Content Nodes
After parsing, the contents
value of each YAML.Document
is the root of an Abstract Syntax Tree of nodes representing the document (or null
for an empty document).
Both scalar and collection values may have an anchor
associated with them; this is rendered in the string representation with a &
prefix, so e.g. in foo: &aa bar
, the value bar
has the anchor aa
.
Anchors are used by Alias nodes to allow for the same value to be used in multiple places in the document.
It is valid to have an anchor associated with a node even if it has no aliases.
Scalar Values
class NodeBase {
comment?: string // a comment on or immediately after this
commentBefore?: string // a comment before this
range?: [number, number, number]
// The `[start, value-end, node-end]` character offsets for the part
// of the source parsed into this node (undefined if not parsed).
// The `value-end` and `node-end` positions are themselves not
// included in their respective ranges.
spaceBefore?: boolean
// a blank line before this node and its commentBefore
tag?: string // a fully qualified tag, if required
clone(): NodeBase // a copy of this node
toJS(doc, options?): any // a plain JS representation of this node
toJSON(): any // a plain JSON representation of this node
}
For scalar values, the tag
will not be set unless it was explicitly defined in the source document; this also applies for unsupported tags that have been resolved using a fallback tag (string, YAMLMap
, or YAMLSeq
).
class Scalar<T = unknown> extends NodeBase {
anchor?: string // an anchor associated with this node
format?: 'BIN' | 'HEX' | 'OCT' | 'TIME' | undefined
// By default (undefined), numbers use decimal notation.
// The YAML 1.2 core schema only supports 'HEX' and 'OCT'.
type?:
'BLOCK_FOLDED' | 'BLOCK_LITERAL' | 'PLAIN' |
'QUOTE_DOUBLE' | 'QUOTE_SINGLE' | undefined
value: T
}
A parsed document’s contents will have all of its non-object values wrapped in Scalar
objects, which themselves may be in some hierarchy of YAMLMap
and YAMLSeq
collections.
However, this is not a requirement for the document’s stringification, which is rather tolerant regarding its input values, and will use doc.createNode()
when encountering an unwrapped value.
When stringifying, the node type
will be taken into account by !!str
and !!binary
values, and ignored by other scalars.
On the other hand, !!int
and !!float
stringifiers will take format
into account.
Collections
class Pair<K = unknown, V = unknown> {
key: K // When parsed, key and value are always
value: V // Node or null, but can be set to anything
}
class Collection extends NodeBase {
anchor?: string // an anchor associated with this node
flow?: boolean // use flow style when stringifying this
schema?: Schema
addIn(path: Iterable<unknown>, value: unknown): void
clone(schema?: Schema): NodeBase // a deep copy of this collection
deleteIn(path: Iterable<unknown>): boolean
getIn(path: Iterable<unknown>, keepScalar?: boolean): unknown
hasIn(path: Iterable<unknown>): boolean
setIn(path: Iterable<unknown>, value: unknown): void
}
class YAMLMap<K = unknown, V = unknown> extends Collection {
items: Pair<K, V>[]
add(pair: Pair<K, V> | { key: K; value: V }, overwrite?: boolean): void
delete(key: K): boolean
get(key: K, keepScalar?: boolean): unknown
has(key: K): boolean
set(key: K, value: V): void
}
class YAMLSeq<T = unknown> extends Collection {
items: T[]
add(value: T): void
delete(key: number | Scalar<number>): boolean
get(key: number | Scalar<number>, keepScalar?: boolean): unknown
has(key: number | Scalar<number>): boolean
set(key: number | Scalar<number>, value: T): void
}
Within all YAML documents, two forms of collections are supported: sequential YAMLSeq
collections and key-value YAMLMap
collections.
The JavaScript representations of these collections both have an items
array, which may (YAMLSeq
) or must (YAMLMap
) consist of Pair
objects that contain a key
and a value
of any type, including null
.
The items
array of a YAMLSeq
object may contain values of any type.
When stringifying collections, by default block notation will be used.
Flow notation will be selected if flow
is true
, the collection is within a surrounding flow collection, or if the collection is in an implicit key.
The yaml-1.1
schema includes additional collections that are based on YAMLMap
and YAMLSeq
: OMap
and Pairs
are sequences of Pair
objects (OMap
requires unique keys & corresponds to the JS Map object), and Set
is a map of keys with null values that corresponds to the JS Set object.
All of the collections provide the following accessor methods:
Method | Returns | Description |
---|---|---|
add(value), addIn(path, value) | void |
Adds a value to the collection. For !!map and !!omap the value must be a Pair instance or a { key, value } object, which may not have a key that already exists in the map. |
delete(key), deleteIn(path) | boolean |
Removes a value from the collection. Returns true if the item was found and removed. |
get(key, [keep]), getIn(path, [keep]) | any |
Returns value at key , or undefined if not found. By default unwraps scalar values from their surrounding node; to disable set keep to true (collections are always returned intact). |
has(key), hasIn(path) | boolean |
Checks if the collection includes a value with the key key . |
set(key, value), setIn(path, value) | any |
Sets a value in this collection. For !!set , value needs to be a boolean to add/remove the item from the set. When overwriting a Scalar value with a scalar, the original node is retained. |
const doc = new YAML.Document({ a: 1, b: [2, 3] }) // { a: 1, b: [ 2, 3 ] }
doc.add({ key: 'c', value: 4 }) // { a: 1, b: [ 2, 3 ], c: 4 }
doc.addIn(['b'], 5) // { a: 1, b: [ 2, 3, 5 ], c: 4 }
doc.set('c', 42) // { a: 1, b: [ 2, 3, 5 ], c: 42 }
doc.setIn(['c', 'x']) // Error: Expected YAML collection at c. Remaining path: x
doc.delete('c') // { a: 1, b: [ 2, 3, 5 ] }
doc.deleteIn(['b', 1]) // { a: 1, b: [ 2, 5 ] }
doc.get('a') // 1
doc.get('a', true) // Scalar { value: 1 }
doc.getIn(['b', 1]) // 5
doc.has(doc.createNode('a')) // true
doc.has('c') // false
doc.hasIn(['b', '0']) // true
For all of these methods, the keys may be nodes or their wrapped scalar values (i.e. 42
will match Scalar { value: 42 }
).
Keys for !!seq
should be positive integers, or their string representations.
add()
and set()
do not automatically call doc.createNode()
to wrap the value.
Each of the methods also has a variant that requires an iterable as the first parameter, and allows fetching or modifying deeper collections.
If any intermediate node in path
is a scalar rather than a collection, an error will be thrown.
If any of the intermediate collections is not found:
getIn
andhasIn
will returnundefined
orfalse
(respectively)addIn
andsetIn
will create missing collections; non-negative integer keys will create sequences, all other keys create mapsdeleteIn
will throw an error
Note that for addIn
the path argument points to the collection rather than the item; for maps its value
should be a Pair
or an object with { key, value }
fields.
Alias Nodes
class Alias extends NodeBase {
source: string
resolve(doc: Document): Scalar | YAMLMap | YAMLSeq | undefined
}
const obj = YAML.parse('[ &x { X: 42 }, Y, *x ]')
// => [ { X: 42 }, 'Y', { X: 42 } ]
obj[2].Z = 13
// => [ { X: 42, Z: 13 }, 'Y', { X: 42, Z: 13 } ]
YAML.stringify(obj)
// - &a1
// X: 42
// Z: 13
// - Y
// - *a1
Alias
nodes provide a way to include a single node in multiple places in a document; the source
of an alias node must be a preceding anchor in the document.
Circular references are fully supported, and where possible the JS representation of alias nodes will be the actual source object.
For ease of use, alias nodes also provide a resolve(doc)
method to dereference its source node.
When nodes are constructed from JS structures (e.g. during YAML.stringify()
), multiple references to the same object will result in including an autogenerated anchor at its first instance, and alias nodes to that anchor at later references.
Creating Nodes
const doc = new YAML.Document(['some', 'values'])
// Document {
// contents:
// YAMLSeq {
// items:
// [ Scalar { value: 'some' },
// Scalar { value: 'values' } ] } }
const map = doc.createNode({ balloons: 99 })
// YAMLMap {
// items:
// [ Pair {
// key: Scalar { value: 'balloons' },
// value: Scalar { value: 99 } } ] }
doc.add(map)
doc.get(0, true).comment = ' A commented item'
String(doc)
// - some # A commented item
// - values
// - balloons: 99
doc.createNode(value, replacer?, options?): Node
To create a new node, use the createNode(value, options?)
document method.
This will recursively wrap any input with appropriate Node
containers.
Generic JS Object
values as well as Map
and its descendants become mappings, while arrays and other iterable objects result in sequences.
With Object
, entries that have an undefined
value are dropped.
If value
is already a Node
instance, it will be directly returned.
To create a copy of a node, use instead the node.clone()
method.
For collections, the method accepts a single Schema
argument,
which allows overwriting the original’s schema
value.
Use a replacer
to apply a replacer array or function, following the JSON implementation.
To force flow styling on a collection, use the flow: true
option.
For all available options, see the CreateNode Options section.
The primary purpose of this method is to enable attaching comments or other metadata to a value, or to otherwise exert more fine-grained control over the stringified output.
To that end, you’ll need to assign its return value to the contents
of a document (or somewhere within said contents), as the document’s schema is required for YAML string output.
If you’re not interested in working with such metadata, document contents
may also include non-Node
values at any level.
doc.createAlias(node, name?): Alias
const alias = doc.createAlias(doc.get(1, true), 'foo')
doc.add(alias)
String(doc)
// - some # A commented item
// - &foo values
// - balloons: 99
// - *foo
Create a new Alias
node, ensuring that the target node
has the required anchor.
If node
already has an anchor, name
is ignored.
Otherwise, the node.anchor
value will be set to name
, or if an anchor with that name is already present in the document, name
will be used as a prefix for a new unique anchor.
If name
is undefined, the generated anchor will use ‘a’ as a prefix.
You should make sure to only add alias nodes to the document after the nodes to which they refer, or the document’s YAML stringification will fail.
new YAMLMap(), new YAMLSeq(), doc.createPair(key, value): Pair
import { Document, YAMLSeq } from 'yaml'
const doc = new Document(new YAMLSeq())
doc.contents.items = [
'some values',
42,
{ including: 'objects', 3: 'a string' }
]
doc.add(doc.createPair(1, 'a number'))
doc.toString()
// - some values
// - 42
// - "3": a string
// including: objects
// - 1: a number
To construct a YAMLSeq
or YAMLMap
, use new Document()
or doc.createNode()
with array, object or iterable input, or create the collections directly by importing the classes from yaml
.
Once created, normal array operations may be used to modify the items
array.
New Pair
objects may created either by importing the class from yaml
and using its new Pair(key, value)
constructor, or by using the doc.createPair(key, value, options?)
method.
The latter will recursively wrap the key
and value
as nodes, and accepts the same options as doc.createNode()
Finding and Modifying Nodes
const doc = YAML.parseDocument(`
- some values
- 42
- "3": a string
including: objects
- 1: a number
`)
const obs = doc.getIn([2, 'including'], true)
obs.type = 'QUOTE_DOUBLE'
YAML.visit(doc, {
Pair(_, pair) {
if (pair.key && pair.key.value === '3') return YAML.visit.REMOVE
},
Scalar(key, node) {
if (
key !== 'key' &&
typeof node.value === 'string' &&
node.type === 'PLAIN'
) {
node.type = 'QUOTE_SINGLE'
}
}
})
String(doc)
// - 'some values'
// - 42
// - including: "objects"
// - 1: 'a number'
In general, it’s safe to modify nodes manually, e.g. splicing the items
array of a YAMLMap
or setting its flow
value to true
.
For operations on nodes at a known location in the tree, it’s probably easiest to use doc.getIn(path, true)
to access them.
For more complex or general operations, a visitor API is provided:
YAML.visit(node, visitor): void
Apply a visitor to an AST node or document.
Walks through the tree (depth-first) starting from node
, calling a visitor
function with three arguments:
key
: For sequence values and mapPair
, the node’s index in the collection. Within aPair
,'key'
or'value'
, correspondingly.null
for the root node.node
: The current node.path
: The ancestry of the current node.
The return value of the visitor may be used to control the traversal:
undefined
(default): Do nothing and continueYAML.visit.SKIP
: Do not visit the children of this node, continue with next siblingYAML.visit.BREAK
: Terminate traversal completelyYAML.visit.REMOVE
: Remove the current node, then continue with the next oneNode
: Replace the current node, then continue by visiting itnumber
: While iterating the items of a sequence or map, set the index of the next step. This is useful especially if the index of the current node has changed.
If visitor
is a single function, it will be called with all values encountered in the tree, including e.g. null
values.
Alternatively, separate visitor functions may be defined for each Map
, Pair
, Seq
, Alias
and Scalar
node.
To define the same visitor function for more than one node type,
use the Collection
(map and seq), Value
(map, seq & scalar) and Node
(alias, map, seq & scalar) targets.
Of all these, only the most specific defined one will be used for each node.
YAML.visitAsync(node, visitor): Promise<void>
The same as visit()
,
but allows for visitor functions that return a promise
which resolves to one of the above-defined control values.
Identifying Node Types
import {
isAlias,
isCollection, // map or seq
isDocument,
isMap,
isNode, // alias, scalar, map or seq
isPair,
isScalar,
isSeq
} from 'yaml'
const doc = new Document({ foo: [13, 42] })
isDocument(doc) === true
isNode(doc) === false
isMap(doc.contents) === true
isNode(doc.contents) === true
isPair(doc.contents.items[0]) === true
isCollection(doc.get('foo')) === true
isScalar(doc.getIn(['foo', 1])) === true
isAlias(x: unknown): boolean
isCollection(x: unknown): boolean
isDocument(x: unknown): boolean
isMap(x: unknown): boolean
isNode(x: unknown): boolean
isPair(x: unknown): boolean
isScalar(x: unknown): boolean
isSeq(x: unknown): boolean
To find out what you’ve got, a family of custom type guard functions is provided.
These should be preferred over other methods such as instanceof
checks, as they’ll work even if the nodes have been created by a different instance of the library.
Internally, node identification uses property symbols that are set on instances during their construction.
Comments and Blank Lines
const doc = YAML.parseDocument(`
# This is YAML.
---
it has:
- an array
- of values
`)
doc.toJS() // { 'it has': [ 'an array', 'of values' ] }
doc.commentBefore // ' This is YAML.'
const seq = doc.get('it has')
seq.spaceBefore // true
seq.items[0].comment = ' item comment'
seq.comment = ' collection end comment'
doc.toString()
// # This is YAML.
//
// it has:
//
// - an array # item comment
//
// - of values
// # collection end comment
A primary differentiator between this and other YAML libraries is the ability to programmatically handle comments, which according to the spec “must not have any effect on the serialization tree or representation graph. In particular, comments are not associated with a particular node.” Similarly to comments, the YAML spec instructs non-content blank lines to be discarded.
This library does allow comments and blank lines to be handled programmatically, and does attach them to particular nodes (most often, the following node).
Each Scalar
, Map
, Seq
and the Document
itself has comment
, commentBefore
members that may be set to a stringifiable value, and a spaceBefore
boolean to add an empty line before the comment.
The string contents of comments are not processed by the library, except for merging adjacent comment and blank lines together.
Document comments will be separated from the rest of the document by a blank line.
In the node member values, comment lines terminating with the #
indicator are represented by a single space, while completely empty lines are represented as empty strings.
Scalar block values with “keep” chomping (i.e. with +
in their header) consider any trailing empty lines to be a part of their content, so the following node’s spaceBefore
or commentBefore
with leading whitespace is ignored.
Note: Due to implementation details, the library’s comment handling is not completely stable, in particular for trailing comments. When creating, writing, and then reading a YAML file, comments may sometimes be associated with a different node.