Issue during my work

script

Here is part of my script. Can you find where the problem is:

const args = process.argv.slice(2)

const data_header = [
    ['id', 'text', 'textA', 'textB']
];

const data_ID_match = [
    [null, '1', 'A', 'B']
];

const dataid_update = ['001', '002', '003', '004']

function data_create() {
    let data_set = data_header
    for(var i = 0;i < 3;i++) {
        data_set = data_set.concat(data_ID_match)
        }
    console.log(data_set)
    console.log("————————— data_set —————————")
    return data_set
}

function xlsx_output(data_set) {
    for(var i = 1;i < dataid_update.length;i++) {
        data_set[i][0] = dataid_update[i - 1]
        console.log(data_set)
        console.log("————————— step " + i + " ——————————")
    }
    
}

xlsx_output(data_create())

output

Name this js file as test.js and run at node test.js command.


Then you can Console log as followed: 

[
  [ 'id', 'text', 'textA', 'textB' ],
  [ null, '1', 'A', 'B' ],
  [ null, '1', 'A', 'B' ],
  [ null, '1', 'A', 'B' ]
]
————————— data_set —————————
[
  [ 'id', 'text', 'textA', 'textB' ],
  [ '001', '1', 'A', 'B' ],
  [ '001', '1', 'A', 'B' ],
  [ '001', '1', 'A', 'B' ]
]
————————— step 1 ——————————
[
  [ 'id', 'text', 'textA', 'textB' ],
  [ '002', '1', 'A', 'B' ],
  [ '002', '1', 'A', 'B' ],
  [ '002', '1', 'A', 'B' ]
]
————————— step 2 ——————————
[
  [ 'id', 'text', 'textA', 'textB' ],
  [ '003', '1', 'A', 'B' ],
  [ '003', '1', 'A', 'B' ],
  [ '003', '1', 'A', 'B' ]
]
————————— step 3 ——————————

From the output, we can see a strange phenomenon that during one single assignment, the whole ‘id’ column of data is assigned.


Why it happens

Shallow Copy:

  • When a reference variable is copied into a new reference variable using the assignment operator, a shallow copy of the referenced object is created. In simple words, a reference variable mainly stores the address of the object it refers to. When a new reference variable is assigned the value of the old reference variable, the address stored in the old reference variable is copied into the new one.
  • This means both the old and new reference variable point to the same object in memory. As a result if the state of the object changes through any of the reference variables it is reflected for both.

From the above example, it is seen that the newly created object has the same memory address as the old one. Hence, any change made to either of them changes the attributes for both. If one of them is removed from memory, the other one ceases to exist. In a way the two objects are interdependent.


How to solve

Deep Copy:

  • Unlike the shallow copy, deep copy makes a copy of all the members of the old object, allocates separate memory location for the new object and then assigns the copied members to the new object.
  • In this way, both the objects are independent of each other and in case of any modification to either one the other is not affected. Also, if one of the objects is deleted the other still remains in the memory.

To overcome this problem, deep copy should be used.


script fixed

So I fixed my script like that:

// deep copy
function deepClone (obj) {
    var objClone = Array.isArray(obj) ? [] : {}; 
    if (obj && typeof obj === 'object') {
        for (var key in obj) {
            if (obj.hasOwnProperty(key)) {
                if (obj[key] && typeof obj[key] === 'object') {
                    objClone[key] = deepClone(obj[key]);
                } else {
                    objClone[key] = obj[key];
                }
            }
        }
    }
    return objClone;
}

function data_create() {
    let data_set = data_header
    for(var i = 0;i < 3;i++) {
        // deep copy here
        data_set = data_set.concat(deepClone(data_ID_match))
        }
    return data_set
}

correct output

Now, the output becomes correct.

[
  [ 'id', 'text', 'textA', 'textB' ],
  [ null, '1', 'A', 'B' ],
  [ null, '1', 'A', 'B' ]
  [ null, '1', 'A', 'B' ]
]
————————— data_set —————————
[
  [ 'id', 'text', 'textA', 'textB' ],
  [ '001', '1', 'A', 'B' ],
  [ null, '1', 'A', 'B' ],
  [ null, '1', 'A', 'B' ]
]
————————— step 1 ——————————
[
  [ 'id', 'text', 'textA', 'textB' ],
  [ '001', '1', 'A', 'B' ],
  [ '002', '1', 'A', 'B' ],
  [ null, '1', 'A', 'B' ]
]
————————— step 2 ——————————
[
  [ 'id', 'text', 'textA', 'textB' ],
  [ '001', '1', 'A', 'B' ],
  [ '002', '1', 'A', 'B' ],
  [ '003', '1', 'A', 'B' ]
]
————————— step 3 ——————————