reactjs the syntaxError: url is readonly

Question

i have this issue in react i have this function but its not right format of react

check(img) {
        console.log(img,typeof img)
        const url="";
        const arrN = ["15","16","35","36","37","38","39","40","n15","n16","n35","n36","n37","n38","n39","n40"];
        for (var i = 0; i < arrN.length; i++) {
            if (img === arrN[i]) {
                 url = "/blah/allIcons/blah"+img+"_en.png";
            }else{
                 url = "/blah/allIcons/blah"+img+".png";
            }
        }
        return url;
    }

it give me this errror

Module build failed: SyntaxError: "url" is read-only

how i can do it ?

Maybe use have defined `url` above. just use `url = ""` instead of `const url = ""`; — Adnan Umer, Feb 09 '17 at 12:32
@AdnanUmer Very bad advice. I guess you don't know why declaring variable is important. — dfsq, Feb 09 '17 at 12:37
const is something that u can't change, use var or let, read the diff between const, var, let. http://stackoverflow.com/questions/762011/whats-the-difference-between-using-let-and-var-to-declare-a-variable — Mayank Shukla, Feb 09 '17 at 12:52

dfsq · Accepted Answer · 2017-02-09T13:18:31.560

If you change url variable then it's should not be declared as a constant. Use let:

check(img) {
  const arrN = ["15", "16", "35", "36", "37", "38", "39", "40", "n15", "n16", "n35", "n36", "n37", "n38", "n39", "n40"];
  let url = "";

  for (var i = 0; i < arrN.length; i++) {
    if (img === arrN[i]) {
      url = "/blah/allIcons/blah" + img + "_en.png";
    } else {
      url = "/blah/allIcons/blah" + img + ".png";
    }
  }
  return url;
}

But you don't seem to need it anyway, as entire for-loop check seems inefficient. Can be optimized like this:

check(img) {
  const arrN = ["15", "16", "35", "36", "37", "38", "39", "40", "n15", "n16", "n35", "n36", "n37", "n38", "n39", "n40"];

  if (arrN.indexOf(img) > -1) { // or if (arrN.includes(img)) {...}
    return "/blah/allIcons/blah" + img + "_en.png";
  }

  return "/blah/allIcons/blah" + img + ".png";
}

it should **not** be declared as a constant :) – Mayank Shukla Feb 09 '17 at 12:55 — Mayank Shukla, Feb 09 '17 at 12:55

IVN · Answer 2 · 2017-02-10T16:34:43.993

0

Use let url="" instead of const url="".

The value of a constant cannot change through re-assignment, and it can't be redeclared.

So if you declare variable const url="", you can't later say url="/blah/allIcons/blah" + img + "_en.png"

edited Feb 10 '17 at 16:34

answered Feb 09 '17 at 12:35

IVN

1,307
15
19

Could you provide more info and details with your answer? Thank you – Robert Feb 09 '17 at 19:15
@Robert, I updated my answer with some more info and references, however I will not go into details explaining `let` and `const` here, because question is not about it. – IVN Feb 10 '17 at 16:36

reactjs the syntaxError: url is readonly

2 Answers2