What is js array flattening? Implementation of js array flattening (with code)

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1. What is array flattening

1. Flattening, as the name suggests, is to reduce complexity decoration and make things It is more concise and simple, highlighting the theme.

2. Array flattening, as you already know from the above meaning, is to transform a complex nested multi-layered array, one layer at a time, into one with fewer layers or only one layer. Array of layers.

Ps: flatten can flatten the array, and the effect will be as follows:

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

As can be seen from this, The array processed using flatten has only one layer. Let's try to implement it.

2. Simple implementation

2.1 Ordinary recursion

• This It is the easiest method to think of, simple and clear!

/* ES6 */
const flatten = (arr) => {
  let result = [];
  arr.forEach((item, i, arr) => {
    if (Array.isArray(item)) {
      result = result.concat(flatten(item));
    } else {
      result.push(arr[i])
    }
  })
  return result;
};

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

/* ES5 */
function flatten(arr) {
  var result = [];
  for (var i = 0, len = arr.length; i < len; i++) {
    if (Array.isArray(arr[i])) {
      result = result.concat(flatten(arr[i]))
    }
    else {
      result.push(arr[i])
    }
  }
  return result;
}

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

2.2 toString()

• This method uses toString to turn the array into A comma-separated string, then iterates through the array and converts each item back to its original type.

Let’s first look at how toString turns an array into a string

[1, [2, 3, [4]]].toString()
// "1,2,3,4"

Complete display

/* ES6 */
const flatten = (arr) => arr.toString().split(',').map((item) => +item);

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

/* ES5 */
function flatten(arr) {
  return arr.toString().split(',').map(function(item){
    return +item;
  });
}

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

This However, the usage scenarios of this method are very limited. All elements in the array must be Number.
It can also be all String, you can realize the specific implementation by yourself.

2.3 [].concat.apply some

• Use arr.some to determine if there is an array in the array , circularly call the flatten flat function (using [].concat.apply flattening), connect with concat, and finally return arr;

/* ES6 */
const flatten = (arr) => {
  while (arr.some(item => Array.isArray(item))){
    arr = [].concat.apply([], arr);
  }
  return arr;
}

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

/* ES5 */
/**
* 封装Array.some
* @param {function} callback    - 回调函数
* @param {any}      currentThis - 回调函数中this指向
*/
Array.prototype.some = function (callback, currentThis){
  let context = this;
  let flag = false;
  currentThis = currentThis || this;
  for (var i = 0, len = context.length; i < len; i++) {
    const res = callback.call(currentThis, context[i], i, context);
    if (res) {
      flag = true;
    } else if (!flag) {
      flag = false;
    }
  }
  return flag;
}

function flatten(arr){
  while(arr.some(item => Array.isArray(item))){
    arr = [].concat.apply([], arr);
  }
  return arr;
}

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

2.4 reduce

• reduce itself is an iterative looper, usually used for accumulation , so according to this feature there are the following:

function flatten(arr){
  return arr.reduce(function(prev, cur){
    return prev.concat(Array.isArray(cur) ? flatten(cur) : cur)
  }, [])
}

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

2.5 Destructuring operator in ES6...

• ... Only the outermost array can be expanded each time. After being [].concat, arr will be flattened once.

function flatten(arr){
  while(arr.some(item => Array.isArray(item))){
    arr = [].concat(...arr);
  }
  return arr;
}

const arr = [1, [2, [3, 4]]];
console.log(flatten(arr));

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