`x = y, z` comma assignment in JavaScript [duplicate]

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Closed 8 years ago.

Possible Duplicate:
Javascript syntax: what comma means?

I came across the code while reading this article (do a Ctrl+F search for Andre Breton):

//function returning array of `umbrella` fibonacci numbers
function Colette(umbrella) {
  var staircase = 0, galleons = 0, brigantines = 1, armada = [galleons, brigantines], bassoon;
  Array.prototype.embrace = [].push;

  while(2 + staircase++ < umbrella) {
    bassoon = galleons + brigantines;
    armada.embrace(brigantines = (galleons = brigantines, bassoon));
  }

  return armada;
}

What does the x = (y = x, z) construct mean? Or more specifically, what does the y = x, z mean? I'm calling it comma assignment because it looks like assignment and has a comma.

In Python, it meant tuple unpacking (or packing in this case). Is it the same case here?

Answer 1

This is the comma operator.

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

The resultant value when a,b,c,...,n is evaluated will always be the value of the rightmost expression, however all expressions in the chain are still evaluated (from left to right).

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So in your case, the assignations would still be evaluated, but the final value would be bassoon.

Result:

galleons = brigantines
brigantines = bassoon
armada.embrace(basson)

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More information: Javascript "tuple" notation: what is its point?

Answer 2

var syntax allows multiple assignment, so when you see the following, you're declaring multiple variables using one var statement.

var a, b, c;

Note that this syntax is not the comma operator.

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The , can be used as the comma operator. It simply evaluates a series of expressions. So when you see the following syntax, you are seeing a series of expressions being evaluated, and the return value of the last one being returned.

x = (y = x, z)

Within the parens, x is assigned to y, then z is evaluated and returned from the () and assigned to x.

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I'd suggest that this syntax is unclear and offers little benefit.

Answer 3

The comma operand evaluates all of its operands and returns the last one. It makes no difference in this case if we had used

x = (y = x, z);

or

y = x;

x = z;

It's there to take away that line of code.