JavaScript reduce の使い方 #4
JavaScript reduce の使い方 #1、JavaScript reduce の使い方 #2、JavaScript reduce の使い方 #3 で reduce の使い方をまとめてきました。map のときと同様、reduce のまとめで使った関数をアロー関数で記述してみます。掲載するソースコードはすべて GAS の開発環境で動作確認をしています。
アロー関数を使おうと使わなかろうと同じ結果となりますし、どちらが正しい、というわけでは無いです。状況により使い分ければと思います。
function myFunction1() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( function( accumulator, currentValue, index, array ) {
console.log( accumulator, currentValue, index, array ) ;
} );
}
function myFunction_arrow1() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( ( accumulator, currentValue, index, array ) => console.log( accumulator, currentValue, index, array ) );
}
// 実行結果
// accumulator currentValue index array
// 1 3 1 [ 1, 3, 10, 17, 20, 31 ]
// undefined 10 2 [ 1, 3, 10, 17, 20, 31 ]
// undefined 17 3 [ 1, 3, 10, 17, 20, 31 ]
// undefined 20 4 [ 1, 3, 10, 17, 20, 31 ]
// undefined 31 5 [ 1, 3, 10, 17, 20, 31 ]function myFunction2() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( function( accumulator, currentValue, index, array ) {
console.log( accumulator, currentValue, index, array ) ;
return currentValue ;
} );
}
function myFunction_arrow2() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( ( accumulator, currentValue, index, array ) => {
console.log( accumulator, currentValue, index, array ) ;
return currentValue ;
}
);
}
// 実行結果
// accumulator currentValue index array
// 1 3 1 [ 1, 3, 10, 17, 20, 31 ]
// 3 10 2 [ 1, 3, 10, 17, 20, 31 ]
// 10 17 3 [ 1, 3, 10, 17, 20, 31 ]
// 17 20 4 [ 1, 3, 10, 17, 20, 31 ]
// 20 31 5 [ 1, 3, 10, 17, 20, 31 ]function sumByReduce() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( function( accumulator, currentValue, index, array ) {
console.log( accumulator, currentValue, index, array ) ;
return accumulator + currentValue ;
} );
}
function sumByReduce_arrow() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( ( accumulator, currentValue, index, array ) => {
console.log( accumulator, currentValue, index, array ) ;
return accumulator + currentValue ;
} );
}
// 実行結果
// accumulator currentValue index array
// 1 3 1 [ 1, 3, 10, 17, 20, 31 ]
// 4 10 2 [ 1, 3, 10, 17, 20, 31 ]
// 14 17 3 [ 1, 3, 10, 17, 20, 31 ]
// 31 20 4 [ 1, 3, 10, 17, 20, 31 ]
// 51 31 5 [ 1, 3, 10, 17, 20, 31 ]function sumX2ByReduceCorrect() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( function( accumulator, currentValue, index, array ) {
console.log( accumulator, currentValue, index, array ) ;
return accumulator + currentValue * 2 ;
}, 0 );
}
function sumX2ByReduceCorrect_arrow() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( ( accumulator, currentValue, index, array ) => {
console.log( accumulator, currentValue, index, array ) ;
return accumulator + currentValue * 2 ;
}, 0 );
}
// 実行結果
// accumulator currentValue index array
// 0 1 0 [ 1, 3, 10, 17, 20, 31 ]
// 2([]+1x2) 3 1 [ 1, 3, 10, 17, 20, 31 ]
// 8(2+3x2) 10 2 [ 1, 3, 10, 17, 20, 31 ]
// 28(8+10x2) 17 3 [ 1, 3, 10, 17, 20, 31 ]
// 62(28+17x2) 20 4 [ 1, 3, 10, 17, 20, 31 ]
// 102(62+20x2) 31 5 [ 1, 3, 10, 17, 20, 31 ]function checkEvenReduce() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( function( accumulator, currentValue, index, array ) {
console.log( accumulator, currentValue, index, array ) ;
if ( currentValue % 2 === 0 )
accumulator.push( currentValue );
return accumulator ;
}, [] );
}
function checkEvenReduce_arrow() {
const inputArray = [ 1, 3, 10, 17, 20, 31 ] ;
inputArray.reduce( ( accumulator, currentValue, index, array ) => {
console.log( accumulator, currentValue, index, array ) ;
if ( currentValue % 2 === 0 )
accumulator.push( currentValue );
return accumulator ;
}, [] );
}
// 実行結果
// accumulator currentValue index array
// [] 1 0 [ 1, 3, 10, 17, 20, 31 ]
// [] 3 1 [ 1, 3, 10, 17, 20, 31 ]
// [] 10 2 [ 1, 3, 10, 17, 20, 31 ]
// [ 10 ] 17 3 [ 1, 3, 10, 17, 20, 31 ]
// [ 10 ] 20 4 [ 1, 3, 10, 17, 20, 31 ]
// [ 10, 20 ] 31 5 [ 1, 3, 10, 17, 20, 31 ]function sampleReduce() {
const inputArray = [ ['A', 50 ], ['B', 50 ], ['C', 50 ], ['A', 70 ], ['B', 80 ], ['C', 90 ], ['A', 100 ] ] ;
const returnValue =
inputArray.reduce( function( accumulator, currentValue, index, array ) {
console.log( accumulator, currentValue, index, array ) ;
if ( currentValue[ 0 ] === 'A' )
accumulator += currentValue[ 1 ];
return accumulator ;
}, 0 );
console.log( returnValue );
}
function sampleReduce() {
const inputArray = [ ['A', 50 ], ['B', 50 ], ['C', 50 ], ['A', 70 ], ['B', 80 ], ['C', 90 ], ['A', 100 ] ] ;
const returnValue =
inputArray.reduce( ( accumulator, currentValue, index, array ) => {
console.log( accumulator, currentValue, index, array ) ;
if ( currentValue[ 0 ] === 'A' )
accumulator += currentValue[ 1 ];
return accumulator ;
}, 0 );
console.log( returnValue );
}
// 実行結果
// accumulator currentValue index arrray
// 0 ['A', 50 ] 0 [['A', 50], ['B', 50], ['C', 50], ['A', 70], ['B', 80], ['C', 90], ['A', 100]]
// 50 ['B', 50 ] 1 [['A', 50], ['B', 50], ['C', 50], ['A', 70], ['B', 80], ['C', 90], ['A', 100]]
// 50 ['C', 50 ] 2 [['A', 50], ['B', 50], ['C', 50], ['A', 70], ['B', 80], ['C', 90], ['A', 100]]
// 50 ['A', 70 ] 3 [['A', 50], ['B', 50], ['C', 50], ['A', 70], ['B', 80], ['C', 90], ['A', 100]]
// 120 ['B', 80 ] 4 [['A', 50], ['B', 50], ['C', 50], ['A', 70], ['B', 80], ['C', 90], ['A', 100]]
// 120 ['C', 90 ] 5 [['A', 50], ['B', 50], ['C', 50], ['A', 70], ['B', 80], ['C', 90], ['A', 100]]
// 120 ['A',100 ] 6 [['A', 50], ['B', 50], ['C', 50], ['A', 70], ['B', 80], ['C', 90], ['A', 100]]
// 220では、ごきげんよう。