group

The group() method groups the elements of an array according to a computed key and returns an object indexed by group name with the associated elements.

Interactive example

Syntax

Classic signature

function group<
	GenericInput extends readonly unknown[],
	GenericOutput extends ArrayGroupFunctionOutput
>(
	input: GenericInput,
	theFunction: (
		element: GenericInput[number], 
		params: ArrayGroupFunctionParams
	) => GenericOutput
): ArrayGroupResult<GenericOutput>

Curried signature

function group<
	GenericInput extends readonly unknown[],
	GenericOutput extends ArrayGroupFunctionOutput
>(
	theFunction: (
		element: GenericInput[number],
		params: ArrayGroupFunctionParams
	) => GenericOutput
): (input: GenericInput) => ArrayGroupResult<GenericOutput>

Auxiliary parameters

interface ArrayGroupFunctionOutput<
	GenericGroupName extends string = string, 
	GenericGroupValue extends unknown = unknown
> {
	group: GenericGroupName;
	value: GenericGroupValue;
}

interface ArrayGroupFunctionParams {
	index: number;
	output: typeof groupOutput;
}

function groupOutput<
	GenericGroupValue extends unknown, 
	GenericGroupName extends string
>(
	group: GenericGroupName
): (
	value: GenericGroupValue
) => ArrayGroupFunctionOutput<GenericGroupName, GenericGroupValue>;

function groupOutput<
	GenericGroupValue extends unknown, 
	GenericGroupName extends string
>(
	group: GenericGroupName,
	value: GenericGroupValue
): ArrayGroupFunctionOutput<GenericGroupName, GenericGroupValue>;

Parameters

• input: The array to traverse to create groups.
• theFunction: Function applied to each element. It returns an object { group, value } describing the target group name and the value to add.
• params.index: Current index in the input array.
• params.output: Shortcut to build an output object via groupOutput, which guarantees typing consistency.

Return value

An object in which each property corresponds to a computed group. Each key contains an array of the values produced for that group. Missing keys correspond to groups never encountered.

See also

• reduce - Aggregates an array into a single value
• sum - Adds all values of an array

Sources

• MDN Web Docs - Array.prototype.group()