我可以创建一个引用对象中节点的变量吗？

Question

I have this piece of code. It increments a value in an array or sets it to 1 if undefined:

if(typeof sum[period][count] === "undefined"){
    sum[period][count] = 1
}else{
    sum[period][count]++;
}

I would like to make it shorter and less repetitive like this:

node = sum[period][count];
if(typeof node === "undefined"){
    node = 1
}else{
    node++;
}

or even

node = typeof node === "undefined"? 1 : node+1;

But it doesn't work, why not and what can I do about it?

Answer 1

since the value sum[period][count] is a primitive, you can't get a reference to it. You could shorten it by assigning the object containing this value to a variable: var node = sum[period] and test for node[count] . However, it is not much shorter:

var node = sum[period];
if(typeof node[count] === "undefined"){
    node[count] = 1
}else{
    node[count]++;
}

Answer 2

因为node的值不是Reference 。

Answer 3

I believe your solution will work in some cases, but in others it will not. This is a consequence of using an array of arrays (two-dimensional array/hash) and only checking if the 2nd order value is undefined.

When you define a list of lists (eg 'sum') you don't create a 2D matrix. Instead, you are creating a 1D list of 1D lists. If the first list index (period) is found, then it will get that list and look for the second index (count) within that 2nd list. But if the first list doesn't contain that first index, then you'll get an error immediately since you are referencing an index in an undefined list.

The solution is to check if the first order index value is undefined before checking the second. For reusability, I'm packaging the code up in a function. You will want to remove the JQuery output; it's just added for my testing.

var sum = {0: {0: 2}};
var period = 0;
var count = 1;

var helper = function(idxa, idxb) {
  if (sum[idxa] === undefined) {
    sum[idxa] = {idxb: 1}; // we can define the new list, knowing idxb must be 1 now.
  } else {
    // you were onto the right code here, but you weren't validating sum[idxa] separately.
    sum[idxa][idxb] = (sum[idxa][idxb] === undefined) ? 1 : sum[idxa][idxb] + 1;
  }
  $('#output').append(" After: " + sum[idxa][idxb]);
};

helper(0, 3); // outputs " After: 1". [0] was defined, but [0][3] wasn't.
helper(2, 3); // outputs " After: 1". Neither [2] nor [2][3] were defined.
helper(0, 0); // outputs " After: 3", because [0][0] was 2
helper(0, 0); // outputs " After: 4", because we already incremented to 3.

You could nest a ternary operation inside a ternary, but I didn't because it's no faster and it wouldn't improve readability.