Why do you not include parentheses when passing a function as an argument?

Question

When passing a function declaration as an argument into another function why are the parentheses unnecessary? Do you not need to call the function to pass it as an argument? From my understanding of functions (which is limited), a function call without a set of parenthesis will cause a syntax error.

Here is the working code:

let html = '';

function random(){
  return Math.floor(Math.random() * 256);
}

function randomRGB(value){
  const color = `rgb(${value()}, ${value()}, ${value()})`;
  return color;
}

for(let i = 1; i <= 10; i++){
  html += `<div style="background-color: ${randomRGB(**random**)}">${i}</div>`;
}

document.querySelector('main').innerHTML = html;

So why does the previous code work but not this?:

html += `<div style="background-color: ${randomRGB(**random()**)}">${i}</div>`;
}

Answer 1 · 2022-01-12T14:20:00Z

January 12, 2022 2:20pm

The parentheses are only used if you want to call the function first, and then pass the return value from it as an argument. Parentheses are not used when you pass a reference to the function itself as the argument.

Here, the random function is being passed to randomRGB, which then calls it three separate times while it runs (as value()) to get different values for each color.

Welcome to the Treehouse Community

Looking to learn something new?

Jeremy Yochum

Jeremy Yochum

Why do you not include parentheses when passing a function as an argument?

1 Answer

Steven Parker

Steven Parker

Jeremy Yochum

Jeremy Yochum