Copy by value does not work in JS

```
var a = newSeq[proc():int]()
for i in [1,2,3,4]:
  var j = i
  var fun = proc():int = j
  a.add(fun)

for fun in a: echo(fun())
```

Produces in C:

```
1
2
3
4
```

Produces in JS:

```
4
4
4
4
```
