Iterate over properties
Need to process both names and values in an object.
JavaScript · Function
Object.entries
Object.entries() returns an array of [key, value] pairs for an object's own enumerable string-keyed properties. It excludes inherited properties and preserves the standard property-enumeration order.
- Since
- ES2017
How it works
Call Object.entries with an object-like value to receive an array of two-item arrays. Each inner array contains an own enumerable string property key followed by its value; inherited properties are excluded, and the entries follow the object's property enumeration order.
Syntax
Object.entries(obj)Parameters
| Name | Type | Description |
|---|---|---|
obj | object | The value converted to an object and inspected for own enumerable string-keyed properties. |
Returns
An array of the object's own enumerable string-keyed property key-value pairs; each pair is a two-element array containing a string key and its value.
Key points
- It is a static Object method that collects an object's own enumerable string-keyed properties into an array.
- Every result element is a two-item array whose first item is the property key.
- Keys in the returned pairs are always strings, while the second item preserves the property value.
- Its direct-property traversal resembles for...in, but it omits enumerable properties inherited through prototypes.
- The output array uses the same property enumeration order as for...in.
- The resulting pairs can be passed directly to Map to construct a map.
Common mistakes
- Passing undefined or null throws a TypeError instead of producing an empty result.
- Primitive arguments are coerced: strings can produce character entries, whereas other primitives produce no entries.
Example: Iterate over an object's entries
const user = { name: "Ada", age: 36 };
for (const [key, value] of Object.entries(user)) {
console.log(`${key}: ${value}`);
}Iterates through each own enumerable string-keyed property as a two-item array.
When to use it
Create a Map from an object
Need an iterable sequence of pairs for a collection constructor.
Transform entries
Need to transform or filter properties before rebuilding an object.
Interview questions
QWhat does Object.entries return?
Object.entries(obj) returns an array of [key, value] pairs for the object's own enumerable string-keyed properties. Each key is a string, and inherited properties are not included.
Follow-up: How does it differ from for...in? Object.entries excludes prototype-chain properties, while for...in includes enumerable inherited properties.
QWhat is the ordering and shape of the result?
The result is ordered like a for...in enumeration of the object's properties. The returned outer array contains one two-element array per matching property.
Follow-up: How would you get only one side of each pair? Use Object.keys for keys or Object.values for values.
QHow can Object.entries convert an object to a Map?
It can be used with the Map constructor because the result is an iterable list of key-value pairs. For example, new Map(Object.entries(obj)) converts an object into a Map.
Follow-up: What is the reverse operation? Object.fromEntries converts a list of key-value pairs into an object, though it can also create symbol-keyed properties.
Related
Object.keys()Object.values()Object.fromEntries()Map()