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Structure-ish provides core protocols for es6 data storage objects. By using structurish you can declare, for example, that your object (obj) implements the KeyedMethods protocol. This means that doing obj.set(key, value) followed by obj.get(key) will result in value. Without this library you may be able to check that methods exist with the names get and set, but the presence of a protocol symbol gives additional information about the behavior you can expect.

Protocol declaration and detection is critical to facilitate multiple data structure implementations playing nicely with each other (and playing nicely with native types), and to promote a long term extensible solution (protocol Symbols) which if broadly adopted would eventually make it unnecessary.

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To use structure-ish, you must have a working implementation of es6 symbols. If you need to support browsers such as IE11 which do not support symbols, it is your responsibiltiy to set up the relevant parts of corejs, including the Symbol constructor and the well-known-symbol Symbol.iterator.

Install with npm install structure-ish or yarn add structure-ish.

Also note that structure-ish expects to be minified. When NODE_ENV === 'production', static analysis should remove the consistency checks implemented to help eliminate bugs and inconsistencies during dev and test.

The Structure-ish API

The most important caveat is that the structurish API is not duck typed. It requires structures to be positively identified.

  • isStructure(shape) Returns true if shape is iterable, and additionally has keys, values, and entries iterators and a forEach(value, key) method.
  • hasKeyedMethods(shape) Returns true if shape has get, set, has, and delete, methods, as well as a size property.
  • hasSetMethods(shape) Returns true if shape has add, has, and delete, methods, as well as a size property.
  • isEntryIterable(shape) Returns true if the default iterator for shape can be expected to yield [key, value] pairs. If Structure is also defined, the entries iterator should yield the same items as the default iterator, and the keys and values iterators should yield the equivalent of Array.from(obj).map(([key,]) => key) and Array.from(obj).map(([, value]) => value) respectively.
  • isMapish(shape) Returns true if shape implements the es6 Map API by being a structure, an entry iterable, and having keyed methods.
  • isSetish(shape) Returns true if shape implements the es6 Set API by being a structure with set methods.
  • isListish(shape) Returns true if shape looks like a List, meaning a structure with keyed methods.

Creating New Types of Structures

If you can, the easiest way to create a new type of structure is to subclass Map, Set, or Array. At the moment however this cannot be done if you need to support IE11, as it cannot even be transpiled correctly.

Therefore this library exports Symbols, which if incorporated into a class will cause it to be treated as a structure by structure-ish.

The following symbols are exported:

Note that where it is specified that methods may throw an error, they will do so when the declared symbol suggests that certain methods will exist but they are not present. These checks only run when process.env.NODE_ENV !== 'production'.

  • Structure: When defined, this symbol causes isStructure to return true (or throw an error).
  • KeyedMethods: When defined, this symbol causes hasKeyedMethods to return true (or throw an error).
  • SetMethods: When defined, this symbol causes hasSetMethods to return true (or throw an error).
  • EntryIterable: When defined, this symbol causes isEntryIterable to return true (or throw an error).

Here's an example in which the correct symbols are applied to create a custom Map class:

import {
} from 'structure-ish';
class MyMap {
  // has, get, set, keys, values, entries and forEach implementations go here
  [Symbol.iterator]() {
    return this.entries();
  // Only these methods interact with structure-ish
  [Structure]() {
    return true;
  [EntryIterable]() {
    return true;
  [KeyedMethods]() {
    return true;
isStructure(new MyMap()); // true
isMapish(new MyMap()); // true
isEntryIterable(new MyMap()); // true

This is a fair bit of boilerplate, but fortunately it should be reasonable to encapsulate the boilerplate in a base class and reuse it.


Question: I assumed that hasKeyedMethods(Immutable.Seq.Keyed()), (or similar) would be true, why isn't it?

Because the get and set methods probably won't have the performance characteristics you would expect, and the size property might not exist at all.

Question: Are typed arrays structures? Uint8Array, for example.

They could be considered structures but for reasons of performance they are not.

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