Huge News!Announcing our $40M Series B led by Abstract Ventures.Learn More
Socket
Sign inDemoInstall
Socket

tassign

Package Overview
Dependencies
Maintainers
1
Versions
1
Alerts
File Explorer

Advanced tools

Socket logo

Install Socket

Detect and block malicious and high-risk dependencies

Install

tassign

Fixed-arity, subset-typed, non-mutating Object.assign.

  • 1.0.0
  • latest
  • Source
  • npm
  • Socket score

Version published
Weekly downloads
2.9K
decreased by-27.02%
Maintainers
1
Weekly downloads
 
Created
Source

tassign

This is a simple wrapper for fixed-arity, subset-typed, non-mutating Object.assign.

Why?

This came about when writing Redux reducers in TypeScript. Since reducers must not mutate the passed in state, a common pattern is to 'set' store properties using Object.assign():

interface IMyState {
  foo: Number;
}

function myReducer(state: IMyState, action: Action) {
  switch(action.type) {
    case SET_FOO_ACTION: 
      return Object.assign({}, state, { foo: 3 });
  }
  
  return state;
}

This works well enough. Object.assign to an empty object produces a new object with the new value of foo. However I wasn't happy with it from a type-enforcement perspective.

The reason is that for a reducer use case, I would like to enforce that the reducer can only set known properties of IMyState.

Using the regular, intersection-based Object.assign typings, this is perfectly legal:

interface IMyState {
  foo: Number;
}

function myReducer(state: IMyState, action: Action) {
  switch(action.type) {
    case SET_FOO_ACTION: 
      // No TS error - I would like there to be.
      return Object.assign({}, state, { foo: 3, bar: 'wat' });
  }
  
  return state;
}

Formally, Object.assign(t:T, u:U) returns a type which is the intersection of T & U, meaning that properties can be added onto the assignee. This is reasonable in the general case for Object.assign, but it's not what I want in a reducer.

In a reducer, I want assign(t:T, u:U) to enforce a subset rule: the return value should be an instance of T, and only subsets of T's type are legal values for U. In short I want to make sure you can only assign members of U to the object returned by the reducer.

So, here's a utility that handles the typings the way I want in a reducer:

interface IMyState {
  foo: Number;
}

function myReducer(state: IMyState, action: Action) {
  switch(action.type) {
    case SET_FOO_ACTION: 
      // Returns a new object with the type of the first argument.
      // Type of the second argument must be a subset of the type of the first argument.
      
      // Fails: bar is not a member of IMyState.
      return tassign(state, { foo: 3, bar: 'wat' });
  }
  
  return state;
}

FAQs

Package last updated on 07 Nov 2016

Did you know?

Socket

Socket for GitHub automatically highlights issues in each pull request and monitors the health of all your open source dependencies. Discover the contents of your packages and block harmful activity before you install or update your dependencies.

Install

Related posts

SocketSocket SOC 2 Logo

Product

  • Package Alerts
  • Integrations
  • Docs
  • Pricing
  • FAQ
  • Roadmap
  • Changelog

Packages

npm

Stay in touch

Get open source security insights delivered straight into your inbox.


  • Terms
  • Privacy
  • Security

Made with ⚡️ by Socket Inc