Argument vs. Parameter, what is the difference?

People (including me) often use them interchangeablly.

However, there is a difference between these two.

A parameter is a piece of data that a function needs to run. It is often declared while declaring a function.

For example,

const alwaysSunny=(day)=>{
    return `${day}'s weather is sunny!`
}

Here day is the parameter of the function alwaysSunny.

An arugument, on the other hand, is the piece of data you pass to a function when you call it. So it is the actual data.

alwaysSunny('Wednesday')

Here the string Wednesday is the argument.

In JavaScript and TypeScript, you could mark a parameter as Optional by using ?.

Let's modify our alwaysSunny function:

const alwaysSunny=(day:string,location?:string)=>{
    
    return `${day}'s weather in ${location||'Barcelona'} is sunny!`
}

location is optional parameter, if not provided, the function will fall back to the default value 'Barcelona'.

caution

In TypeScript, optional parameter needs to be at the end of your parameter list.

One cool and very useful feature is the rest parameter.

Imagine you don't know how many arguments are going to be passed to a function, you could potentially save all arguments to an array and then pass the array. But there's a better way to do it.

const sum(...numbers)=>{
    return numbers.reduce((total,n)=>total+n,0)
}

sum() // zero parameter returns 0

sum(1) // returns 1

sum(1,2,3) //returns 6

tip

To make it type safe, you could simplly annotate the parameter type ...numbers:numbers[]