What are the commonly used array functions?

Some methods of arrays

The array of JavaScript is a very familiar type and has very powerful functions, but not everyone without front-end learning can master it proficiently. Let’s follow Let’s talk about some methods of arrays

• Array deduplication

  (1) Give an array arr=[1,2,3 ,3,4], how to remove duplicates?

• There is a new data structure Set in es6 var newArr=new Set(arr);

• ## The #es5 method is to declare a new array obj, traverse arr, and check whether the elements exist in the new array. The code is as follows:

  var newArr = [];
  for (var i = 0; i < arr.length; i++) {
      if(newArr.indexOf(arr[i]) === -1){
          newArr.push(arr[i]);
      }
  }
  console.log(newArr); //[1,2,3,4];

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• ##Array.forEach

  foreach executes a given function once for each element in the array, but the original array remains unchanged and has no return value

      var arr = [1,2,3,4];
      arr.forEach(function(item, index, origin){
          item += 1;
          console.log(item);//2,3,4,5
      })
      console.log(arr);//[1,2,3,4]

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• Array.map

  map executes a given function once for each element in the array. The original array remains unchanged and a new array is returned.

      var arr = [1,2,3,4];
      var newArr=arr.map(function(item, index, origin){
          return ++item;
      })
      console.log(newArr);//[2,3,4,5]

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• Array.filter

  filter is to execute a given function once for each element in the array, leaving the original array unchanged and returning a new array that meets the conditions

      var arr = [1,2,3,4];
      var newArr=arr.filter(function(item, index, origin){
          return item>2;
      })
      console.log(newArr);//[3,4]

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• Array .reduce

  reduce is that the first parameter is a function. The parameters of this function are the last execution function result pre and the value and index of the current element. It is usually used to find the sum of arrays. The second parameter is The first parameter is the value of pre when the function is executed for the first time. If there is no such parameter, the first value will be regarded as the value of pre

      var arr = [1,2,3,4];
      var res=arr.reduce(function(pre, cur, curIndex, origin){
          console.log(curIndex);//1,2,3
          return pre + cur;
      })
      console.log(res);//10
  
      var res=arr.reduce(function(pre, cur, curIndex, origin){
          console.log(curIndex);//0,1,2,3
          return pre + cur;
      }, 5)
      console.log(res);//15

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• Array.every

  every executes a given function once for each element in the array. If one result is false, false is returned;

      var arr = [1,2,3,4];
      var res = arr.every(function(item, index, origin){
          return item > 2;
      })
      console.log(res);//false

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##Array. some

• Some is opposite to every, which executes a given function once for each element in the array. If one result is true, it returns true;

      var arr = [1,2,3,4];
      var res = arr.some(function(item, index, origin){
          return item > 2;
      })
      console.log(res);//true

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Convert class array to array
The
Array.from
• method is provided in es6
  You can use the
Array.prototype.slice.call(arr)
• method in es5

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