Group Objects by Property in JavaScript

• Adds current object into that city array.
• Real World Use Cases
• Group employees by department
• Group students by class
• Group products by category
• Group jobs by city
• Group candidates by skills

const users = [
    { name: "Raj", city: "Bangalore" },
    { name: "Amit", city: "Delhi" },
    { name: "John", city: "Bangalore" }
];

const grouped = users.reduce((acc, curr) => {
	
	//console.log(curr);
	
    if (!acc[curr.city]) {
        acc[curr.city] = [];
    }

    acc[curr.city].push(curr);

    return acc;
}, {});

console.log(grouped);

/*
  output : 
	  {
		  Bangalore: [
			{ name: 'Raj', city: 'Bangalore' },
			{ name: 'John', city: 'Bangalore' }
		  ],
		  Delhi: [ { name: 'Amit', city: 'Delhi' } ]
	  }
*/

Understanding Step by Step
acc

acc means accumulator.

Initially:

{}

It stores the final grouped result.

curr

curr means current object.

First iteration:

{ name: "Raj", city: "Bangalore" }
PART 1
if (!acc[curr.city]) {
    acc[curr.city] = [];
}
What is curr.city ?

For first object:

curr.city

Value:

"Bangalore"

So this becomes:

acc["Bangalore"]
First Iteration

Initially:

acc = {}

Check:

if (!acc["Bangalore"])

Since it does not exist:

undefined

Condition becomes TRUE.

Then:

acc["Bangalore"] = [];

Now accumulator becomes:

{
   Bangalore: []
}
PART 2
acc[curr.city].push(curr);

This becomes:

acc["Bangalore"].push(curr)

Meaning:

[].push({ name: "Raj", city: "Bangalore" })

Now:

{
   Bangalore: [
      { name: "Raj", city: "Bangalore" }
   ]
}
Second Iteration

Current object:

{ name: "Amit", city: "Delhi" }

Check:

if (!acc["Delhi"])

Delhi does not exist.

Create:

acc["Delhi"] = [];

Then push:

acc["Delhi"].push(curr)

Now:

{
   Bangalore: [
      { name: "Raj", city: "Bangalore" }
   ],

   Delhi: [
      { name: "Amit", city: "Delhi" }
   ]
}
Third Iteration

Current:

{ name: "John", city: "Bangalore" }

Check:

if (!acc["Bangalore"])

Now Bangalore already exists:

[
   { name: "Raj", city: "Bangalore" }
]

Condition becomes FALSE.

So it skips array creation.

Then:

acc["Bangalore"].push(curr)

Final Output:

{
   Bangalore: [
      { name: "Raj", city: "Bangalore" },
      { name: "John", city: "Bangalore" }
   ],

   Delhi: [
      { name: "Amit", city: "Delhi" }
   ]
}
Simple Meaning
This line:
acc[curr.city] = [];

Creates a new array if city does not exist.

This line:
acc[curr.city].push(curr);