Deno - Read and Write YAML FIles Examples

This tutorial shows you how to parse a YAML file in Deno and also how to write data to a file in YAML format.

YAML (YAML Ain't Markup Language) is a human-readable data-serialization language. YAML uses indentation to indicate nesting, [...] for lists and {...} for maps. It also has libraries available for many programming languages.

Deno itself has a module named yaml as part of Deno Standard Modules. It has the functionalities for formatting an object to YAML format and parsing a string in YAML format.

For this tutorial, let's assume we have these two files, one with a single document and the other with multi-documents.

data.yaml

  playerName: tuwaise
  monsters:
    - number: 1
      type: fire
    - number: 2
      type: water
    - number: 3
      type: grass

data2.yaml

  playerName: tuwaise
  monsters:
    - number: 1
      type: fire
    - number: 2
      type: water
    - number: 3
      type: grass
  ---
  playerName: chibi
  monsters:
    - number: 1
      type: ice
    - number: 2
      type: psychic

Using Deno `std` yaml Module

First, import and re-export the functions of yaml module in deps.ts file. There are three functions we are going to use.

parse: used to parse a yaml string to an object.
parseAll: used to parse a yaml string containing multi-documents to a list of objects.
stringify: used to format an object to YAML format.

deps.ts

  import {
    parse as yamlParse,
    parseAll as yamlParseAll,
    stringify as yamlStringify,
  } from 'https://deno.land/std@0.82.0/encoding/yaml.ts';

Parse YAML File

Below is the function that allows you to parse a string in YAML format to a JSON object.

  function parse(content: string, options?: ParseOptions): unknown

ParseOptions has the following fields:

legacy?: boolean: qqq.
listener?: ((...args: Any[]) => void) | null: qqq.
filename?: string: string to be used as a file path in error/warning message.
schema?: SchemaDefinition: qqq.
json?: boolean: Compatibility with JSON.parse behaviour.
onWarning?(this: null, e?: YAMLError): void: function to call on warning messages.

The function requires you to pass the content as a string. If you need to read the contents from a file, you can use Deno.readTextFile. Since the code requires access to read files, you need to add --allow-read flag while running deno run command.

Example:

  import { yamlParse } from './deps.ts';

  const data = yamlParse(await Deno.readTextFile('data.yaml'));
  console.log(data);

Output:

  {
    playerName: "tuwaise",
    monsters: [
      { number: 1, type: "fire" },
      { number: 2, type: "water" },
      { number: 3, type: "grass" }
    ]
  }

Make sure the file to be parsed contains a valid YAML format. Otherwise YAMLError will be thrown.

The parse function only works if the file contains a single document. If the file contains multi-documents, you need to use the parseAll function:

  function parseAll(
    content: string,
    iterator?: CbFunction,
    options?: ParseOptions,
  ): unknown

Example:

  import { yamlParseAll } from './deps.ts';

  const data = yamlParseAll(await Deno.readTextFile('data2.yaml'));
  console.log(data);

Output:

  [
    {
      playerName: "tuwaise",
      monsters: [
        { number: 1, type: "fire" },
        { number: 2, type: "water" },
        { number: 3, type: "grass" }
      ]
    },
    {
      playerName: "chibi",
      monsters: [ { number: 1, type: "ice" }, { number: 2, type: "psychic" } ]
    }
  ]

Write YAML File

The module also has a function named stringify that can be used to format an object to YAML format.

  function stringify(
    obj: Record<string, unknown>,
    options?: DumpOptions,
  ): string

You need to pass the object as the first argument. Optionally, you can also pass a second argument which is an object whose type is DumpOptions to customize the output format. DumpOptions has the following fields:

indent?: number: Indentation with in spaces.
noArrayIndent?: boolean: If true, it will not add an indentation level to array elements.
skipInvalid?: boolean: If true, it will not throw on invalid types and skip pairs and single values with such types.
flowLevel?: number: Specifies level of nesting, when to switch from block to flow style for collections. -1 means block style everywhere.
styles?: ArrayObject<StyleVariant> | null: Each tag may have its own set of styles. - "tag" => "style" map.
schema?: SchemaDefinition: The schema to use.
sortKeys?: boolean | ((a: string, b: string) => number): If true, sort keys when dumping YAML in ascending, ASCII character order. If a function, use the function to sort the keys. Defaults to false.
lineWidth?: number: Set the maximum line width. Defaults to 80.
noRefs?: boolean: If true, don't convert duplicate objects into references. Defaults to false.
noCompatMode?: boolean: If true, don't try to be compatible with older yaml versions.
condenseFlow?: boolean: If true flow sequences will be condensed, omitting the space between `key: value` or `a, b`.

Below is a basic usage example that doesn't pass the options argument. --allow-write flag needs to be added if you want to write the output to a file.

  const yaml = yamlStringify({
    playerName: 'tuwaise',
    monsters: [
      { number: 1, type: 'fire' },
      { number: 2, type: 'water' },
      { number: 3, type: 'grass' },
    ]
  });
  console.log(await Deno.writeTextFile('output.yaml', yaml));

Output:

  playerName: tuwaise
  monsters:
    - number: 1
      type: fire
    - number: 2
      type: water
    - number: 3
      type: grass

Here's another example with custom indent size and sorted keys.

  const yaml = yamlStringify(
    {
      playerName: 'tuwaise',
      monsters: [
          { number: 1, type: 'fire' },
          { number: 2, type: 'water' },
          { number: 3, type: 'grass' },
      ]
    },
    {
      indent: 3,
      sortKeys: true,
    }
  );
  console.log(await Deno.writeTextFile('output.yaml', yaml));

Output:

  monsters:
     -
        number: 1
        type: fire
     -
        number: 2
        type: water
     -
        number: 3
        type: grass
  playerName: tuwaise

Summary

That's how to parse data in YAML format and how to format data to YAML format. You can utilize yaml module which is part of Deno Standard Modules.

Deno - Read and Write YAML FIles Examples

Using Deno std yaml Module

Parse YAML File

Write YAML File

Summary

Ivan Andrianto

Using Deno `std` yaml Module