for...in

variable

Receives a string property name on each iteration. May be either a declaration with const, let, or var, or an assignment target (e.g. a previously declared variable or an object property).

object

Object whose non-symbol enumerable properties are iterated over.

statement

A statement to be executed on every iteration. May reference variable. You can use a block statement to execute multiple statements.

The loop will iterate over all enumerable properties of the object itself and those the object inherits from its prototype chain (properties of nearer prototypes take precedence over those of prototypes further away from the object in its prototype chain).

A for...in loop only iterates over enumerable, non-symbol properties. Objects created from built–in constructors like Array and Object have inherited non–enumerable properties from Array.prototype and Object.prototype, such as Array's indexOf() method or Object's toString() method, which will not be visited in the for...in loop.

The traversal order, as of modern ECMAScript specification, is well-defined and consistent across implementations. Within each component of the prototype chain, all non-negative integer keys (those that can be array indices) will be traversed first in ascending order by value, then other string keys in ascending chronological order of property creation.

The variable part of for...in accepts anything that can come before the = operator. You can use const to declare the variable as long as it's not reassigned within the loop body (it can change between iterations, because those are two separate variables). Otherwise, you can use let. You can use destructuring or an object property like for (x.y in iterable) as well.

A legacy syntax allows var declarations of the loop variable to have an initializer. This throws a syntax error in strict mode and is ignored in non–strict mode.

If a property is modified in one iteration and then visited at a later time, its value in the loop is its value at that later time. A property that is deleted before it has been visited will not be visited later. Properties added to the object over which iteration is occurring may either be visited or omitted from iteration.

In general, it is best not to add, modify, or remove properties from the object during iteration, other than the property currently being visited. There is no guarantee whether an added property will be visited, whether a modified property (other than the current one) will be visited before or after it is modified, or whether a deleted property will be visited before it is deleted.

Array indexes are just enumerable properties with integer names and are otherwise identical to general object properties. The for...in loop will traverse all integer keys before traversing other keys, and in strictly increasing order, making the behavior of for...in close to normal array iteration. However, the for...in loop will return all enumerable properties, including those with non–integer names and those that are inherited. Unlike for...of, for...in uses property enumeration instead of the array's iterator. In sparse arrays, for...of will visit the empty slots, but for...in will not.

It is better to use a for loop with a numeric index, Array.prototype.forEach(), or the for...of loop, because they will return the index as a number instead of a string, and also avoid non-index properties.

If you only want to consider properties attached to the object itself, and not its prototypes, you can use one of the following techniques:

• Object.keys(myObject)
• Object.getOwnPropertyNames(myObject)

Object.keys will return a list of enumerable own string properties, while Object.getOwnPropertyNames will also contain non-enumerable ones.

Many JavaScript style guides and linters recommend against the use of for...in, because it iterates over the entire prototype chain which is rarely what one wants, and may be a confusion with the more widely-used for...of loop. for...in is most practically used for debugging purposes, being an easy way to check the properties of an object (by outputting to the console or otherwise). In situations where objects are used as ad hoc key-value pairs, for...in allows you check if any of those keys hold a particular value.

The for...in loop below iterates over all of the object's enumerable, non-symbol properties and logs a string of the property names and their values.

const obj = { a: 1, b: 2, c: 3 };

for (const prop in obj) {
  console.log(`obj.${prop} = ${obj[prop]}`);
}

// Logs:
// "obj.a = 1"
// "obj.b = 2"
// "obj.c = 3"

The following function illustrates the use of Object.hasOwn(): the inherited properties are not displayed.

const triangle = { a: 1, b: 2, c: 3 };

function ColoredTriangle() {
  this.color = "red";
}

ColoredTriangle.prototype = triangle;

const obj = new ColoredTriangle();

for (const prop in obj) {
  if (Object.hasOwn(obj, prop)) {
    console.log(`obj.${prop} = ${obj[prop]}`);
  }
}

// Logs:
// "obj.color = red"

• for...of
• for
• Enumerability and ownership of properties
• Object.getOwnPropertyNames()
• Object.hasOwn()
• Array.prototype.forEach()