They told me to use const/let instead of JavaScript var.

The let scope is valid among subblocks, so
Declare outside the if subblock.

 function hoge(hoge1,hoge2){
    let hoge3 = "aaa";
    if(hoge1==="abc"){
        // hoge3 declared in line 2
        hoge3 = "def";
    } else if(hoge1==="def"){
        // hoge3 declared in line 2
        hoge3 = "ghi";
    } else{
        // Declaring a variable of the same name in a subblock is a different variable
        // Does not affect hoge3 in line 2
        let hoge3 = "jkl";
    }

    return hoge1+hoge2+hoge3;
}

hoge("abc", "xyz"; // = "abcxyzdef"
hoge("def", "xyz"; // = "defxyzghi"
hoge("ghi", "xyz"; // = "ghixyzaaa"

"If the purpose of the if...else part is to ""calculate one value according to the conditions,"" I think it is better to make it into a function."

 function hoge(hoge1,hoge2){

    consthoge3=(function calcHoge3(hoge1){
        if(hoge1==="abc"){
            return "def";
        } else if(hoge1==="def"){
            return "ghi";
        }
    }) (hoge1);

    return hoge1+hoge2+hoge3;

}

• Good points

  • Small scope

    • It's convenient to rely solely on the argument because it works even if you copy it
    • When logic grows, functions can be separated quickly
    • If there are many variables that depend on it, you can't do this

  • Calculating the value is the only purpose

    • If you want to change the outside scope variables (hoge1,hoge2), you can't do this

  • I don't use var that tends to behave strangely, so I'm relieved to know that strange behavior doesn't occur

• What's wrong

  • Depth indentation
  • It's messy and hard to read

Good point

• Small scope

  • It's convenient to rely solely on the argument because it works even if you copy it
  • When logic grows, functions can be separated quickly
  • If there are many variables that depend on it, you can't do this

• Calculating the value is the only purpose

  • If you want to change the outside scope variables (hoge1,hoge2), you can't do this

• I don't use var that tends to behave strangely, so I'm relieved to know that strange behavior doesn't occur

Small scope

• It's convenient to rely solely on the argument because it works even if you copy it
• When logic grows, functions can be separated quickly
• If there are many variables that depend on it, you can't do this

You can make it clear that the only purpose is to calculate the value

• If you want to change the outside scope variables (hoge1,hoge2), you can't do this

I don't use var, which tends to behave strangely, so I'm relieved to know that it doesn't happen

Bad points

• Depth indentation
• It's messy and hard to read

If you have an if expression like Ruby and Scala, it's easy to write.

How about this one?

export const hoge=(hoge1,hoge2)=>{
  let hoge3 = ' '
  if(hoge1==='aiueo')hoge3='kakikukeko'
  else if(hoge1==='oeuia')hoge3='aiueo'
  return hoge1+hoge2+hoge3
}

// or
export default {
  hoge(hoge1,hoge2){
    // same process...
  }
}