/**
 * Maps each chunk as an object with a "label" and a "value".
 *
 * The label can be the result of a property on the chunk.
 *
 * @group Transformers
 * @see {@link groupBy:function}
 * @example
 * ```
 * --one--------------------two--------------------three-------------------
 *
 * label('length')
 *
 * --{label:3,value:'one'}--{label:3,value:'two'}--{label:5,value:'three'}-
 * ```
 */
export declare function label<T, K extends keyof T>(propName: K): TransformStream<T, {
    label: T[K];
    value: T;
}>;
/**
 * The label can be the result of calling a provided function.
 *
 * @group Transformers
 * @example
 * ```
 * --6.1------------------4.2------------------6.3------------------
 *
 * label(Math.floor)
 *
 * --{label:6,value:6.1}--{label:4,value:4.2}--{label:6,value:6.3}--
 * ```
 */
export declare function label<T, L>(fn: (chunk: T) => L): TransformStream<T, {
    label: L;
    value: T;
}>;
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