Warning: expression closures are deprecated

The JavaScript warning "expression closures are deprecated" occurs when the non-standard expression closure syntax (shorthand function syntax) is used.

Message

Warning: expression closures are deprecated

Error type

Warning. JavaScript execution won't be halted.

What went wrong?

The non-standard expression closure syntax (shorthand function syntax) is deprecated and shouldn't be used anymore. This syntax will be removed entirely in bug 1083458 and scripts using it will throw a SyntaxError then.

Examples

Deprecated syntax

Expression closures omit curly braces or return statements from function declarations or from method definitions in objects.

var x = function() 1;

var obj = {
  count: function() 1
};

Standard syntax

To convert the non-standard expression closures syntax to standard ECMAScript syntax, you can add curly braces and return statements.

var x = function() { return 1; }

var obj = {
  count: function() { return 1; }
};

Standard syntax using arrow functions

Alternatively, you can use arrow functions:

var x = () => 1;

Standard syntax using shorthand method syntax

Expression closures can also be found with getter and setter, like this:

var obj = {
  get x() 1,
  set x(v) this.v = v
};

With ES2015 method definitions, this can be converted to:

var obj = {
  get x() { return 1 },
  set x(v) { this.v = v }
};