去重

Object

开辟一个外部存储空间用于标示元素是否出现过。

const unique = (array)=> {
    var container = {};
    return array.filter((item, index) =>  container.hasOwnProperty(item) ? false : (container[item] = true));
}

indexOf + filter

const unique = arr => arr.filter((e,i) => arr.indexOf(e) === i);

Set

const unique = arr => Array.from(new Set(arr));

const unique = arr => [...new Set(arr)];

排序

通过比较相邻数字是否重复，将排序后的数组进行去重。

    const unique = (array) => {
      array.sort((a, b) => a - b);
      let pre = 0;
      const result = [];
      for (let i = 0; i < array.length; i++) {
        if (!i || array[i] != array[pre]) {
          result.push(array[i]);
        }
        pre = i;
      }
      return result;
    }

去除重复的值

不同于上面的去重，这里是只要数字出现了重复次，就将其移除掉。

const filterNonUnique = arr => arr.filter(i => 
  arr.indexOf(i) === arr.lastIndexOf(i)
)

扁平

基本实现

    const flat = (array) => {
      let result = [];
      for (let i = 0; i < array.length; i++) {
        if (Array.isArray(array[i])) {
          result = result.concat(flat(array[i]));
        } else {
          result.push(array[i]);
        }
      }
      return result;
    }

使用reduce简化

    function flatten(array) {
      return array.reduce(
        (target, current) =>
          Array.isArray(current) ?
            target.concat(flatten(current)) :
            target.concat(current)
        , [])
    }

根据指定深度扁平数组

    function flattenByDeep(array, deep = 1) {
      return array.reduce(
        (target, current) =>
          Array.isArray(current) && deep > 1 ?
            target.concat(flattenByDeep(current, deep - 1)) :
            target.concat(current)
        , [])
    }

最值

reduce

array.reduce((c,n)=>Math.max(c,n))

Math.max

Math.max参数原本是一组数字，只需要让他可以接收数组即可。

const array = [3,2,1,4,5];
Math.max.apply(null,array);
Math.max(...array);

使用reduce实现map

    Array.prototype.reduceToMap = function (handler) {
      return this.reduce((target, current, index) => {
        target.push(handler.call(this, current, index))
        return target;
      }, [])
    };

使用reduce实现filter

    Array.prototype.reduceToFilter = function (handler) {
      return this.reduce((target, current, index) => {
        if (handler.call(this, current, index)) {
          target.push(current);
        }
        return target;
      }, [])
    };