JavaScriptにおけるメソッドの追加と削除

1.Adding/removing elements push() Adds one or more elements at the end of an array Returns the new array’s length. Modifies the original array and returns the new length property. Examples: Append one element to an array let numbers = [ 10 , 20 , 30 ]; const length = numbers . push ( 40 ); console . log ({ numbers }); console . log ({ length }); Output: { numbers: [ 10 , 20 , 30 , 40 ] } { length: 4 } Add multiple elements to the end of an array let numbers = [ 10 , 20 , 30 ]; const length = numbers . push ( 40 , 50 ); console . log ({ numbers }); console . log ({ length }); Output: { numbers: [ 10 , 20 , 30 , 40 , 50 ] } { length: 5 } Append elements of an array to another array // Using a for...of loop let colors = [ ' red ' , ' green ' , ' blue ' ]; let cmyk = [ ' cyan ' , ' magenta ' , ' yellow ' , ' back ' ]; for ( const color of cmyk ) { colors . push ( color ); } console . log ( colors ); Output: [ 'red' , 'green' , 'blue' , 'cyan' , 'magenta' , 'yellow' , 'back' ] // Using a spread operator (...) let colors = [ ' red ' , ' green ' , ' blue ' ]; let cmyk = [ ' cyan ' , ' magenta ' , ' yellow ' , ' back ' ]; colors . push (... cmyk ); console . log ({ colors }); unshift() Adds one or more elements to the beginning of an array Returns the new array’s length. It is slow for large arrays because it needs to reindex the existing elements. Examples: Prepend an element to an array let numbers = [ 30 , 40 ]; const length = numbers . unshift ( 20 ); console . log ({ length }); console . log ({ numbers }); Output: { length: 3 } { numbers: [ 20 , 30 , 40 ] } Prepend multiple elements to an array let numbers = [ 30 , 40 ]; const length = numbers . unshift ( 10 , 20 ); console . log ({ length }); console . log ({ numbers }); Output: { length: 4 } { numbers: [ 10 , 20 , 30 , 40 ] } Prepend elements of an array to another array const days = [ ' Mon ' , ' Tue ' , ' Wed ' , ' Thu ' , ' Fri ' ]; const weekends = [ ' Sat ' , ' Sun ' ]; for ( const weekend of weekends ) { days . unshift ( weekend ); } console . log ( days ); Output: [ 'Sun' , 'Sat' , 'Mon' , 'Tue' , 'Wed' , 'Thu' , 'Fri' ] To make it more concise, you can use the spread operator like this: let days = [ ' Mon ' , ' Tue ' , ' Wed ' , ' Thu ' , ' Fri ' ]; let weekends = [ ' Sat ' , ' Sun ' ]; days . unshift (... weekends ); console . log ( days ); pop() Removes the last element from an array Returns the removed element. Modifies the original array by changing its length property. If the array is empty, the pop() returns undefined . Examples: Removing the last element of an array const numbers = [ 10 , 20 , 30 ]; const last = numbers . pop (); console . log ({ last }); console . log ({ length : numbers . length }); Output: { last: 30 } { length: 2 } Calling the pop() method on an empty array const numbers = []; const last = numbers . pop (); console . log ({ last }); console . log ({ length : numbers . length }); Output: { last: undefined } { length: 0 } shift() Removes the first element from an array Returns that element. Example: const numbers = [ 10 , 20 , 30 ]; let number = numbers . shift (); console . log ({ number }); console . log ({ numbers }); console . log ({ length : numbers . length }); Output: { number: 10 } { numbers: [ 20 , 30 ] } { length: 2 } slice() slice() is used to clone an array Returns a new array that contains the elements of the original array. Syntax: slice ( start , stop ); // start includes, stop excluding.. Example: Clone an array var numbers = [ 1 , 2 , 3 , 4 , 5 ]; var newNumbers = numbers . slice (); Copy a portion of an array var colors = [ ' red ' , ' green ' , ' blue ' , ' yellow ' , ' purple ' ]; var rgb = colors . slice ( 0 , 3 ); console . log ( rgb ); // ["red", "green", "blue"] splice() To delete existing elements, insert new elements, and replace elements in an array. Modifies (or muate) the original array. To create a new array from the original with some element inserted, deleted, and replaced, you can use the toSpliced() method. Example: let scores = [ 1 , 2 , 3 , 4 , 5 ]; let deletedScores = scores . splice ( 0 , 3 ); console . log ({ scores }); console . log ({ deletedScores }); Output: { scores: [ 4 , 5 ] } { deletedScores: [ 1 , 2 , 3 ] } References: https://www.javascripttutorial.net/javascript-array-methods/ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array 1.Adding/removing elements push() Adds one or more elements at the end of an array Returns the new array’s length. Modifies the original array and returns the new length property. Examples: Append one element to an array let numbers = [ 10 , 20 , 30 ]; const length = numbers . push ( 40 ); console . log ({ numbers }); console . log ({ length }); Output: { numbers: [ 10 , 20 , 30 , 40 ] } { length: 4 } Add multiple elements to the end of an array let numbers = [ 10 , 20 , 30 ]; const length = numbers . push ( 40 , 50 ); console . log ({ numbers }); console . log ({ length }); Output: { numbers: [ 10 , 20 , 30 , 40 , 50 ] } { length: 5 } Append elements of an array to another array // Using a for...of loop let colors = [ ' red ' , ' green ' , ' blue ' ]; let cmyk = [ ' cyan ' , ' magenta ' , ' yellow ' , ' back ' ]; for ( const color of cmyk ) { colors . push ( color ); } console . log ( colors ); Output: [ 'red' , 'green' , 'blue' , 'cyan' , 'magenta' , 'yellow' , 'back' ] // Using a spread operator (...) let colors = [ ' red ' , ' green ' , ' blue ' ]; let cmyk = [ ' cyan ' , ' magenta ' , ' yellow ' , ' back ' ]; colors . push (... cmyk ); console . log ({ colors }); unshift() Adds one or more elements to the beginning of an array Returns the new array’s length. It is slow for large arrays because it needs to reindex the existing elements. Examples: Prepend an element to an array let numbers = [ 30 , 40 ]; const length = numbers . unshift ( 20 ); console . log ({ length }); console . log ({ numbers }); Output: { length: 3 } { numbers: [ 20 , 30 , 40 ] } Prepend multiple elements to an array let numbers = [ 30 , 40 ]; const length = numbers . unshift ( 10 , 20 ); console . log ({ length }); console . log ({ numbers }); Output: { length: 4 } { numbers: [ 10 , 20 , 30 , 40 ] } Prepend elements of an array to another array const days = [ ' Mon ' , ' Tue ' , ' Wed ' , ' Thu ' , ' Fri ' ]; const weekends = [ ' Sat ' , ' Sun ' ]; for ( const weekend of weekends ) { days . unshift ( weekend ); } console . log ( days ); Output: [ 'Sun' , 'Sat' , 'Mon' , 'Tue' , 'Wed' , 'Thu' , 'Fri' ] To make it more concise, you can use the spread operator like this: let days = [ ' Mon ' , ' Tue ' , ' Wed ' , ' Thu ' , ' Fri ' ]; let weekends = [ ' Sat ' , ' Sun ' ]; days . unshift (... weekends ); console . log ( days ); pop() Removes the last element from an array Returns the removed element. Modifies the original array by changing its length property. If the array is empty, the pop() returns undefined . Examples: Removing the last element of an array const numbers = [ 10 , 20 , 30 ]; const last = numbers . pop (); console . log ({ last }); console . log ({ length : numbers . length }); Output: { last: 30 } { length: 2 } Calling the pop() method on an empty array const numbers = []; const last = numbers . pop (); console . log ({ last }); console . log ({ length : numbers . length }); Output: { last: undefined } { length: 0 } shift() Removes the first element from an array Returns that element. Example: const numbers = [ 10 , 20 , 30 ]; let number = numbers . shift (); console . log ({ number }); console . log ({ numbers }); console . log ({ length : numbers . length }); Output: { number: 10 } { numbers: [ 20 , 30 ] } { length: 2 } slice() slice() is used to clone an array Returns a new array that contains the elements of the original array. Syntax: slice ( start , stop ); // start includes, stop excluding.. Example: Clone an array var numbers = [ 1 , 2 , 3 , 4 , 5 ]; var newNumbers = numbers . slice (); Copy a portion of an array var colors = [ ' red ' , ' green ' , ' blue ' , ' yellow ' , ' purple ' ]; var rgb = colors . slice ( 0 , 3 ); console . log ( rgb ); // ["red", "green", "blue"] splice() To delete existing elements, insert new elements, and replace elements in an array. Modifies (or muate) the original array. To create a new array from the original with some element inserted, deleted, and replaced, you can use the toSpliced() method. Example: let scores = [ 1 , 2 , 3 , 4 , 5 ]; let deletedScores = scores . splice ( 0 , 3 ); console . log ({ scores }); console . log ({ deletedScores }); Output: { scores: [ 4 , 5 ] } { deletedScores: [ 1 , 2 , 3 ] } References: https://www.javascripttutorial.net/javascript-array-methods/ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array