js array entries() Get iteration method

1. Detailed syntax of entires() method

entries() method returns an array iterator object that contains the array's (key/value) .

The index value of the array in the iteration object is used as key and the array element is used as value . Its prototype （__proto__:Array Iterator） has a next method that can be used to traverse the iterator to obtain [key,value] of the original array. Here you need to know the relevant iterator knowledge.

2. Common uses and precautions of the entires() method

2.1 Returning an iterator object

    var arr = ["red", "blue", "green"]
    var x = arr.entries()
    console.log(x); // Array Iterator {}
    console.log(x.next()) //{value: Array:[0, "red"],done:false}
    console.log(x.next()) //{value: Array:[1, "blue"],done:false}
    console.log(x.next()) //{value: Array:[2, "green"],done:false}
    console.log(x.next()) //{value: undefined, done: true}

2.2 Use of for...of...

    const options = [1, , , , 5];
    for (const [index, value] of options.entries()) {
      console.log(value);
    }
    // 0 1
    // 1 undefined
    // 2 undefined
    // 3 undefined
    // 4 5

2.3 Sorting rows of a two-dimensional array

    function sortTwo(arr) {
      var entries = arr.entries()
      var flag = true
      while (flag) {
        var res = entries.next()
        if (!res.done) {
          res.value[1].sort((a, b) => a - b);
          flag = true
        } else {
          flag = false
        }
      }
      return arr
    }
    var arr = [[1, 3, 2], [44, 33], [11, 55, 44, 33]]
    sortTwo(arr)
    console.log(arr); // [[1, 2, 3], [33, 44], [11, 33, 44, 55]]

In the above code, sortTwo method first obtains the iteration object of the passed array, and then defines an initialization flag of true , recursively calls the next method of the iteration object entires to assign it to res object, and judges the d one attribute of res object. If the value is true , it means recursion is possible. res.value corresponds to each row of the two-dimensional array, and the item can be sorted. If the value is flase , it means the recursion is ended.

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