# Expressions

To enable custom calculations, Vega includes its own *expression language* for writing basic formulas. For example, these expressions are used by the filter and formula transforms to modify data, and within signal definitions to calculate updated values in response to user input.

The expression language is a restricted subset of JavaScript. All basic arithmetic, logical and property access expressions are supported, as are boolean, number, string, object (`{}`

) and array (`[]`

) literals. Ternary operators (`ifTest ? thenValue : elseValue`

) and a special `if(test, thenValue, elseValue)`

function are supported.

To keep the expression language simple, secure and free of unwanted side effects, the following elements are **not** allowed: assignment operators (`=`

, `+=`

etc), pre/postfix updates (`++`

), `new`

expressions, and most control flow statements (`for`

, `while`

, `switch`

, etc). In addition, function calls involving nested properties (`foo.bar()`

) are not allowed. Instead, the expression language supports a collection of functions defined in the top-level scope.

This page documents the expression language. If you are interested in implementation aspects, the bulk of the expression language – including parsing, code generation, and some of the constant and function definitions – are maintained in the vega-expression module.

## Expression Language Reference

- Bound Variables
- Constants
- Type Checking Functions
- Type Coercion Functions
- Control Flow Functions
- Math Functions
- Date/Time Functions
- Array Functions
- String Functions
- Object Functions
- Formatting Functions
- Regular Expression Functions
- Color Functions
- Event Functions
- Data Functions
- Scale & Projection Functions
- Geographic Functions
- Tree (Hierarchy) Functions
- Browser Functions
- Logging Functions

## Bound Variables

The expression language includes a number of automatically-bound named variables.

#
**datum**

The current input data object, available within data transform and event handler expressions. To lookup object properties, use normal JavaScript syntax: `datum.value`

or `datum['My Value']`

.

#
**event**

If the expression is being invoked in response to an event, an *event* variable is defined. This variable consists of a standard JavaScript DOM event, providing access to bound properties of the event, such as `event.metaKey`

or `event.keyCode`

.

#
*signal names*

Any in-scope signal value can be referenced directly by name. For example, if you have defined a signal named `hover`

within your Vega specification, you can refer to it directly within an expression (e.g., `hover.value`

).

## Constants

Constant values that can be referenced by name within expressions.

#
**NaN**

Not a number. Same as the JavaScript literal `NaN`

.

#
**E**

The transcendental number *e*. Same as JavaScript’s `Math.E`

.

#
**LN2**

The natural log of 2. Same as JavaScript’s `Math.LN2`

.

#
**LN10**

The natural log of 10. Same as JavaScript’s `Math.LN10`

.

#
**LOG2E**

The base 2 logarithm of *e*. Same as JavaScript’s `Math.LOG2E`

.

#
**LOG10E**

The base 10 logarithm *e*. Same as JavaScript’s `Math.LOG10E`

.

#
**MAX_VALUE**

The largest positive numeric value. Same as JavaScript’s `Number.MAX_VALUE`

.

#
**MIN_VALUE**

The smallest positive numeric value. Same as JavaScript’s `Number.MIN_VALUE`

.

#
**PI**

The transcendental number *π*. Same as JavaScript’s `Math.PI`

.

#
**SQRT1_2**

The square root of 0.5. Same as JavaScript’s `Math.SQRT1_2`

.

#
**SQRT2**

The square root of 2. Same as JavaScript’s `Math.SQRT2`

.

## Type Checking Functions

Predicate functions for checking value types.

#
**isArray**(*value*)

Returns true if *value* is an array, false otherwise.

#
**isBoolean**(*value*)

Returns true if *value* is a boolean (`true`

or `false`

), false otherwise.

#
**isDate**(*value*)

Returns true if *value* is a Date object, false otherwise. This method will return false for timestamp numbers or date-formatted strings; it recognizes Date objects only.

#
**isNumber**(*value*)

Returns true if *value* is a number, false otherwise. `NaN`

and `Infinity`

are considered numbers.

#
**isObject**(*value*)

Returns true if *value* is an object (including arrays and Dates), false otherwise.

#
**isRegExp**(*value*)

Returns true if *value* is a RegExp (regular expression) object, false otherwise.

#
**isString**(*value*)

Returns true if *value* is a string, false otherwise.

## Type Coercion Functions

Functions for coercing values to a desired type.

#
**toBoolean**(*value*)

Coerces the input *value* to a string. Null values and empty strings are mapped to `null`

.

#
**toDate**(*value*)

Coerces the input *value* to a Date instance. Null values and empty strings are mapped to `null`

. If an optional *parser* function is provided, it is used to perform date parsing, otherwise `Date.parse`

is used. Be aware that `Date.parse`

has different implementations across browsers!

#
**toNumber**(*value*)

Coerces the input *value* to a number. Null values and empty strings are mapped to `null`

.

#
**toString**(*value*)

Coerces the input *value* to a string. Null values and empty strings are mapped to `null`

.

## Control Flow Functions

#
**if**(*test*, *thenValue*, *elseValue*)

If *test* is truthy, returns *thenValue*. Otherwise, returns *elseValue*. The *if* function is equivalent to the ternary operator `a ? b : c`

.

## Math Functions

Basic mathematical functions.

#
**isNaN**(*value*)

Returns true if *value* is not a number. Same as JavaScript’s `isNaN`

.

#
**isFinite**(*value*)

Returns true if *value* is a finite number. Same as JavaScript’s `isFinite`

.

#
**abs**(*value*)

Returns the absolute value of *value*. Same as JavaScript’s `Math.abs`

.

#
**acos**(*value*)

Trigonometric arccosine. Same as JavaScript’s `Math.acos`

.

#
**asin**(*value*)

Trigonometric arcsine. Same as JavaScript’s `Math.asin`

.

#
**atan**(*value*)

Trigonometric arctangent. Same as JavaScript’s `Math.atan`

.

#
**atan2**(*dy*, *dx*)

Returns the arctangent of *dy / dx*. Same as JavaScript’s `Math.atan2`

.

#
**ceil**(*value*)

Rounds *value* to the nearest integer of equal or greater value. Same as JavaScript’s `Math.ceil`

.

#
**clamp**(*value*, *min*, *max*)

Restricts *value* to be between the specified *min* and *max*.

#
**cos**(*value*)

Trigonometric cosine. Same as JavaScript’s `Math.cos`

.

#
**exp**(*exponent*)

Returns the value of *e* raised to the provided *exponent*. Same as JavaScript’s `Math.exp`

.

#
**floor**(*value*)

Rounds *value* to the nearest integer of equal or lower value. Same as JavaScript’s `Math.floor`

.

#
**log**(*value*)

Returns the natural logarithm of *value*. Same as JavaScript’s `Math.log`

.

#
**max**(*value1*, *value2*, …)

Returns the maximum argument value. Same as JavaScript’s `Math.max`

.

#
**min**(*value1*, *value2*, …)

Returns the minimum argument value. Same as JavaScript’s `Math.min`

.

#
**pow**(*value*, *exponent*)

Returns *value* raised to the given *exponent*. Same as JavaScript’s `Math.pow`

.

#
**random**()

Returns a pseudo-random number in the range [0,1). Same as JavaScript’s `Math.random`

.

#
**round**(*value*)

Rounds *value* to the nearest integer. Same as JavaScript’s `Math.round`

.

#
**sin**(*value*)

Trigonometric sine. Same as JavaScript’s `Math.sin`

.

#
**sqrt**(*value*)

Square root function. Same as JavaScript’s `Math.sqrt`

.

#
**tan**(*value*)

Trigonometric tangent. Same as JavaScript’s `Math.tan`

.

## Date/Time Functions

Functions for working with date/time values.

#
**now**()

Returns the timestamp for the current time.

#
**datetime**(*year*, *month*[, *day*, *hour*, *min*, *sec*, *millisec*])

Returns a new `Date`

instance. The *month* is 0-based, such that `1`

represents February.

#
**date**(*datetime*)

Returns the day of the month for the given *datetime* value, in local time.

#
**day**(*datetime*)

Returns the day of the week for the given *datetime* value, in local time.

#
**year**(*datetime*)

Returns the year for the given *datetime* value, in local time.

#
**quarter**(*datetime*)

Returns the quarter of the year (0-3) for the given *datetime* value, in local time.

#
**month**(*datetime*)

Returns the (zero-based) month for the given *datetime* value, in local time.

#
**hours**(*datetime*)

Returns the hours component for the given *datetime* value, in local time.

#
**minutes**(*datetime*)

Returns the minutes component for the given *datetime* value, in local time.

#
**seconds**(*datetime*)

Returns the seconds component for the given *datetime* value, in local time.

#
**milliseconds**(*datetime*)

Returns the milliseconds component for the given *datetime* value, in local time.

#
**time**(*datetime*)

Returns the epoch-based timestamp for the given *datetime* value.

#
**timezoneoffset**(*datetime*)

Returns the timezone offset from the local timezone to UTC for the given *datetime* value.

#
**utc**(*year*, *month*[, *day*, *hour*, *min*, *sec*, *millisec*])

Returns a timestamp for the given UTC date. The *month* is 0-based, such that `1`

represents February.

#
**utcdate**(*datetime*)

Returns the day of the month for the given *datetime* value, in UTC time.

#
**utcday**(*datetime*)

Returns the day of the week for the given *datetime* value, in UTC time.

#
**utcyear**(*datetime*)

Returns the year for the given *datetime* value, in UTC time.

#
**utcquarter**(*datetime*)

Returns the quarter of the year (0-3) for the given *datetime* value, in UTC time.

#
**utcmonth**(*datetime*)

Returns the (zero-based) month for the given *datetime* value, in UTC time.

#
**utchours**(*datetime*)

Returns the hours component for the given *datetime* value, in UTC time.

#
**utcminutes**(*datetime*)

Returns the minutes component for the given *datetime* value, in UTC time.

#
**utcseconds**(*datetime*)

Returns the seconds component for the given *datetime* value, in UTC time.

#
**utcmilliseconds**(*datetime*)

Returns the milliseconds component for the given *datetime* value, in UTC time.

## Array Functions

Functions for working with arrays of values.

#
**extent**(*array*) ≥ 4.0

Returns a new *[min, max]* array with the minimum and maximum values of the input array, ignoring `null`

, `undefined`

, and `NaN`

values.

#
**clampRange**(*range*, *min*, *max*)

Clamps a two-element *range* array in a span-preserving manner. If the span of the input *range* is less than *(max - min)* and an endpoint exceeds either the *min* or *max* value, the range is translated such that the span is preserved and one endpoint touches the boundary of the *[min, max]* range. If the span exceeds *(max - min)*, the range *[min, max]* is returned.

#
**indexof**(*array*, *value*)

Returns the first index of *value* in the input *array*.

#
**inrange**(*value*, *range*)

Tests whether *value* lies within (or is equal to either) the first and last values of the *range* array.

#
**join**(*array*[, *separator*]) ≥ 5.3

Returns a new string by concatenating all of the elements of the input *array*, separated by commas or a specified *separator* string.

#
**lastindexof**(*array*, *value*)

Returns the last index of *value* in the input *array*.

#
**length**(*array*)

Returns the length of the input *array*.

#
**lerp**(*array*, *fraction*)

Returns the linearly interpolated value between the first and last entries in the *array* for the provided interpolation *fraction* (typically between 0 and 1). For example, `lerp([0, 50], 0.5)`

returns 25.

#
**peek**(*array*)

Returns the last element in the input *array*. Similar to the built-in `Array.pop`

method, except that it does not remove the last element. This method is a convenient shorthand for `array[array.length - 1]`

.

#
**reverse**(*array*) ≥ 5.3

Returns a new array with elements in a reverse order of the input *array*. The first array element becomes the last, and the last array element becomes the first.

#
**sequence**([*start*, ]*stop*[, *step*])

Returns an array containing an arithmetic sequence of numbers. If *step* is omitted, it defaults to 1. If *start* is omitted, it defaults to 0. The *stop* value is exclusive; it is not included in the result. If *step* is positive, the last element is the largest *start + i * step* less than *stop*; if *step* is negative, the last element is the smallest *start + i * step* greater than *stop*. If the returned array would contain an infinite number of values, an empty range is returned. The arguments are not required to be integers.

#
**slice**(*array*, *start*[, *end*])

Returns a section of *array* between the *start* and *end* indices. If the *end* argument is negative, it is treated as an offset from the end of the array (*length(array) + end*).

#
**span**(*array*)

Returns the span of *array*: the difference between the last and first elements, or *array[array.length-1] - array[0]*.

## String Functions

Functions for modifying text strings.

#
**indexof**(*string*, *substring*)

Returns the first index of *substring* in the input *string*.

#
**lastindexof**(*string*, *substring*)

Returns the last index of *substring* in the input *string*.

#
**length**(*string*)

Returns the length of the input *string*.

#
**lower**(*string*)

Transforms *string* to lower-case letters.

#
**pad**(*string*, *length*[, *character*, *align*])

Pads a *string* value with repeated instances of a *character* up to a specified *length*. If *character* is not specified, a space (‘ ‘) is used. By default, padding is added to the end of a string. An optional *align* parameter specifies if padding should be added to the `'left'`

(beginning), `'center'`

, or `'right'`

(end) of the input string.

#
**parseFloat**(*string*)

Parses the input *string* to a floating-point value. Same as JavaScript’s `parseFloat`

.

#
**parseInt**(*string*)

Parses the input *string* to an integer value. Same as JavaScript’s `parseInt`

.

#
**replace**(*string*, *pattern*, *replacement*)

Returns a new string with some or all matches of *pattern* replaced by a *replacement* string. The *pattern* can be a string or a regular expression. If *pattern* is a string, only the first instance will be replaced. Same as JavaScript’s String.replace.

#
**slice**(*string*, *start*[, *end*])

Returns a section of *string* between the *start* and *end* indices. If the *end* argument is negative, it is treated as an offset from the end of the string (*length(string) + end*).

#
**split**(*string*, *separator*[, *limit*]) ≥ 4.3

Returns an array of tokens created by splitting the input *string* according to a provided *separator* pattern. The result can optionally be constrained to return at most *limit* tokens.

#
**substring**(*string*, *start*[, *end*])

Returns a section of *string* between the *start* and *end* indices.

#
**trim**(*string*) ≥ 5.3

Returns a trimmed string with preceding and trailing whitespace removed.

#
**truncate**(*string*, *length*[, *align*, *ellipsis*])

Truncates an input *string* to a target *length*. The optional *align* argument indicates what part of the string should be truncated: `'left'`

(the beginning), `'center'`

, or `'right'`

(the end). By default, the `'right'`

end of the string is truncated. The optional *ellipsis* argument indicates the string to use to indicate truncated content; by default the ellipsis character `…`

(`\u2026`

) is used.

#
**upper**(*string*)

Transforms *string* to upper-case letters.

## Object Functions

Functions for manipulating object instances.

#
**merge**(*object1*[, *object2*, …]) ≥ 4.0

Merges the input objects *object1*, *object2*, etc into a new output object. Inputs are visited in sequential order, such that key values from later arguments can overwrite those from earlier arguments. Example: `merge({a:1, b:2}, {a:3}) -> {a:3, b:2}`

.

## Formatting Functions

Functions for formatting number and datetime values as strings.

#
**dayFormat**(*day*)

Formats a (0-6) *weekday* number as a full week day name, according to the current locale. For example: `dayFormat(0) -> "Sunday"`

.

#
**dayAbbrevFormat**(*day*)

Formats a (0-6) *weekday* number as an abbreviated week day name, according to the current locale. For example: `dayAbbrevFormat(0) -> "Sun"`

.

#
**format**(*value*, *specifier*)

Formats a numeric *value* as a string. The *specifier* must be a valid d3-format specifier (e.g., `format(value, ',.2f')`

.

#
**monthFormat**(*month*)

Formats a (zero-based) *month* number as a full month name, according to the current locale. For example: `monthFormat(0) -> "January"`

.

#
**monthAbbrevFormat**(*month*)

Formats a (zero-based) *month* number as an abbreviated month name, according to the current locale. For example: `monthAbbrevFormat(0) -> "Jan"`

.

#
**timeFormat**(*value*, *specifier*)

Formats a datetime *value* (either a `Date`

object or timestamp) as a string, according to the local time. The *specifier* must be a valid d3-time-format specifier. For example: `timeFormat(timestamp, '%A')`

.

#
**timeParse**(*string*, *specifier*)

Parses a *string* value to a Date object, according to the local time. The *specifier* must be a valid d3-time-format specifier. For example: `timeParse('June 30, 2015', '%B %d, %Y')`

.

#
**utcFormat**(*value*, *specifier*)

Formats a datetime *value* (either a `Date`

object or timestamp) as a string, according to UTC time. The *specifier* must be a valid d3-time-format specifier. For example: `utcFormat(timestamp, '%A')`

.

#
**utcParse**(*value*, *specifier*)

Parses a *string* value to a Date object, according to UTC time. The *specifier* must be a valid d3-time-format specifier. For example: `utcParse('June 30, 2015', '%B %d, %Y')`

.

## RegExp Functions

Functions for creating and applying regular expressions.

#
**regexp**(*pattern*[, *flags*]) -
Creates a regular expression instance from an input *pattern* string and optional *flags*. Same as JavaScript’s `RegExp`

.

#
**test**(*regexp*[, *string*]) -
Evaluates a regular expression *regexp* against the input *string*, returning `true`

if the string matches the pattern, `false`

otherwise. For example: `test(/\\d{3}/, "32-21-9483") -> true`

.

## Color Functions

Functions for representing colors in various color spaces. Color functions return objects that, when coerced to a string, map to valid CSS RGB colors.

#
**rgb**(*r*, *g*, *b*[, *opacity*]) |
**rgb**(*specifier*)

Constructs a new RGB color. If *r*, *g* and *b* are specified, these represent the channel values of the returned color; an *opacity* may also be specified. If a CSS Color Module Level 3 *specifier* string is specified, it is parsed and then converted to the RGB color space. Uses d3-color’s rgb function.

#
**hsl**(*h*, *s*, *l*[, *opacity*]) |
**hsl**(*specifier*)

Constructs a new HSL color. If *h*, *s* and *l* are specified, these represent the channel values of the returned color; an *opacity* may also be specified. If a CSS Color Module Level 3 *specifier* string is specified, it is parsed and then converted to the HSL color space. Uses d3-color’s hsl function.

#
**lab**(*l*, *a*, *b*[, *opacity*]) |
**lab**(*specifier*)

Constructs a new CIE LAB color. If *l*, *a* and *b* are specified, these represent the channel values of the returned color; an *opacity* may also be specified. If a CSS Color Module Level 3 *specifier* string is specified, it is parsed and then converted to the LAB color space. Uses d3-color’s lab function.

#
**hcl**(*h*, *c*, *l*[, *opacity*]) |
**hcl**(*specifier*)

Constructs a new HCL (hue, chroma, luminance) color. If *h*, *c* and *l* are specified, these represent the channel values of the returned color; an *opacity* may also be specified. If a CSS Color Module Level 3 *specifier* string is specified, it is parsed and then converted to the HCL color space. Uses d3-color’s hcl function.

## Event Functions

Functions for processing input event data. These functions are only legal in expressions evaluated in response to an event (for example a signal event handler). Invoking these functions elsewhere can result in errors.

#
**item**()

Returns the current scenegraph item that is the target of the event.

#
**group**([*name*])

Returns the scenegraph group mark item in which the current event has occurred. If no arguments are provided, the immediate parent group is returned. If a group name is provided, the matching ancestor group item is returned.

#
**xy**([*item*])

Returns the x- and y-coordinates for the current event as a two-element array. If no arguments are provided, the top-level coordinate space of the view is used. If a scenegraph *item* (or string group name) is provided, the coordinate space of the group item is used.

#
**x**([*item*])

Returns the x coordinate for the current event. If no arguments are provided, the top-level coordinate space of the view is used. If a scenegraph *item* (or string group name) is provided, the coordinate space of the group item is used.

#
**y**([*item*])

Returns the y coordinate for the current event. If no arguments are provided, the top-level coordinate space of the view is used. If a scenegraph *item* (or string group name) is provided, the coordinate space of the group item is used.

#
**pinchDistance**(*event*)

Returns the pixel distance between the first two touch points of a multi-touch event.

#
**pinchAngle**(*event*)

Returns the angle of the line connecting the first two touch points of a multi-touch event.

#
**inScope**(*item*)

Returns true if the given scenegraph *item* is a descendant of the group mark in which the event handler was defined, false otherwise.

## Data Functions

Functions for accessing Vega data sets.

#
**data**(*name*)

Returns the array of data objects for the Vega data set with the given *name*. If the data set is not found, returns an empty array.

#
**indata**(*name*, *field*, *value*)

Tests if the data set with a given *name* contains a datum with a *field* value that matches the input *value*. For example: `indata('table', 'category', value)`

.

## Scale and Projection Functions

Functions for working with Vega scale transforms and cartographic projections.

#
**scale**(*name*, *value*[, *group*])

Applies the named scale transform (or projection) to the specified *value*. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the scale or projection.

#
**invert**(*name*, *value*[, *group*])

Inverts the named scale transform (or projection) for the specified *value*. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the scale or projection.

#
**copy**(*name*[, *group*])

Returns a copy (a new cloned instance) of the named scale transform of projection, or `undefined`

if no scale or projection is found. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the scale or projection.

#
**domain**(*name*[, *group*])

Returns the scale domain array for the named scale transform, or an empty array if the scale is not found. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the scale.

#
**range**(*name*[, *group*])

Returns the scale range array for the named scale transform, or an empty array if the scale is not found. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the scale.

#
**bandwidth**(*name*[, *group*])

Returns the current band width for the named band scale transform, or zero if the scale is not found or is not a band scale. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the scale.

#
**bandspace**(*count*[, *paddingInner*, *paddingOuter*])

Returns the number of steps needed within a band scale, based on the *count* of domain elements and the inner and outer padding values. While normally calculated within the scale itself, this function can be helpful for determining the size of a chart’s layout.

#
**gradient**(*scale*, *p0*, *p1*[, *count*])

Returns a linear color gradient for the *scale* (whose range must be a continuous color scheme) and starting and ending points *p0* and *p1*, each an *[x, y]* array. The points *p0* and *p1* should be expressed in normalized coordinates in the domain [0, 1], relative to the bounds of the item being colored. If unspecified, *p0* defaults to `[0, 0]`

and *p1* defaults to `[1, 0]`

, for a horizontal gradient that spans the full bounds of an item. The optional *count* argument indicates a desired target number of sample points to take from the color scale.

#
**panLinear**(*domain*, *delta*)

Given a linear scale *domain* array with numeric or datetime values, returns a new two-element domain array that is the result of panning the domain by a fractional *delta*. The *delta* value represents fractional units of the scale range; for example, `0.5`

indicates panning the scale domain to the right by half the scale range.

#
**panLog**(*domain*, *delta*)

Given a log scale *domain* array with numeric or datetime values, returns a new two-element domain array that is the result of panning the domain by a fractional *delta*. The *delta* value represents fractional units of the scale range; for example, `0.5`

indicates panning the scale domain to the right by half the scale range.

#
**panPow**(*domain*, *delta*, *exponent*)

Given a power scale *domain* array with numeric or datetime values and the given *exponent*, returns a new two-element domain array that is the result of panning the domain by a fractional *delta*. The *delta* value represents fractional units of the scale range; for example, `0.5`

indicates panning the scale domain to the right by half the scale range.

#
**panSymlog**(*domain*, *delta*, *constant*)

Given a symmetric log scale *domain* array with numeric or datetime values parameterized by the given *constant*, returns a new two-element domain array that is the result of panning the domain by a fractional *delta*. The *delta* value represents fractional units of the scale range; for example, `0.5`

indicates panning the scale domain to the right by half the scale range.

#
**zoomLinear**(*domain*, *anchor*, *scaleFactor*)

Given a linear scale *domain* array with numeric or datetime values, returns a new two-element domain array that is the result of zooming the domain by a *scaleFactor*, centered at the provided fractional *anchor*. The *anchor* value represents the zoom position in terms of fractional units of the scale range; for example, `0.5`

indicates a zoom centered on the mid-point of the scale range.

#
**zoomLog**(*domain*, *anchor*, *scaleFactor*)

Given a log scale *domain* array with numeric or datetime values, returns a new two-element domain array that is the result of zooming the domain by a *scaleFactor*, centered at the provided fractional *anchor*. The *anchor* value represents the zoom position in terms of fractional units of the scale range; for example, `0.5`

indicates a zoom centered on the mid-point of the scale range.

#
**zoomPow**(*domain*, *anchor*, *scaleFactor*, *exponent*)

Given a power scale *domain* array with numeric or datetime values and the given *exponent*, returns a new two-element domain array that is the result of zooming the domain by a *scaleFactor*, centered at the provided fractional *anchor*. The *anchor* value represents the zoom position in terms of fractional units of the scale range; for example, `0.5`

indicates a zoom centered on the mid-point of the scale range.

#
**zoomSymlog**(*domain*, *anchor*, *scaleFactor*, *constant*)

Given a symmetric log scale *domain* array with numeric or datetime values parameterized by the given *constant*, returns a new two-element domain array that is the result of zooming the domain by a *scaleFactor*, centered at the provided fractional *anchor*. The *anchor* value represents the zoom position in terms of fractional units of the scale range; for example, `0.5`

indicates a zoom centered on the mid-point of the scale range.

## Geographic Functions

Functions for analyzing geographic regions represented as GeoJSON features.

#
**geoArea**(*projection*, *feature*[, *group*])

Returns the projected planar area (typically in square pixels) of a GeoJSON *feature* according to the named *projection*. If the *projection* argument is `null`

, computes the spherical area in steradians using unprojected longitude, latitude coordinates. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the projection. Uses d3-geo’s geoArea and path.area methods.

#
**geoBounds**(*projection*, *feature*[, *group*])

Returns the projected planar bounding box (typically in pixels) for the specified GeoJSON *feature*, according to the named *projection*. The bounding box is represented by a two-dimensional array: [[*x₀*, *y₀*], [*x₁*, *y₁*]], where *x₀* is the minimum x-coordinate, *y₀* is the minimum y-coordinate, *x₁* is the maximum x-coordinate, and *y₁* is the maximum y-coordinate. If the *projection* argument is `null`

, computes the spherical bounding box using unprojected longitude, latitude coordinates. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the projection. Uses d3-geo’s geoBounds and path.bounds methods.

#
**geoCentroid**(*projection*, *feature*[, *group*])

Returns the projected planar centroid (typically in pixels) for the specified GeoJSON *feature*, according to the named *projection*. If the *projection* argument is `null`

, computes the spherical centroid using unprojected longitude, latitude coordinates. The optional *group* argument takes a scenegraph group mark item to indicate the specific scope in which to look up the projection. Uses d3-geo’s geoCentroid and path.centroid methods.

## Tree (Hierarchy) Functions

Functions for processing hierarchy data sets constructed with the stratify or nest transforms.

#
**treePath**(*name*, *source*, *target*)

For the hierarchy data set with the given *name*, returns the shortest path through from the *source* node id to the *target* node id. The path starts at the *source* node, ascends to the least common ancestor of the *source* node and the *target* node, and then descends to the *target* node.

#
**treeAncestors**(*name*, *node*)

For the hierarchy data set with the given *name*, returns the array of ancestors nodes, starting with the input *node*, then followed by each parent up to the root.

## Browser Functions

Functions for accessing web browser facilities.

#
**containerSize**()

Returns the current CSS box size (`[el.clientWidth, el.clientHeight]`

) of the parent DOM element that contains the Vega view. If there is no container element, returns `[undefined, undefined]`

.

#
**screen**()

Returns the `window.screen`

object, or `{}`

if Vega is not running in a browser environment.

#
**windowSize**()

Returns the current window size (`[window.innerWidth, window.innerHeight]`

) or `[undefined, undefined]`

if Vega is not running in a browser environment.

## Logging Functions

Logging functions for writing messages to the console. These can be helpful when debugging expressions.

#
**warn**(*value1*[, *value2*, …])

Logs a warning message and returns the last argument. For the message to appear in the console, the visualization view must have the appropriate logging level set.

#
**info**(*value1*[, *value2*, …])

Logs an informative message and returns the last argument. For the message to appear in the console, the visualization view must have the appropriate logging level set.

#
**debug**(*value1*[, *value2*, …])

Logs a debugging message and returns the last argument. For the message to appear in the console, the visualization view must have the appropriate logging level set.