Run results for: Build vs copy object

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:123.0) Gecko/20100101 Firefox/123.0

Browser: Firefox 123

Operating system: Mac OS X 10.15

Device Platform: Desktop

Date tested: 2 years ago

Test name	Executions per second
Build up object	10228327.0 Ops/sec
Use fromEntries	3587368.0 Ops/sec
Copy and fill	5112843.0 Ops/sec
Assign and fill	8851860.0 Ops/sec

Script Preparation code:

keys = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
values = [1, 2, 3, 4, 5, 6, 7, 9, 10];
templateObj = Object.fromEntries(keys.map(k => [k, undefined]));
pairs = keys.map((k, i) => [k, values[i]]);

Tests:

Build up object
const obj = {}; for (const [k, v] of pairs) { obj[k] = v; }
Use fromEntries
Object.fromEntries(pairs)
Copy and fill
const obj = {...templateObj}; for (const [k, v] of pairs) { obj[k] = v; }
Assign and fill
const obj = {}; Object.assign(obj, templateObj); for (const [k, v] of pairs) { obj[k] = v; }

Run results for: Build vs copy object

Is it faster to add properties to an object one by one, or create an object with all of them and set values one by one?

Build up object

Use fromEntries

Copy and fill

Assign and fill