Delta
Deltas are a simple, yet expressive format that can be used to describe contents and changes. The format is JSON based, and is human readable, yet easily parsible by machines. Deltas can describe any rich text document, includes all text and formatting information, without the ambiguity and complexity of HTML.
A Delta is made up of an Array of Operations, which describe changes to a document. They can be an insert
, delete
or retain
. Note operations do not take an index. They always describe the change at the current index. Use retains to "keep" or "skip" certain parts of the document.
Don’t be confused by its name Delta—Deltas represents both documents and changes to documents. If you think of Deltas as the instructions from going from one document to another, the way Deltas represent a document is by expressing the instructions starting from an empty document.
Quick Example
const delta = new Delta([
{ insert: 'Gandalf', attributes: { bold: true } },
{ insert: ' the ' },
{ insert: 'Grey', attributes: { color: '#ccc' } }
]);
const death = new Delta().retain(12)
.insert('White', { color: '#fff' })
.delete(4);
const restored = delta.compose(death);
This README describes Deltas in its general form and API functionality. Additional information on the way Quill specifically uses Deltas can be found on its own Delta docs. A walkthough of the motivation and design thinking behind Deltas are on Designing the Delta Format.
This format is suitable for Operational Transform and defines several functions to support this use case.
Contents
Operations
Construction
Documents
These methods called on or with non-document Deltas will result in undefined behavior.
Utility
Operational Transform
Operations
Insert Operation
Insert operations have an insert
key defined. A String value represents inserting text. Any other type represents inserting an embed (however only one level of object comparison will be performed for equality).
In both cases of text and embeds, an optional attributes
key can be defined with an Object to describe additonal formatting information. Formats can be changed by the retain operation.
{ insert: "Text", attributes: { bold: true } }
{ insert: "Google", attributes: { link: 'https://www.google.com' } }
{
insert: { image: 'https://octodex.github.com/images/labtocat.png' },
attributes: { alt: "Lab Octocat" }
}
{
insert: { video: 'https://www.youtube.com/watch?v=dMH0bHeiRNg' },
attributes: {
width: 420,
height: 315
}
}
Delete Operation
Delete operations have a Number delete
key defined representing the number of characters to delete. All embeds have a length of 1.
{ delete: 10 }
Retain Operation
Retain operations have a Number retain
key defined representing the number of characters to keep (other libraries might use the name keep or skip). An optional attributes
key can be defined with an Object to describe formatting changes to the character range. A value of null
in the attributes
Object represents removal of that key.
Note: It is not necessary to retain the last characters of a document as this is implied.
{ retain: 5 }
{ retain: 5, attributes: { bold: true } }
{ retain: 5, attributes: { bold: null } }
Construction
constructor
Creates a new Delta object.
Methods
new Delta()
new Delta(ops)
new Delta(delta)
Parameters
ops
- Array of operationsdelta
- Object with an ops
key set to an array of operations
Note: No validity/sanity check is performed when constructed with ops or delta. The new delta's internal ops array will also be assigned from ops or delta.ops without deep copying.
Example
const delta = new Delta([
{ insert: 'Hello World' },
{ insert: '!', attributes: { bold: true }}
]);
const packet = JSON.stringify(delta);
const other = new Delta(JSON.parse(packet));
const chained = new Delta().insert('Hello World').insert('!', { bold: true });
insert()
Appends an insert operation. Returns this
for chainability.
Methods
insert(text, attributes)
insert(embed, attributes)
Parameters
text
- String representing text to insertembed
- Object representing embed type to insertattributes
- Optional attributes to apply
Example
delta.insert('Text', { bold: true, color: '#ccc' });
delta.insert({ image: 'https://octodex.github.com/images/labtocat.png' });
delete()
Appends a delete operation. Returns this
for chainability.
Methods
Parameters
length
- Number of characters to delete
Example
delta.delete(5);
retain()
Appends a retain operation. Returns this
for chainability.
Methods
retain(length, attributes)
Parameters
length
- Number of characters to retainattributes
- Optional attributes to apply
Example
delta.retain(4).retain(5, { color: '#0c6' });
Documents
concat()
Returns a new Delta representing the concatenation of this and another document Delta's operations.
Methods
Parameters
other
- Document Delta to concatenate
Returns
Delta
- Concatenated document Delta
Example
const a = new Delta().insert('Hello');
const b = new Delta().insert('!', { bold: true });
const concat = a.concat(b);
diff()
Returns a Delta representing the difference between two documents. Optionally, accepts a suggested index where change took place, often representing a cursor position before change.
Methods
diff(other)
diff(other, index)
Parameters
other
- Document Delta to diff againstindex
- Suggested index where change took place
Returns
Delta
- difference between the two documents
Example
const a = new Delta().insert('Hello');
const b = new Delta().insert('Hello!');
const diff = a.diff(b);
eachLine()
Iterates through document Delta, calling a given function with a Delta and attributes object, representing the line segment.
Methods
eachLine(predicate, newline)
Parameters
predicate
- function to call on each line groupnewline
- newline character, defaults to \n
Example
const delta = new Delta().insert('Hello\n\n')
.insert('World')
.insert({ image: 'octocat.png' })
.insert('\n', { align: 'right' })
.insert('!');
delta.eachLine((line, attributes, i) => {
console.log(line, attributes, i);
});
invert()
Returned an inverted delta that has the opposite effect of against a base document delta. That is base.compose(delta).compose(inverted) === base
.
Methods
Parameters
base
- Document delta to invert against
Returns
Delta
- inverted delta against the base delta
Example
const base = new Delta().insert('Hello\n')
.insert('World');
const delta = new Delta().retain(6, { bold: true }).insert('!').delete(5);
const inverted = delta.invert(base);
Utility
filter()
Returns an array of operations that passes a given function.
Methods
Parameters
predicate
- Function to test each operation against. Return true
to keep the operation, false
otherwise.
Returns
Array
- Filtered resulting array
Example
const delta = new Delta().insert('Hello', { bold: true })
.insert({ image: 'https://octodex.github.com/images/labtocat.png' })
.insert('World!');
const text = delta
.filter((op) => typeof op.insert === 'string')
.map((op) => op.insert)
.join('');
forEach()
Iterates through operations, calling the provided function for each operation.
Methods
Parameters
predicate
- Function to call during iteration, passing in the current operation.
Example
delta.forEach((op) => {
console.log(op);
});
length()
Returns length of a Delta, which is the sum of the lengths of its operations.
Methods
Example
new Delta().insert('Hello').length();
new Delta().insert('A').retain(2).delete(1).length();
map()
Returns a new array with the results of calling provided function on each operation.
Methods
Parameters
predicate
- Function to call, passing in the current operation, returning an element of the new array to be returned
Returns
Array
- A new array with each element being the result of the given function.
Example
const delta = new Delta().insert('Hello', { bold: true })
.insert({ image: 'https://octodex.github.com/images/labtocat.png' })
.insert('World!');
const text = delta
.map((op) => {
if (typeof op.insert === 'string') {
return op.insert;
} else {
return '';
}
})
.join('');
partition()
Create an array of two arrays, the first with operations that pass the given function, the other that failed.
Methods
Parameters
predicate
- Function to call, passing in the current operation, returning whether that operation passed
Returns
Array
- A new array of two Arrays, the first with passed operations, the other with failed operations
Example
const delta = new Delta().insert('Hello', { bold: true })
.insert({ image: 'https://octodex.github.com/images/labtocat.png' })
.insert('World!');
const results = delta.partition((op) => typeof op.insert === 'string');
const passed = results[0];
const failed = results[1];
reduce()
Applies given function against an accumulator and each operation to reduce to a single value.
Methods
reduce(predicate, initialValue)
Parameters
predicate
- Function to call per iteration, returning an accumulated valueinitialValue
- Initial value to pass to first call to predicate
Returns
any
- the accumulated value
Example
const delta = new Delta().insert('Hello', { bold: true })
.insert({ image: 'https://octodex.github.com/images/labtocat.png' })
.insert('World!');
const length = delta.reduce((length, op) => (
length + (op.insert.length || 1);
), 0);
slice()
Returns copy of delta with subset of operations.
Methods
slice()
slice(start)
slice(start, end)
Parameters
start
- Start index of subset, defaults to 0end
- End index of subset, defaults to rest of operations
Example
const delta = new Delta().insert('Hello', { bold: true }).insert(' World');
const copy = delta.slice();
const world = delta.slice(6);
const space = delta.slice(5, 6);
Operational Transform
compose()
Returns a Delta that is equivalent to applying the operations of own Delta, followed by another Delta.
Methods
Parameters
Example
const a = new Delta().insert('abc');
const b = new Delta().retain(1).delete(1);
const composed = a.compose(b);
transform()
Transform given Delta against own operations.
Methods
transform(other, priority = false)
transform(index, priority = false)
- Alias for transformPosition
Parameters
other
- Delta to transformpriority
- Boolean used to break ties. If true
, then this
takes priority
over other
, that is, its actions are considered to happen "first."
Returns
Delta
- transformed Delta
Example
const a = new Delta().insert('a');
const b = new Delta().insert('b').retain(5).insert('c');
a.transform(b, true);
a.transform(b, false);
transformPosition()
Transform an index against the delta. Useful for representing cursor/selection positions.
Methods
transformPosition(index, priority = false)
Parameters
index
- index to transform
Returns
Number
- transformed index
Example
const delta = new Delta().retain(5).insert('a');
delta.transformPosition(4);
delta.transformPosition(5);