JavaScript Parameters, Arguments, Default & Rest Parameters

When I started learning JavaScript functions, I kept mixing up parameters and arguments. Then I came across default and rest parameters.

Here’s how I understand them now.

1. Parameters

Parameters are the variables we define when creating a function.

function greet(name) {
    console.log("Hello " + name);
}

Here, name is a parameter.

2. Arguments

Arguments are the actual values we pass when calling the function.

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greet("John");

Here, "John" is the argument.

Parameter → placeholder
Argument → actual value

3. Default Parameters

We can give a parameter a default value.

function greet(name = "John") {
    console.log("Hello " + name);
}

greet();

Output:

Hello John

If we provide a value, the default value isn't used:

greet("David");

Output:

Hello David

4. Rest Parameters

What if we don't know how many arguments will be passed?

We can use ... to collect them.

function add(...numbers) {
    return numbers.reduce((sum, num) => sum + num, 0);
}

console.log(add(10, 20, 30));

Output:

60

The ...numbers collects the arguments into an array.

Quick way to remember

Parameter → variable in the function
Argument → value passed to the function
Default → fallback value
Rest → collects multiple arguments

Understanding these four concepts made JavaScript functions much clearer for me.

At first, parameters, arguments, default parameters, and rest parameters looked like separate confusing topics. But once I understood how they work together, functions started making much more sense.

Still learning, still practicing, and slowly building my JavaScript fundamentals one concept at a time.

What JavaScript concept confused you when you first started?