What are the possible side-effects in javascript?

A side effect is the modification of the state through the invocation of a function or expression. These side effects make our function impure by default. Below are some side effects which make function impure,

• Making an HTTP request. Asynchronous functions such as fetch and promise are impure.
• DOM manipulations
• Mutating the input data
• Printing to a screen or console: For example, console.log() and alert()
• Fetching the current time
• Math.random() calls: Modifies the internal state of Math object

Practical Code Example

// Common Side-effects in JavaScript:
// 1. Mutating an input parameter
function mutateUser(user) { user.updated = true; }

// 2. Making an API/Network call
// 3. Writing to DOM or console.log
// 4. Writing to localStorage or cookies