CODEWITHSUNDEEP
javascriptecmascript-6destructuring
Steve Cockram
Steve Cockram

Reputation: 169

Destructuring assignment in while loop in ES6 function doesn't propogate out of loop?

I was implementing a simple GCD algorithm in ES6 (through node-esml) and came upon (to me) strange behaviour with updating values of variables inside a while loop. This code works fantastically:

function gcdWithTemp(x, y) {
  let [r, rdash] = [x, y]
  while (r != 0) {
    q = Math.floor(rdash / r)
    temp = r
    r = rdash - q * r
    rdash = temp
  }
  return(rdash)
}
console.log(gcdWithTemp(97, 34))

Returning the expected answer of 1. However, if I remove the temporary variable and instead use destructuring assignment to try and achieve the same results:

function gcdWithDestructuredAssignment(x, y) {
  let [r, rdash] = [x, y]
  while (r != 0) {
    q = Math.floor(rdash / r)
    [r, rdash] = [rdash - q * r, r]
  }
  return(rdash)
}
console.log(gcdWithDestructuredAssignment(97, 34))

It never completes, further debugging shows that r will always have the first value assigned to, x. It seems that these two implementations should be identical? see Swapping variables

I've also tried with using var instead of let to no avail. Am I drastically misunderstanding the point of destructuring assignment or missing something subtle? Or is it a bug?

Upvotes: 6

Views: 454

Answers (1)

Michał Perłakowski
Michał Perłakowski

Reputation: 92471

That's a problem not with destructuring assignment, but with ASI (automatic semicolon insertion). These two lines:

q = Math.floor(rdash / r)
[r, rdash] = [rdash - q * r, r]

in practice mean this:

q = Math.floor(rdash / r)[r, rdash] = [rdash - q * r, r]

which obviously is not what you meant. To fix that, add a semicolon in front of [:

function gcdWithDestructuredAssignment(x, y) {
  let [r, rdash] = [x, y]
  while (r != 0) {
    q = Math.floor(rdash / r)
    ;[r, rdash] = [rdash - q * r, r]
  }
  return(rdash)
}
console.log(gcdWithDestructuredAssignment(97, 34))

Of course you can add the missing semicolon at the end of the previous line instead (q = Math.floor(rdash / r);), but since you generally don't use semicolons, I assumed that you're using npm coding style.

Upvotes: 8

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