This is an error I've never seen before.

Here's a simple repro: /

const x = myVar => {

  console.log(myVar);

  for(let myVar of [1,2,3]){
    console.log(myVar);
  }

};

x(10);

Output in Chrome: 10, 1, 2, 3
Output in Safari: SyntaxError: Cannot declare a let variable twice: 'myVar'.

Ideas? Is this a bug in Safari?

Edit - Worth noting, I wouldn't ever do this on purpose. I noticed it because I use the ASP.NET bundling & minification system, and that system did this (so my site broke in Safari)

Edit 2 - Interestingly, this works in both browsers

let x = 10;

for(let x of [1,2,3])
{
    console.log(x);
}

/

This is an error I've never seen before.

Here's a simple repro: https://jsfiddle/jakelauer/qr0ysmst/3/

const x = myVar => {

  console.log(myVar);

  for(let myVar of [1,2,3]){
    console.log(myVar);
  }

};

x(10);

Output in Chrome: 10, 1, 2, 3
Output in Safari: SyntaxError: Cannot declare a let variable twice: 'myVar'.

Ideas? Is this a bug in Safari?

Edit - Worth noting, I wouldn't ever do this on purpose. I noticed it because I use the ASP.NET bundling & minification system, and that system did this (so my site broke in Safari)

Edit 2 - Interestingly, this works in both browsers

let x = 10;

for(let x of [1,2,3])
{
    console.log(x);
}

https://jsfiddle/jakelauer/aw37pd2s/1/

• javascript
• google-chrome
• safari
• ecmascript-6

• 1 I'm not sure whether this should be an error or not, but I think it's bad coding to shadow a parameter like that. You should call the myVar variable in your loop something else. – Freyja Commented Feb 25, 2017 at 0:30
• @Frxstrem I certainly agree - I wouldn't ever do it on purpose. I noticed it because I use the ASP.NET bundling & minification system, and that system did this (so my site broke in Safari). – Jake Commented Feb 25, 2017 at 0:39
• What Safari version? – Andrew Li Commented Feb 25, 2017 at 0:40
• @AndrewLi I'm using 10.0.3 – Jake Commented Feb 25, 2017 at 0:41
• 4 Definitely seems like a Safari bug to me. The let in the loop should be running in its own iteration-specific environment. – loganfsmyth Commented Feb 25, 2017 at 0:42

2 Answers 2

Looks like it's a Safari bug. I filed a bug report.

There is a mention of 'shadow' variables, but this is the case that tripped things up for me:

I am reusing key inside the loop and 'shadowing' the parameter for the function.

I was able to get both these errors: cannot declare a const variable twice or cannot declare a let variable twice by changing the declaration in the for loop.

So the fix for me was easy - to just change const key to const k for the loop.