JavaScript/文法/配列・連想配列/forEach map filter reduce

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forEach map filter reduce

 let nums = [ 1, 2, 3, 4 ];
 let ret = nums.map((n) => { return n * 2; });
 console.log(ret);
 
  => [ 2, 4, 6, 8 ]

 let nums = [ 1, 2, 3, 4 ];
 let ret = nums.filter((n) => { return n % 2 === 0 ? true : false; });
 console.log(ret);
 
  => [ 2, 4 ]

 let nums = [ 1, 2, 3, 4 ];
 let ret = nums.filter((n) => { return true; });
 console.log(ret);
 
  => [ 1, 2, 3, 4 ]

 let nums = [ 1, 2, 3, 4 ];
 let ret = nums.filter((n) => { return false; });
 console.log(ret);
 
  => []

 let total = [0, 1, 2, 3].reduce((sum, value) => sum + value);
 console.log(total);
 
  => 6 // = 0 + 1 + 2 + 3

 let total = [0, 1, 2, 3].reduce((sum, value) => sum + value, 100);
 console.log(total);
 
  => 106 // = 100 + 0 + 1 + 2 + 3

 let result = ['A', 'B', 'C'].reduce((str, value) => str + value);
 console.log(result);
 
  => ABC

 let result = ['A', 'B', 'C'].reduce((str, value) => str + value, 'X');
 console.log(result);
 
  => XABC