wiki/content/20201126095525-idempotent.md

---
id: 48609306-f2e9-4d01-b774-7d3ec34703aa
title: idempotent
---

# Origin

Idem is latin for 'same'.

# Definition

An operation or function, that when called multiple times will always
produce the same result.

# Examples

## Idempotent

``` javascript
console.log(1 * 1 * 1 * 1)
```

``` javascript
const set = new Set()

set.add('blaat')
set.add('blaat')
set.add('blaat')

console.log(set) // { 'blaat' }
```

### Practial examples

-   Get, Put & Delete endpoints on a REST api
-   Payment operations. Reposting the same form should not result in
    multiple charges to the customer.

## Non idempotent

``` javascript
console.log(2 * 2 * 2 * 2)
```

``` javascript
const arr = []

arr.push('blaat')
arr.push('blaat')
arr.push('blaat')

console.log(arr) // [ 'blaat', 'blaat', 'blaat' ]
```