Javascript – how to increment values on unique keys

I’m trying to make an array with unique keys and assigned incremented values, in order to end up with a weighted list.

In PHP, I do it like this…

$crit1 = ['a','b','c'];
$crit2 = ['c','d','e'];
$weight = [];
foreach ( $crit1 as $item ) {$weight[$item] += 1;}
foreach ( $crit2 as $item ) {$weight[$item] += 2;}   
// print_r($weight); => array([a] => 1 [b] => 1 [c] => 3 [d] => 2 [e] => 2)

…with ‘c’ ending up with the most points: 1 for showing up in the first foreach and another 2 for being featured in the second.

Trying to translate this to JS…

const crit1 = ['a','b','c'];
const crit2 = ['c','d','e'];
let weight = [];   
crit1.forEach(function (item) { weight.push({ [item]: +1 }) });
crit2.forEach(function (item) { weight.push({ [item]: +2 }) });
// console.log(weight) => [{ a: 1}, { b: 1}, { c: 1}, { c: 2}, { d: 2}, { e: 2}]

…no longer works – the values for ‘c’ are assigned separately so the array comes up with 6 items instead of just 5.

What am I doing wrong?

>Solution :

1. Instead of an array, use an object so the keys value can be bumped

2. Since we’re using an object, we can’t use push, but we’ll need to address the key

3. weight[item] = (weight[item] || 0) + 1)
   

   Increments the current item key in the weight object by 1, if it’s not defined yet, we set it to 0.

const crit1 = ['a','b','c'];
const crit2 = ['c','d','e'];
let weight = {};   

crit1.forEach((item) => weight[item] = (weight[item] || 0) + 1);
crit2.forEach((item) => weight[item] = (weight[item] || 0) + 2);

let arr = Object.keys(weight).reduce((prev, cur) => [ ...prev, { [cur]: weight[cur] } ], []);

console.log(weight);
console.log(arr);

{
  "a": 1,
  "b": 1,
  "c": 3,
  "d": 2,
  "e": 2
}

If you really want an array containing objects with just a single key, as shown in your comment, we can use reduce() to do the conversion:

let arr = Object.keys(weight).reduce((prev, cur) => [ ...prev, { [cur]: weight[cur] } ], []);

Will produce:

[
  {
    "a": 1
  },
  {
    "b": 1
  },
  {
    "c": 3
  },
  {
    "d": 2
  },
  {
    "e": 2
  }
]