Reading puzzle outputs
Predicting the output of Object.keys(), for..in, or similar enumeration.
JS Puzzles · Const
Object key order
JavaScript objects use a special key order: integer property names come first in ascending numeric order, while other string keys follow creation order. Thus, object enumeration is not always simple insertion order.
How it works
When JavaScript enumerates an object’s own keys, integer-index-like property names come first in ascending numeric order. Other string keys retain the order in which they were created, so insertion order applies only to that second group.
Key points
- Object enumeration is ordered, but not by a single universal insertion-order rule.
- Integer property names are emitted in ascending order; ordinary string properties use their creation order.
- An object stores data as properties, each pairing a key with a value.
- Object property keys are strings or symbols; values may have any type.
- Known property names can be accessed with dot notation.
- Bracket notation lets code compute or obtain the property name dynamically.
Common mistakes
- An object does not use pure insertion order: integer properties are reordered numerically before ordinary string keys.
Example: Object key enumeration order
const object = { 100: "a", 2: "b", 7: "c", apple: 1, banana: 2 };
console.log(Object.keys(object));
// ["2", "7", "100", "apple", "banana"]Integer-index-like keys are listed first in numeric order; other string keys follow their creation order.
When to use it
Controlling object presentation
Designing serializable records whose displayed keys have predictable ordering.
Selecting a collection
Choosing between an object and Map when insertion order is a core requirement.
Interview questions
QAre JavaScript object keys always returned in insertion order?
Object keys follow a special order: integer properties are sorted numerically, while other properties appear in creation order. Therefore, adding "2" after "100" can still make it appear first.
Follow-up: How would you preserve straightforward insertion order for arbitrary key types? Use Map, whose entries iterate in insertion order.
QHow can you inspect an object’s key order?
For own enumerable string keys, Object.keys() returns keys in the same ordering used by object enumeration. Numeric-looking keys therefore appear in ascending numeric order, followed by ordinary string keys.
Follow-up: What does Object.getOwnPropertyNames() add? It also includes non-enumerable own string properties, while preserving the corresponding enumeration ordering.
QDoes the rule apply identically to every kind of property key?
The documented object-key rule distinguishes integer properties from other properties. Only the non-integer string-key group follows creation order; symbols are a separate property-key category.
Follow-up: What collection supports keys of any value and insertion-order iteration? Map supports arbitrary key values and remembers insertion order.
Related
Object.keys()Object.getOwnPropertyNames()for..inMapin operator