JavaScript one-liner function: groupArr2d and groupConcatArr2d (>=ES2017)

groupArr2d_groupConcatArr2d.js

// for ES2017
{
  const groupArr2d = (arr2d, {swap=false} = {}) => arr2d.reduce((a,c)=>Object.assign(a,{[c[+swap]]:a[c[+swap]]?[...a[c[+swap]],c[+!swap]]:[c[+!swap]]}),{});

  const groupConcatArr2d = (arr2d, {swap=false, glue=","} = {}) => Object.assign({},...Object.entries(arr2d.reduce((a,c)=>Object.assign(a,{[c[+swap]]:a[c[+swap]]?[...a[c[+swap]],c[+!swap]]:[c[+!swap]]}),{})).map(e=>({[e[0]]:e[1].join(glue)})));

  // two-dimensional arrays can represent object.
  // in each array, the first value can be regarded as a key, and the next value can be regarded as a value.
  // `Object.entries(obj)` is easy to understand as an example.
  // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries#Examples
  
  const testArr2d = [
    ["sameKey","anotherValue1"],
    ["sameKey","anotherValue2"],
    ["anotherKey1","sameValue"],
    ["anotherKey2","sameValue"],
  ];

  /*=========================================
   * groupArr2d test
   *=======================================*/
  // get object: {key: array of value that grouped by key}
  console.log(groupArr2d(testArr2d));

  // get object: {value: array of key that grouped by value}
  console.log(groupArr2d(testArr2d, {swap: true}));

  /*=========================================
   * groupConcatArr2d test
   *=======================================*/
  // get object: {key: concat value string with ","(default glue) that grouped by key}
  console.log(groupConcatArr2d(testArr2d));

  // get object: {value: concat key string with ","(default glue) that grouped by value}
  console.log(groupConcatArr2d(testArr2d, {swap: true}));

  // get object: {key: concat value string with "|glue|"(specified glue) that grouped by key}
  console.log(groupConcatArr2d(testArr2d, {glue: "|glue|"}));

  // get object: {value: concat key string with "|glue|"(specified glue) that grouped by value}
  console.log(groupConcatArr2d(testArr2d, {swap: true, glue: "|glue|"}));
}