forEach map filter reduce

map

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

 => [ 2, 4, 6, 8 ]

filter

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);

 => []

reduce

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

参考

https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce


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Last-modified: 2018-08-31 (金) 06:18:35 (51d)