The field position starting at 0.
records (number)
The number of records which have been fully processed.

Field level options

By returning an object instead of a string, a cast function can overwrite the default options on a field level. The field value must be provided in the value property. The supported options are: delimiter, escape, quote, quoted, quoted_empty, quoted_string, quoted_match and record_delimiter.

The following cast example disable the quoting of the field, leaving it to the responsibility of the cast function:

import assert from 'node:assert';
import { stringify } from 'csv-stringify/sync';

const data = stringify([ [1], [2] ], {
  cast: {
    number: function(value){
      return {value: `="${value}"`, quote: false};
    }
  }
});

assert.equal(data, '="1"\n="2"\n');

./packages/csv-stringify/samples/option.cast.js

Run this example with the command node samples/option.cast.js.

Default behaviour

boolean

A value of true is converted to the string 1 while a value of false to converted to an empty string. The default implementation is:

function(value){
  return value ? '1': ''
}

date

Date objects are converted to timestamps. The default implementation is:

function(value){
  return '' + value.getTime()
}

number

Number are converted into string. The default implementation is:

function(value){
  return '' + value
}

object

Object literals are converted into JSON strings. The default implementation is:

function(value){
  return JSON.stringify(value)
}

string

String are as returned without any modification. The default implementation is:

function(value){
  return value
}

Casting a date

This cast example convert date objects into ISO 8601 dates.

import { stringify } from 'csv-stringify';
import assert from 'node:assert';

stringify([{
  name: 'foo',
  date: new Date('1970-01-01T00:00:00.000Z')
},{
  name: 'bar',
  date: new Date('1971-01-01T00:00:00.000Z')
}],{
  cast: {
    date: function(value) {
      return value.toISOString();
    }
  }
}, function(err, data) {
  assert.equal(
    data,
    'foo,1970-01-01T00:00:00.000Z\n' +
    'bar,1971-01-01T00:00:00.000Z\n'
  );
});