feat: add MODE aggregate function (#2482)

src/55functions.js

@@ -389,6 +389,57 @@ alasql.aggr.median = alasql.aggr.MEDIAN = function (v, s, stage) {
 	}
 };
 
+alasql.aggr.mode = alasql.aggr.MODE = function (v, s, stage) {
+	if (stage === 2) {
+		if (v !== undefined && v !== null) {
+			s.push(v);
+		}
+		return s;
+	}
+
+	if (stage === 1) {
+		if (v === undefined || v === null) {
+			return [];
+		}
+		return [v];
+	}
+
+	if (!s.length) {
+		return undefined;
+	}
+
+	let counts = new Map();
+	let maxCount = 0;
+
+	for (let i = 0; i < s.length; i++) {
+		let val = s[i];
+		let count = (counts.get(val) || 0) + 1;
+		counts.set(val, count);
+		if (count > maxCount) {
+			maxCount = count;
+		}
+	}
+
+	let candidates = [];
+	for (let [val, count] of counts.entries()) {
+		if (count === maxCount) {
+			candidates.push(val);
+		}
+	}
+
+	if (candidates.length === 1) {
+		return candidates[0];
+	}
+
+	candidates.sort((a, b) => {
+		if (a < b) return -1;
+		if (a > b) return 1;
+		return 0;
+	});
+
+	return candidates[0];
+};
+
 alasql.aggr.QUART = function (v, s, stage, nth) {
 	//Quartile (first quartile per default or input param)
 	if (stage === 2) {

test/test999_mode.js

@@ -0,0 +1,49 @@
+if (typeof exports === 'object') {
+	var assert = require('assert');
+	var alasql = require('..');
+}
+
+describe('Test MODE aggregate function', function () {
+	it('1. Basic numbers mode', function () {
+		var res = alasql('SELECT MODE(a) AS m FROM ?', [[{a: 1}, {a: 2}, {a: 2}, {a: 3}]]);
+		assert.deepStrictEqual(res, [{m: 2}]);
+	});
+
+	it('2. Mode with tie breaking (smallest value)', function () {
+		var res = alasql('SELECT MODE(a) AS m FROM ?', [[{a: 3}, {a: 1}, {a: 3}, {a: 1}]]);
+		assert.deepStrictEqual(res, [{m: 1}]);
+	});
+
+	it('3. Mode with strings', function () {
+		var res = alasql('SELECT MODE(a) AS m FROM ?', [
+			[{a: 'apple'}, {a: 'banana'}, {a: 'apple'}, {a: 'orange'}],
+		]);
+		assert.deepStrictEqual(res, [{m: 'apple'}]);
+	});
+
+	it('4. Mode ignores NULL and undefined values', function () {
+		var res = alasql('SELECT MODE(a) AS m FROM ?', [[{a: null}, {a: 5}, {a: undefined}, {a: 5}]]);
+		assert.deepStrictEqual(res, [{m: 5}]);
+	});
+
+	it('5. Mode with GROUP BY', function () {
+		var data = [
+			{g: 1, v: 10},
+			{g: 1, v: 20},
+			{g: 1, v: 20},
+			{g: 2, v: 30},
+			{g: 2, v: 30},
+			{g: 2, v: 40},
+		];
+		var res = alasql('SELECT g, MODE(v) AS m FROM ? GROUP BY g ORDER BY g', [data]);
+		assert.deepStrictEqual(res, [
+			{g: 1, m: 20},
+			{g: 2, m: 30},
+		]);
+	});
+
+	it('6. Empty input returns undefined/null', function () {
+		var res = alasql('SELECT MODE(a) AS m FROM ?', [[]]);
+		assert.deepStrictEqual(res, [{m: undefined}]);
+	});
+});