Array

🤖WARNING

The English translation was done by AI.

Array Deduplication

Method 1

// Naive method
const arr = [1, 1, 2, 5, 2, 6, 8];
let newArr = [];
for (let i = 0; i < arr.length; i++) {
  if (newArr.includes(arr[i])) {
    continue;
  }
  newArr.push(arr[i]);
}

Method 2

const arr = [1, 3, 45, 6, 3, 2, 0];
const newArr = arr.filter((item, index) => {
  return arr.indexOf(item) === index;
});
console.log(arr); // [1, 3, 45, 6, 3, 2, 0]
console.log(newArr); // [1, 3, 45, 6, 2, 0]

Method 3

const arr = [1, 3, 45, 6, 3, 2, 0];
const newArr = [...new Set(arr)];
console.log(arr); // [1, 3, 45, 6, 3, 2, 0]
console.log(newArr); // [1, 3, 45, 6, 2, 0]

Array Filling

new Array(10).fill(1);

Array.from({ length: 10 }, item => 1);

Array Flattening

Method 1

// Array.prototype.flat([depth])
let arr = [1, 2, 7, [2, [2, 3], 6]];

console.log(arr.flat(Infinity));

Method 2

// Recursive with for...of
let arr = [1, 2, 7, [2, [2, 3], 6]];
function flat(arr) {
  let newArr = [];
  for (const item of arr) {
    if (Array.isArray(item)) {
      newArr = newArr.concat(flat(item));
    } else {
      newArr.push(item);
    }
  }
  return newArr;
}
console.log(flat(arr));

Method 3

// Spread operator
const arr = [1, 2, 7, [2, [2, 3], 6]];
function flat(arr) {
  while (arr.some(item => Array.isArray(item))) {
    arr = [].concat(...arr);
  }
  return arr;
}
console.log(flat(arr));

Method 4

// Clever operation
const arr = [1, 2, 7, [2, [2, 3], 6]];
function flat(arr) {
  const result = arr
    .toString()
    .split(',')
    .map(item => {
      return +item;
    });
  return result;
}
console.log(flat(arr));

Finding the Maximum Value in an Array

const arr = [1, 2, 1, 4, 2, 10];
console.log(Math.max.apply(null, arr));

const arr = [1, 2, 1, 4, 2, 10];
console.log(arr.sort((a, b) => a - b)[arr.length - 1]);

const arr = [1, 2, 1, 4, 2, 10];
console.log(Math.max(...arr));

Sorting Character Arrays

const arr1 = ['A1', 'A2', 'B1', 'B2'];
const arr2 = ['A', 'B'];

const c = [...arr1, ...arr2].sort(
  (a, b) =>
    a.charCodeAt(0) - b.charCodeAt(0) || a.length - b.length || a.charCodeAt(1) - b.charCodeAt(1)
);

// ['A', 'A1', 'A2', 'B', 'B1', 'B2']

Sorting Object Arrays

const users = [
  {
    name: 'Alan',
    age: 19
  },
  {
    name: 'Bob',
    age: 25
  }
];
const userList = users.sort((a, b) => b.age - a.age);
// Sort based on age, note: sort will modify the original array

Map Related

const a = ['1', '2', '3'].map(parseInt);
// The '1' in array a is converted to decimal.
console.log(a); // [1, NaN, NaN]
// map has three parameters (item, index, array)
/* parseInt(string, radix)
When radix equals 0 or undefined or is not specified, if string starts with '0x' or '0X', then radix is 16
If it starts with '0', radix is 10/8 depending on the actual situation. */

// Breakdown process
parseInt('1', 0);
parseInt('2', 1);
parseInt('3', 2);