ES6: Default parameters explained

Introduction

ES2015(ES6) introduced default parameters. Let's jump right in and learn about default parameters.

Default parameters

What would happen if we called a function with all or some parameters missing? It turns out JavaScript assigns undefined to the missing arguments.

Let's see example in code:

const add = (num1, num2) => num1 + num2

const sum = add(2) // one argument is missing // gets called as 2 + undefined
console.log(sum) // prints NaN

Default parameters allow us to define a default parameter value and will be used when no argument is provided for the paramter during funcion call:

const main = (port = 3000) => {
  // possible code ommitted here
}
main() //port will default to value of 3000
main(5000) // call main with 5000

Another dummy example:

// add default parameters at the end of parameter list
const restoreWallet = (privateKey, dumpToJson=true) => {
  // posible code omitted
}

const myWallet = restoreWallet('0xFEEDBEEFFEEDBEEF', false) // dumpToJson supplied as false
const myWalletTwo = restoreWallet('0x05417') // dumpToJson defaults to true if not supplied


const fetchItems = async (storeName, keys = []) => {
  // possible code omitted here

}
const itemStore = await fetchItems('Electronics') // keys defaults to an empty array object

Summary

Default parameters allow us to provide a default value for the argument when not supplied during the function call.

Default parameters are added at the end of the paremeter list.

Comments

[Jon Randy 🎖️]

const myWalletTwo = restoreWallet('0x05417') // json defaults to true if not supplied

This is not correct.

In this case, dumpToJson will be '0x05417' and privateKey will be undefined

[Jatin Sharma]

Exactly we pass default parameters in the end of all the other parameters.

[Naftali Murgor]

Thanks for the feedback. Will update to fix the error