Add key value to an array elements in JS

Question

I can't add a key value pair to my array objects:

 const arr = [{'a' :1, 'b':2},{'a':2, 'b':4}]
 arr.map( item => {item.price = 1
 document.getElementById("body").innerHTML += 'a : '+ item.price + ' ' });

I want arr to be :

{'a' :1, 'b':2, 'price' : 1},{'a':2, 'b':4, 'price' : 1}

Why are you using document if you only want to edit the array? — evolutionxbox
– evolutionxbox, Commented May 5, 2021 at 20:02
I was trying to display the result, It's not the problem, consider it a console.log.. — Marwen Hizaoui
– Marwen Hizaoui, Commented May 5, 2021 at 20:03
map needs to return a new item. item.price = 1 will amend the incoming item, and then return 1, making the array [1,1] — evolutionxbox
– evolutionxbox, Commented May 5, 2021 at 20:04

jsh · Accepted Answer · 2021-05-05 20:07:56Z

4

The map function doesn't modify the array you do it to, it returns a new modified array. So you must assign the output to a variable. I would suggest reading a bit more how mapping arrays works on the MDN Docs.

Here's how I would implement what you're looking for:

const arr = [{'a' :1, 'b':2},{'a':2, 'b':4}];
const newarr = arr.map( item => ({ ...item, price: 1 }) )

answered May 5, 2021 at 20:07

jsh

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