JavaScript中最常用的几种数组操作

const arr = [1, 2, 3]; 
console.log(arr.length); // 3

const arr = [1, 2, 3]; 
arr.length = 2; 
arr.forEach(i => console.log(i)); // 1 2

const arr = []; 
arr.length = 100; 
console.log(arr) // [undefined, undefined, undefined ...]

const arr = [1, 2]; 
arr.length = 0; 
console.log(arr)  // []

const arr = [1, 2, 3]; 
arr.splice(2, 1, 4); // 将索引 2 开始的 1 元素更改为 4
console.log(arr); // [1, 2, 4] 
arr.splice(0, 2, 5, 6) // 将索引 0 开始的 2 个元素更改为 5 和 6 
console.log(arr); // [5, 6, 4]

const arr = [1, 2, 3, 4, 5, 6]; 
// 所有奇数的平方
const arr2 = arr.map(item => item % 2 == 0 ? item : item*item); 
console.log(arr2); // [1, 2, 9, 4, 25, 6];

const arr = [1, 2, 3, 4, 5, 6, 7]; 
// 过滤掉所有奇数
const arr2 = arr.filter(item => item % 2 == 0); 
console.log(arr2); // [2, 4, 6];

const arr1 = [1, 2, 3]; 
const arr2 = [4, 5, 6]; 
const arr3 = arr1.concat(arr2);
console.log(arr3 ); // [1, 2, 3, 4, 5, 6]

const arr1 = [1, 2, 3]; 
const arr2 = [4, 5, 6]; 
const arr3 = [...arr1, ...arr2];
console.log(arr3 ); // [1, 2, 3, 4, 5, 6]

const arr1 = [1, 2, 3]; 
const arr2 = [4, 5, 6]; 
Array.prototype.push.apply(arr1, arr2);
console.log(arr1); // [1, 2, 3, 4, 5, 6]

const arr1 = [1, 2, 3]; 
const arr2 = arr1; 
arr2[0] = 4; 
arr2[1] = 2; 
arr2[2] = 0; 
console.log(arr1); // [4, 2, 0]

const arr1 = [1, 2, 3]; 
const arr2 = arr1.slice(0); 
arr2[0] = 4; 
arr2[1] = 2; 
arr2[2] = 0;
console.log(arr1); // [1, 2, 3]
console.log(arr2); // [4, 2, 0]

const arr1 = [1, 2, 3]; 
const arr2 = [...arr1]; 
arr2[0] = 4; 
arr2[1] = 2; 
arr2[2] = 0;
console.log(arr1); // [1, 2, 3]
console.log(arr2); // [4, 2, 0]

const arr1 = [1, 2, 3]; 
const arr2 = [].concat(arr1); 
arr2[0] = 4; 
arr2[1] = 2; 
arr2[2] = 0;
console.log(arr1); // [1, 2, 3]
console.log(arr2); // [4, 2, 0]

const arr1 = [1, 1, 2, 3, 1, 5, 9, 4, 2]; 
const arr2 = arr1.filter((item, index, arr) => arr.indexOf(item) == index);
console.log(arr2); // [1, 2, 3, 5, 9, 4]

const arr1 = [1, 1, 2, 3, 1, 5, 9, 4, 2]; 
const arr2 = arr1.reduce(
  (acc, item) =>  acc.indexOf(item) == -1 ? [...acc, item]: acc,
  []   // 初始化当前值
);
console.log(arr2); // [1, 2, 3, 5, 9, 4]

const arr1 = [1, 1, 2, 3, 1, 5, 9, 4, 2]; 
const arr2 = [...(new Set(arr1))]; 
console.log(arr2); // [1, 2, 3, 5, 9, 4]

Array.prototype.slice.call(arguments);
Array.prototype.concat.apply([], arguments);

console.log('string'.split('')); // ["s", "t", "r", "i", "n", "g"]
console.log(Array.from('string'));  // ["s", "t", "r", "i", "n", "g"]

console.log(Array.from(new Set(1,2,3))); // [1,2,3]
console.log([...(new Set(1,2,3))]); // [1,2,3]

const arr = [1, 2, 3]; 
for (let i = 0; i < arr.length; i++) { 
  console.log(arr[i]); 
} 
// 1 2 3

const arr = [1, 2, 3]; 
for (let i in arr) {
   if(arr.hasOwnProperty(i)) {
      console.log(arr[i]); 
  }
} 
// 1 2 3

const arr = [1, 2, 3]; 
for (let i of arr) {
  console.log(i); 
} 
// 1 2 3

[1, 2, 3].forEach(i => console.log(i))
// 1 2 3

const arr = [1,2,3];
let i = -1;
const length = arr.length;
while(++i < length) {
    console.log(arr[i])
}
// 1 2 3

while > for > for-of > forEach > for-in

// ES5
[1, 2, 3].some((i) => i == 1);
// ES6
[1, 2, 3].find((i) => i == 1);
[1, 2, 3].findIndex((i) => i == 1);

const arr1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
const arr2 = [].concat.apply([], arr1);
console.log(arr2); // [1, 2, 3, 4, 5, 6, 7, 8, 9]

const arr1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]];
const arr2 = [].concat.apply([], arr1);
console.log(arr2); // [1, 2, 3, 4, 5, 6, 7, 8, 9, [1, 2, 3], [4, 5, 6], [7, 8, 9]]

function flatten(arr) {
  return arr.reduce((flat, toFlatten) => {
    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}
const arr1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]];
const arr2 = flatten(arr1);
console.log(arr2); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9]

const arr1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
const arr2 = arr1.flat();
console.log(arr2); // [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

const arr1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]];
const arr2 = arr1.flat(Infinity);
console.log(arr2); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9]

const arr1 = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]];
const arr2 = arr1.toString().split(',').map(n => parseInt(n, 10));
console.log(arr2); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 2, 3, 4, 5, 6, 7, 8, 9]

const arr = [1, 2, 3, 4, 5, 6];
arr.push(7)
console.log( arr ); // [1, 2, 3, 4, 5, 6, 7]

const arr = [1, 2, 3, 4, 5, 6];
arr.unshift(0)
console.log( arr ); // [0, 1, 2, 3, 4, 5, 6]

const arr = [1, 2, 3, 4, 5];
arr.splice(1, 0, 10)
console.log(arr); // [1, 10, 2, 3, 4, 5]

const arr1 = [1, 2, 3, 4, 5];
const arr2 = arr1.concat(6);
console.log(arr2); // [1, 2, 3, 4, 5, 6]

const arr = [1, 2, 3, 4, 5, 6];
arr.length = 4; 
console.log( arr ); // [1, 2, 3, 4]

const arr = [1, 2, 3, 4, 5, 6];
arr.pop(); 
console.log( arr ); // [1, 2, 3, 4, 5]

const arr = [1, 2, 3, 4, 5, 6];
arr.shift(); 
console.log( arr ); // [2, 3, 4, 5, 6]

const arr1 = [1, 2, 3, 4, 5];
arr1.splice(1);   
console.log(arr1); // [1];
const arr2 = [1, 2, 3, 4, 5];
arr2.splice(1, 2) 
console.log(arr2); // [1, 4, 5]
const arr3 = [1, 2, 3, 4, 5];
arr3.splice(1, 1) 
console.log(arr3); // [1,3, 4, 5]

const arr = [1, 2, 3, 4, 5];
arr.splice(1, 0, 10)
console.log(arr); // [1, 10, 2, 3, 4, 5]

const arr = [1, 2, 3, 4, 5];
arr.splice(1, 1, 10)
console.log(arr); // [1, 10, 3, 4, 5]

const arr = [1, 2, 3, 4, 5];
delete arr[1]
console.log(arr); // [1, empty, 3, 4, 5]

const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
const filtered = arr.filter((value, index, arr) => value > 5);
console.log(filtered); // [6, 7, 8, 9]
console.log(arr); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]

let arr = [1,2,3];
arr = [];

let arr = [1,2,3];
arr.length = 0;

let arr = [1,2,3];
arr.splice(0);

const arr = [1, 2, 3, 4, 5, 6];
while (arr.length) { arr.pop(); }
console.log(arr); // []

[1, false, '', NaN, 0, [], {}, '123'].filter(Boolean) // [1, [], {}, '123']

[1, false, '', NaN, 0, [], {}, '123'].some(Boolean)  // true

[1, false, '', NaN, 0, [], {}, '123'].every(Boolean) // false

Array(6).join('0');      // '00000'  注意：如果要补5个0，要写6，而不是5。
Array(5).fill('0').join('')  // '00000'

Math.max.apply(null, [1, 2, 3, 4, 5])  // 5
Math.min.apply(null, [1, 2, 3, 4, 5])  // 1

const str1 = 'string';
const str2 = str1.split('').reverse().join('');
console.log(str1 === str2); // false 

const arr = [1];
// 入队
arr.push(2); 
console.log('入队元素：', arr[arr.length -1]); // 2
// 出队
console.log('出队元素：', arr.shift()); // 1

const arr = [1, 2, 3, 4, 5];
console.log(arr[arr.length - 1]);   // 5 

const arr1 = [1, 2, 3, 4, 5];
function createNegativeArrayProxy(array) {
    if (!Array.isArray(array)) {     
       throw new TypeError('Expected an array'); 
    }
    return new Proxy(array, {
      get: (target, prop, receiver) => { 
        prop = +prop;
        return Reflect.get(target, prop < 0 ? target.length + prop : prop, receiver);; 
      }    
    })
}
const arr2 = createNegativeArrayProxy(arr1);
console.log(arr1[-1]) // undefined
console.log(arr1[-2]) // undefined
console.log(arr2[-1]) // 5
console.log(arr2[-2]) // 4