js中的reduce()函数讲解

  // 1.数组求和
  var arr = [1,5,8,6,15,78,65,25,48,55]
  var sum = arr.reduce(function(total,currentValue){
    return total+currentValue;
  });
  console.log(sum);//306
  var eachSum = 0;
  arr.forEach(function(currentValue){
    eachSum += currentValue;
  })
  console.log(eachSum);//306

  //2.合并二维数组
  var twoArr = [['mu','zi'],['dig','big'],['lucky','jiji']];
  var oneArr = twoArr.reduce(function(total,currentValue){
    // console.log(total)
    return total.concat(currentValue);
  })
  console.log(oneArr);//["mu", "zi", "dig", "big", "lucky", "jiji"]

  //3.统计一个数组中有多少个不重复的单词：
  // 不用reduce时： 
  var arr = ["apple","orange","apple","orange","pear","orange"]; 
  function getWordCnt(){ 
    var obj = {}; 
    for(var i= 0, l = arr.length; i< l; i++){ 
      var item = arr[i]; 
      obj[item] = (obj[item] +1 ) || 1; 
    } 
    return obj; 
  }
  console.log(getWordCnt());//{apple: 2, orange: 3, pear: 1}
  // 用reduce时： 
  var arr = ["apple","orange","apple","orange","pear","orange"]; 
  function getWordCnt(){ 
    return arr.reduce(function(prev,next){ 
      prev[next] = (prev[next] + 1) || 1; 
      return prev; 
    },{}); 
  } 
  console.log(getWordCnt());//{apple: 2, orange: 3, pear: 1}

  // 4.对reduce的理解：
  var arr = ["apple","orange"]; 
  function noPassValue(){ 
    return arr.reduce(function(prev,next){ 
      console.log("prev:",prev); 
      console.log("next:",next); 
      return prev; 
    }); 
  } 
  function passValue(){ 
    return arr.reduce(function(prev,next){ 
      console.log("prev:",prev); 
      console.log("next:",next); 
      prev[next] = 1; 
      return prev; 
    },{});
  } 
  console.log("No Additional parameter:",noPassValue()); 
  console.log("----------------"); 
  console.log("With {} as an additional parameter:",passValue()); 
  /*
  VM415673:4 prev: apple 
  VM415673:5 next: orange 
  VM415673:4 prev: apple 
  VM415673:5 next: orange 
  VM415673:19 No Additional parameter: apple 
  VM415673:20 ---------------- 
  VM415673:13 prev: {} 
  VM415673:14 next: apple 
  VM415673:13 prev: {apple: 1} 
  VM415673:14 next: orange 
  VM415673:21 With {} as an additional parameter: {apple: 1, orange: 1}
  */