Hoisting

In JavaScript, you've two options to declare a function:

javascript
// 1. Function declaration
function sum(a, b){

    return a + b

}

// 2. Function expression
const sum = (a, b) => {
    
    return  a + b

}

JavaScript hoists function declarations (raised to the top), which means, that you can use them before the code executes.

For example:

javascript
// ✅  res = 7
let res = sum(5, 2)

function sum(a, b){

    return a + b

}

But if you try the following:

javascript
// ❌ Uncaught ReferenceError: sum is not defined
let res = sum(5, 2)

const sum = (a, b) => {

    return a + b

}

In this case, you need to declare the function before calling it.