Rework coordReduce Meta

I’m trying to use @turf/meta coordReduce and it doesn’t seem to have the same flow & expected output as Array.reduce().

Issues:

• Missing Index
• Missing Array
• Number of iterations are different (first set of coordinates should be excluded)

Using Array.reduce()

var line = lineString([[126, -11], [129, -21], [135, -31]]);
var coordinates = line.geometry.coordinates;
coordinates.reduce(function (previous, current, index, array) {
    console.log(previous, current, index, array);
    return current;
});

Output

[ 126, -11 ] [ 129, -21 ] 1 [ [ 126, -11 ], [ 129, -21 ], [ 135, -31 ] ]
[ 129, -21 ] [ 135, -31 ] 2 [ [ 126, -11 ], [ 129, -21 ], [ 135, -31 ] ]

Using Turf Meta coordReduce()

var line = lineString([[126, -11], [129, -21], [135, -31]]);
coordReduce(line, function (previous, current, index, array) {
    console.log(previous, current, index, array);
    return current;
});

Output

undefined [ 126, -11 ] undefined undefined
[ 126, -11 ] [ 129, -21 ] undefined undefined
[ 129, -21 ] [ 135, -31 ] undefined undefined

CC: @tmcw

[tmcw]

Yep! index and at least trying to implement the special case of Array.reduce sounds good.

[DenisCarriere]

@tmcw OK FINALLY… I’ve got BOTH examples working. We were both right, you can now use coordReduce in both scenarios we mentioned above:

previous & current coordinates (my example)

var line = lineString([[126, -11], [129, -21], [135, -31]]);
coordReduce(line, function (previousCoords, currentCoords, currentIndex) {
    console.log(previousCoords, currentCoords);
    return currentCoords;
})

output

[ 126, -11 ] [ 129, -21 ] 1
[ 129, -21 ] [ 135, -31 ] 2

memo calculation (your example)

var line = lineString([[126, -11], [129, -21], [135, -31]]);
var sum = coordReduce(line, function (previousValue, currentCoords) {
    console.log('previous:', previousValue);
    var currentValue = previousValue + currentCoords[0];
    return currentValue;
}, 0);
console.log('total:', sum);

output

previous: 0
previous: 126
previous: 255
total: 390

Final JSDocs

/**
 * Callback for coordReduce
 *
 * The first time the callback function is called, the values provided as arguments depend
 * on whether the reduce method has an initialValue argument.
 *
 * If an initialValue is provided to the reduce method:
 *  - The previousValue argument is initialValue.
 *  - The currentValue argument is the value of the first element present in the array.
 *
 * If an initialValue is not provided:
 *  - The previousValue argument is the value of the first element present in the array.
 *  - The currentValue argument is the value of the second element present in the array.
 *
 * @private
 * @callback coordReduceCallback
 * @param {*} accumulator The accumulated value previously returned in the last invocation
 * of the callback, or initialValue, if supplied.
 * @param {[number, number]} currentCoords The current coordinate being processed.
 * @param {number} currentIndex The index of the current element being processed in the
 * array.Starts at index 0, if an initialValue is provided, and at index 1 otherwise.
 */

/**
 * Reduce coordinates in any GeoJSON object into a single value,
 * similar to how Array.reduce works. However, in this case we lazily run
 * the reduction, so an array of all coordinates is unnecessary.
 *
 * @name coordReduce
 * @param {Object} layer any GeoJSON object
 * @param {coordReduceCallback} callback a method that takes (previousValue, currentCoords, currentIndex)
 * @param {*} [initialValue] Value to use as the first argument to the first call of the callback.
 * @param {boolean=} excludeWrapCoord whether or not to include
 * the final coordinate of LinearRings that wraps the ring in its iteration.
 * @returns {*} The value that results from the reduction.
 */
function coordReduce(layer, callback, initialValue, excludeWrapCoord) {