JavaScript数组返回是否添加双引号？

Question

Here is my code:

function iLoveThree (array) {
  var threes = [];
  var x;
  for (x in array) {
    if (x % 3 == 0) {
      threes.push(x)
    }
  }
  return threes
}

When I pass the array [1,2,3,4,5,6,7,8,9] I get the following:

Function returned 
["0","3","6"]
instead of 
[3,6,9]

My question is, where are these double quotes coming from?

Answer 1

for...in is a bad way of iterating array indices. Better use filter :

[1,2,3,4,5,6,7,8,9].filter(function(x) {
  return x % 3 == 0;
}); // [3, 6, 9]

Answer 2

So basically in x in array x is the index not the array value. Because anyway 0 is not in the array but your function is returning it as well. You should instead access the values using array[x]

There are various approaches, one of them is using .filter

function iLoveThree(array){
    return array.filter(function(x){
        return (x%3==0?1:0);
    });
}

Or

function iLoveThree (array) {
  var threes = [];
  var x;
  [].forEach.call(array, function(x){
    if (x % 3 == 0) {
      threes.push(x)
    }
  }
  return threes
}

Answer 3

You're using a for..in loop which gives you the keys in an object, or in this case and array. Keys are always strings. Instead, you want to use a standard for loop.

for (var i = 0; i < array.length; i++) {
  var x = array[i];
  if (x % 3 === 0) {
    threes.push(x);
  }
}

Or if you want to use a more functional approach, you could use Array.prototype.filter .

return array.filter(function(x) {
  return x % 3 === 0;
});

Answer 4

A for..in loop does not loop through the array elements, it loops through the indices of the array. So for:

var arr = ["a", "b", "c"]
for ( x in arr ) console.log( x );

You'll get the string keys of ["0", "1", "2"]

You can fix your code by replacing your loop with a native for loop:

for ( var x = 0; x < array.length; x++ ) {
    if (array[i] % 3 == 0)
        threes.push(array[i]);
}

Answer 5

It seems that you are pushing in the indexes and not the actual values, go ahead and try the following:

function iLoveThree(array) {
    var threes = [];
    var x;
    for (x in array) {
        if (((x-2) % 3) == 0) {
            threes.push(array[x])
        }
    }
    return threes;
}

Another option, shorter, is:

function iLoveThree(arr) {
    var threes = [];
    for (var i = 2; i < arr.length; i = i + 3) {
        threes.push(arr[i]);
    };
    return threes;
}

if you are comfortable with callback/predicate based loops, you could make stuff even shorter by filtering the array, instead of creating a new one:

function iLoveThree(arr) {
    return arr.filter(function(x) {
        return (x % 3) == 0;
    });
}

Answer 6

不是直接回答您的问题，而是一种从数字数组中提取3的每一个倍数的好方法：

[1,2,3,4,5,6,7,8,9].filter(item => item % 3 === 0)

Answer 7

Before you read the answer below, please read: Why is using “for…in” with array iteration such a bad idea? (Thanks to @Oriol for this link.)

Douglas Crockford has also discouraged the use of for..in . He recommends using array.forEach instead.

 function iLoveThree (array) { var threes = []; array.forEach(function(item, i){ // use forEach, 'item' is the actual value and 'i' is the index if (item % 3 === 0) { threes.push(item); // you missed ; here } }); return threes; // you missed ; here } console.log(iLoveThree([1,2,3,4,5,6,7,8,9])); 

Read up: Array.prototype.forEach() | MDN

Answer 8

If you read the for...in documentation, you will realize that you are pushing to threes the indexes (also called keys ) not the values , because the variable x represents the index, so the value should be accessed by array[x] .

function iLoveThree (array) {
  var threes = [];

  for (var x in array) {
    if (array[x] % 3 == 0) {
      threes.push(array[x])
    }
  }
  return threes
}

There are several ways to achieve this, the best one is by using a filter , but that way was already explained by someone else, therefore I will use an exotic implementation using a reduce

[1, 2, 3, 4, 5, 6, 7, 8, 9].reduce(function(acc, e){return e % 3 == 0 ? acc.concat(e) : acc}, [])

Outputs 3, 6, 9