A code chunk's lines of code must be in the same order you want them to be when the code is converted for the computer.
```
<<a>>=
console.log(foo)
@

<<a>>+
var foo = "bar"
@

Foo needs to be set as bar because of blah blah blah...
```

This is because the above chunk will be tangled into:

console.log(foo)
var foo = "bar"

which is not legal JavaScript.