Method of the array (Array.prototype.reduce () method)

Reduce function

reduce()Method applies a function to each element in the accumulator and array (left to right), reducing it to a single value.

invokes the specified callback function for all elements in the array. cumulative result , and this in Next call the callback function as parameter provided.

<script>  = [1, 2, 3, 4];   = (accumulator, currentvalue) + =    {+ ' | ' + currentvalue)    ; return accumulator + currentvalue  };   // 1 + 2 + 3 + 4  Console.log (Array1.reduce (reducer)); // Ten  // 5 + 1 + 2 + 3 + 4  Console.log (Array1.reduce (Reducer, 5));  //  the  </script>

The output is as follows:

Grammar:

callback a function that executes each value in the array, containing four parameters:

• accumulator:The return value of the accumulator summation callback; It is the cumulative value returned when the callback was last called, or initialValue (as shown below).
• currentValue: The element being processed in the array.

• Optional, the index of the current element being processed in the array. currentIndex:  If provided initialValue , the index number is 0, otherwise the index is 1.
• an optional , called array. array:  reduce

initialValue：Optional, used as the callback value of the first parameter of the first call. If no initial value is provided, the first element in the array is used. A call to reduce on an empty array without an initial value will error.

Use the following :

1. Common usage:

  var t = [0, 1, 2, 3, 4].reduce (function(accumulator, CurrentValue, Currentindex, array) {       + ' | ' + currentvalue+ '--+ ' + currentindex + ' + ' + array      ); return accumulator + currentvalue;    });    Console.log (' t: ', T);

The output is as follows:

2. If you provide an initial value as reduce the second parameter of the method, the following is the running process and the result:

var t = [0, 1, 2, 3, 4].reduce ((accumulator, CurrentValue, currentindex, array) = =      {+ ' | ' + Curr Entvalue+ '--+ ' + currentindex + ' + ' + array      ); return accumulator + currentvalue;     (ten );    Console.log (' t: ', T);

The output is as follows:

3. Convert Two-dimensional arrays to one-dimensional

var flattened = [[0, 1], [2, 3], [4, 5]].reduce (      function(A, b) {        return  A.concat (b);      },[]);   Console.log (flattened);

The output is as follows:

4. count the occurrences of each element in the array

var names = [' Alice ', ' Bob ', ' Tiff ', ' Bruce ', ' Alice '];   var countednames = Names.reduce (function  (allnames, name) {     ' | ' + name ')    ; if inch allnames) {      Allnames[name]++    ; Else {      = 1;    }     return allnames;   }, {}); Console.log (countednames);

The output is as follows:

5. array de-weight

Let arr = [1,2,1,2,3,5,4,5,3,4,4,4,4];   = Arr.sort (). Reduce (init, current) ={      if(init.length===0 | | init[init.length-1]!==  Current) {          Init.push (current);      }       return init;  }, []);   // [1,2,3,4,5]

The output is as follows:

Method of the array (Array.prototype.reduce () method)