object destructuring without var


Why does object destructuring throw an error if there is no var keyword in front of it?

{a, b} = {a: 1, b: 2};

throws SyntaxError: expected expression, got '='

The following three examples work without problems

var {a, b} = {a: 1, b: 2};
var [c, d] = [1, 2];
    [e, f] = [1, 2];

Bonus question: Why do we not need a var for array destructuring?

I ran into the problem doing something like

function () {
  var {a, b} = objectReturningFunction();

  // Now a and b are local variables in the function, right?
  // So why can't I assign values to them?

  {a, b} = objectReturningFunction();
}

The issue stems from the {...} operators having multiple meanings in JavaScript.

When { appears at the start of a Statement, it'll always represent a block, which can't be assigned to. If it appears later in the Statement as an Expression, then it'll represent an Object.

The var helps make this distinction, since it can't be followed by a Statement, as will grouping parenthesis:

( {a, b} = objectReturningFunction() );

From their docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Assignment_without_declaration

Notes: The parentheses ( ... ) around the assignment statement are required when using object literal destructuring assignment without a declaration.

{a, b} = {a: 1, b: 2} is not valid stand-alone syntax, as the {a, b} on the left-hand side is considered a block and not an object literal.

However, ({a, b} = {a: 1, b: 2}) is valid, as is var {a, b} = {a: 1, b: 2}

Your ( ... ) expression needs to be preceded by a semicolon or it may be used to execute a function on the previous line.


If you write JavaScript without semicolons, then the 'assignment without declaration' syntax should be preceded with a semicolon for it to work predictably

let a, b

;({a, b} = objectReturningFunction()) // <-- note the preceding ;

Just wanted to highlight this as it caught me out, and hopefully can save others some time figuring out why it doesn't work and/or produces weird results with code formatters etc.

Indeed, it's actually right there in the accepted answer (last line of the quoted docs) but easy to miss, especially without seeing an example!


Here's another way:

let {} = {a, b} = objectReturningFunction()

Pros:

  • No parenthesis needed
  • No semicolons needed

Cons:

  • Looks a bit weird, although in my opinion no weirder than the !(){...}() IIFE
  • Might be confusing as to why it's there