memoize-bind

Memoized function binding

memoize-bind performs the same job as Function.prototype.bind(), however it memoizes the result for future reference.

This ensures that calling memoize-bind repeatedly with the same arguments will always return the same bound function, which can be particularly useful within React.js render methods (see below).

Installation

npm install memoize-bind

Usage

bind(fn, [context], [...args])

Create a new function that, when called, will invoke fn with the this keyword set to context.

If any args are specified, they will be prepended to the list of arguments supplied when calling the newly-created function.

Example

import bind from 'memoize-bind';
import assert from 'assert';

const context = { name: 'foo' };
const fn = function(...args) { return [this.name, ...args]; };
const args = ['bar', 'baz']

const boundFn = bind(fn, context, ...args); // Identical to fn.bind(context, ...args)
const boundFn2 = bind(fn, context, ...args); // Returns the same function
assert(boundFn === boundFn2);

boundFn('qux'); // Returns ['foo', 'bar', 'baz', 'qux']

Why memoize-bind?

Function.prototype.bind() is often used within React components, where callbacks need to be bound to the current component instance (optionally with arguments pre-applied).

While convenient, using .bind() within a render function can be problematic for React's dirty-checking algorithm, seeing as a new function instance would be returned on each render, leaving the React reconciler unable to tell that two functions generated during subsequent renders are indeed identical.

memoize-bind fixes this problem by always returning the same bound function when called with the same arguments, allowing bound functions to be used within straightforward equality checks in the component's shouldComponentUpdate method (as used by PureComponent / PureRenderMixin).

This means that you can keep all the convenience of the .bind() method, without having to worry about affecting performance.

React Example

import bind from 'memoize-bind';
import React, { PropTypes, PureComponent } from 'react';

class TodoList extends PureComponent {
  render() {
    const { items } = this.props;
    return (
      <ul>
        {items.map((item, index) => (
          <li onClick={bind(this.handleItemClicked, this, item)} key={item.id}>{item.label}</li>
        ))}
      </ul>
    );
  }

  handleItemClicked(item) {
    if (!item.isCompleted) { this.props.onComplete(item); }
  }

  static propTypes = {
    items: PropTypes.arrayOf(PropTypes.shape({
      id: PropTypes.number,
      label: PropTypes.string,
      isCompleted: PropTypes.bool,
    })),
    onComplete: PropTypes.func.isRequired,
  };
}

Using memoize-bind in ES5 applications

memoize-bind uses memoize-weak internally to ensure that any memoized arguments and values can be properly garbage-collected.

memoize-weak requires that Map and WeakMap are globally available. This means that these will have to be polyfilled for use in an ES5 environment.

Some examples of Map and WeakMap polyfills for ES5:

• Babel Polyfill
• CoreJS
• es6-map and es6-weak-map