更改子阵列何时更新子阵列与创建新子阵列？ [JavaScript]

Question

I have a [Edit: JavaScript] code that uses a sub-array dataSelectedArray that tracks from my main data set mainArray . The main data is stored as an array of arrays.

mainArray = [ [1,2,3],[3,4,2],[5,1,0] ];
dataSelectedArray=[];
for (var i=0; i<dataArray.length; i++) {
    if ( someLogic ) {
        dataSelectedArray.push(mainArray[i]);
    }
}

Thus, dataSelectedArray holds pointers to the values stored by mainArray . Later, I need to update mainArray by replacing it with data from updatedData . I need dataSelectedArray to also be updated. I can think of two ways to do this: update each mini-array as a set, or update each component.

 var mainArray = [ [1,2,3],[3,4,2],[5,1,0] ]; var dataSelectedArray=[]; for (var i=0; i<mainArray.length; i++) { if ( i<2 ) { dataSelectedArray.push(mainArray[i]); } } updatedData = [ [ 1,0,0],[7,4,2] ]; for (var index= 0; index < updatedData.length; index++) { // this doesn't update dataSelectedArray mainArray[index] = updatedData[index]; } console.log(mainArray[0]); console.log(dataSelectedArray[0]);

 var mainArray = [ [1,2,3],[3,4,2],[5,1,0] ]; var dataSelectedArray=[]; for (var i=0; i<mainArray.length; i++) { if ( i<2 ) { dataSelectedArray.push(mainArray[i]); } } updatedData = [ [ 1,0,0],[7,4,2] ]; for (var index= 0; index < updatedData.length; index++) { // this *does* update dataSelectedArray mainArray[index][0] = updatedData[index][0]; mainArray[index][1] = updatedData[index][1]; mainArray[index][2] = updatedData[index][2]; } console.log(mainArray[0]); console.log(dataSelectedArray[0]);

The first method doesn't update dataSelectedArray ...why not? And is there a shorter (one-liner) way to do the second method?

Answer 1

When you do:

dataSelectedArray.push(mainArray[i]);

dataSelectedArray contains a reference to the sub-array. This is not a reference to the location in mainArray where the original reference came from. So reassigning mainArray[i] has no effect on the value in dataSelectedArray .

It's no different from when you have multiple variables referring to the same array:

var a = [1, 2, 3];
var b = a; // a and b now refer to the same array
b[1] = 6;
console.log(a[1]); // Outputs 6
b = [7, 8, 9]; // b now refers to a different array
console.log(a[1]); // Still 6 -- a hasn't been modified