The JavaScript warning "JavaScript 1.6's for-each-in loops are deprecated; consider
using ES6 for-of instead" occurs when a for each
(variable in obj)
statement is used.
Message
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
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:
var object = { a: 10, b: 20 };
for (var key in object) {
var x = object[key];
console.log(x); // 10
// 20
}
Or, using for...of
(ES2015) and
Object.values
(ES2017), you can get an array of the specified object
values and iterate over the array like this:
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
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.
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
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
.
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.
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:
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 for...of
(ES2015) and
Object.entries
(ES2017), you can get an array of the specified object
values and iterate over the array like this:
var object = { a: 10, b: 20 };
for (var [key, value] of Object.entries(object)) {
console.log(key, value); // "a", 10
// "b", 20
}