Warning: JavaScript 1.6's for-each-in loops are deprecated

Message

JavaScript
Warning: JavaScript 1.6's for-each-in loops are deprecated; consider using ES6 for-of instead

Error Type

Warning

What went wrong?

JavaScript 1.6's for each (variable in obj) statement is deprecated, and will be removed in the near future.

Examples

Object iteration

for each...in has been used to iterate over the specified object values.

Deprecated syntax

JavaScript
var object = { a: 10, b: 20 };

for each (var x in object) {
  console.log(x);        // 10
                         // 20
}

Alternative standard syntax

You can now use the standard for...in loop to iterate over specified object keys, and get each value inside the loop:

JavaScript
var object = { a: 10, b: 20 };

for (var key in object) {
  var x = object[key];
  console.log(x);        // 10
                         // 20
}

Or, using {jsxref("Statements/for...of", "for...of")}} (ES2015) and Object.values (ES2017), you can get an array of the specified object values and iterate over the array like this:

JavaScript
var object = { a: 10, b: 20 };

for (var x of Object.values(object)) {
  console.log(x);        // 10
                         // 20
}

Array iteration

for each...in has been used to iterate over specified array elements.

Deprecated syntax

JavaScript
var array = [10, 20, 30];

for each (var x in array) {
  console.log(x);        // 10
                         // 20
                         // 30
}

Alternative standard syntax

This is now possible with for...of (ES2015) loops as well.

JavaScript
var array = [10, 20, 30];

for (var x of array) {
  console.log(x);        // 10
                         // 20
                         // 30
}

Iterating over a null-able array

for each...in does nothing if the specified value is null or undefined, but for...of will throw an exception in these cases.

Deprecated syntax

JavaScript
function func(array) {
  for each (var x in array) {
    console.log(x);
  }
}
func([10, 20]);        // 10
                       // 20
func(null);            // prints nothing
func(undefined);       // prints nothing

Alternative standard syntax

To rewrite for each...in statements so that values can be null or undefined with for...of as well, you need to guard around for...of.

JavaScript
function func(array) {
  if (array) {
    for (var x of array) {
      console.log(x);
    }
  }
}
func([10, 20]);        // 10
                       // 20
func(null);            // prints nothing
func(undefined);       // prints nothing

Iterating over an object's key-value pair

Deprecated syntax

There's a deprecated idiom to iterate over the specified object's key-value pairs using for each...in and the deprecated Iterator object.

JavaScript
var object = { a: 10, b: 20 };

for each (var [key, value] in Iterator(object)) {
  console.log(key, value);  // "a", 10
                            // "b", 20
}

Alternative standard syntax

You can now use the standard for...in loop to iterate over specified object keys, and get each value inside the loop:

JavaScript
var object = { a: 10, b: 20 };

for (var key in object) {
  var value = object[key];
  console.log(key, value);  // "a", 10
                            // "b", 20
}

Or, using {jsxref("Statements/for...of", "for...of")}} (ES2015) and Object.values (ES2017), you can get an array of the specified object values and iterate over the array like this:

JavaScript
var object = { a: 10, b: 20 };

for (var [key, value] of Object.entries(object)) {
  console.log(key, value);  // "a", 10
                            // "b", 20
}

See also

License

© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/javascript/reference/errors/for-each-in_loops_are_deprecated

JavaScript Warning