Pushing new values to an array is writing over the values of previous elements

Question

I am trying to push multiple data sets(arrays) to my array called colors. As I continue to push new data it seems to overwrite the values in previous elements of the colors array. I have included a simple example showing that push#3 overwrites the values in the 2nd element of the colors array. I do not understand why that is happening.

I am expecting the colors array to contain 3 elements [[R, Red],[B, Blue],[G, Green]] but what I get is [[G, Green],[G, Green],[G,Green]]

function myFunction() {
  var colors = []
  var pair = []

  //===============================
  pair[0]= 'R'
  pair[1]= 'Red'
  // push 1
  colors.push(pair)

  Logger.log('pair=')
  Logger.log(pair)
  Logger.log('colors= ')
  Logger.log(colors)

  //================================
  pair[0] = 'B'
  pair[1] = 'Blue'
  // push #2
  colors.push(pair)

  Logger.log('\n\n')
  Logger.log('pair=')
  Logger.log(pair)
  Logger.log('colors= ')
  Logger.log(colors)

  //================================
  pair[0] = 'G'
  pair[1] = 'Green'
  // push #3
  
  colors.push(pair)
  Logger.log('\n\n')
  Logger.log('pair=')
  Logger.log(pair)
  Logger.log('colors= ')
  Logger.log(colors)
}

Which programming language are you using?

Progman
– Progman

2025-05-23 21:49:06 +00:00
Commented May 23 at 21:49 — Progman
– Progman, Commented May 23 at 21:49

Carcigenicate · Accepted Answer · 2025-05-23 21:47:56Z

0

You keep pushing the same pair over and over. Javascript won't make a copy of pair just because you added it to an array. So, if you do pair[0] = 'G', that affects all the references to pair that are already in the array, because they're all referencing the same object.

Just create a new object each time:

colors.push(['R', 'Red']);
colors.push(['B', 'Blue']);
colors.push(['G', 'Green']);

Note how I'm using the literal syntax ([]) each time, which in this context creates a new array each time. You could have also done something like:

let pair = ['R', 'Red']
colors.push(pair);

pair = ['B', 'Blue']
colors.push(pair);

pair = ['G', 'Green']
colors.push(pair);

In this case, the variable is reused, but is reassigned a new object for each color channel.

answered May 23 at 21:47

Carcigenicate

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1 Comment

glenn May 23 at 22:25

Thank you @Carcigenicate ! I get it now, I was thinking the array would keep a snapshot of the values of pair. Makes sense that it is a reference to the pair object.

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