Spread syntax modifies array

[mohitmayank]

• Version: v10.15.1
• Platform: Linux 62865c8f9a62 4.15.0-43-generic Tracking / Assuring Compatibility #46~16.04.1-Ubuntu SMP Fri Dec 7 13:31:08 UTC 2018 x86_64 GNU/Linux
• Subsystem:

const assert = require('assert'); 
 
const array = [ 
  {   
    id : 5 
  } ,
  {  
    id : 7 
  }   
]; 
 
const last = array.pop(); 
const obj = Object.assign(...array, last); 
// const obj = Object.assign(last, ...array); 
 
assert(array[0].id === 5);  //AssertionError

[mscdex]

This is expected due to the following:

• Object.assign() takes as its first argument the target object to assign properties to. All of the other arguments are source values/objects.
• The spread syntax is equivalent to passing each value of the array to the function as separate arguments
• Objects in javascript are passed/assigned by reference

With all of this in mind, the Object.assign() is equivalent to:

const obj = Object.assign(array[0], last);

So the properties of last are copied to array[0]. Since both have the same, single property, that means the value of 5 in array[0] is overwritten with the value of 7 from last.

[mohitmayank]

Er. Now when i look at your comment. it seems obvious. Thanks.