CODEWITHSUNDEEP
typescriptangular6
Thanny
Thanny

Reputation: 604

Angular 6 array gets updated, without any updation made

I have the following code whose job is to remove specific properties from a JSON so that only some selected fields can be displayed in an a table

    var tempColumnDefs = [];
  var tempRowData = array;
  for( var i = 0;i<columns.length;i++)
    {
      if(this.dynoArray[this.header[i]])
      {tempColumnDefs.push(columns[i]);}
    }
    for( var i = 0;i<tempRowData.length;i++)
    {
      for(var j = 0;j<this.header.length;j++)
      {
        if(!this.dynoArray[this.header[j]])
        {
          delete tempRowData[i][this.header[j]];
        }
      }
    }

    this.cols = tempColumnDefs;
 this.cars = tempRowData;

As you can see "array" is never on the left hand side so it shouldn't be updated at any case,only "tempRowData" should, but it somehow updates array too.

  1. Can someone explain why this happens?
  2. How can I stop array from being updated?

Upvotes: 0

Views: 693

Answers (2)

user8401765
user8401765

Reputation:

You can also try copying the array using the '...' operator.

var tempRowData = [...array];

this does exactly what you want to do, i.e. copy each element one by one. the 

...array

will hold out the elements of the array for you. And putting it inside a 

[ ];

will give you an array with the same elements but a different array binding.

BEWARE: Both the copying by loop and the ... method will still copy the elements by reference. Which means Mutating(adding deleting elements) the array might not have any effect on the original, but in case of object elements, changing any element's properties will affect the element in the original array.

Upvotes: 2

firegloves
firegloves

Reputation: 5709

This is probably because with this instruction:

var tempRowData = array;

you are making two variables that point to the same memory address, so updating one will update also the other one.

You need to create a new array and copy each value of the old array into the new one.

Generally primitive data are passed by value, non primitive data are passed by reference

Upvotes: 1

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