@bind Decorator

Lazily binds a class prototype method to the instance (this) the first time it is accessed, avoiding the need for manual constructor binding while preserving prototype memory efficiency.

---

Why

Common patterns like passing methods as callbacks (event listeners, timers, Promise chains) require stable this binding. Manual solutions:

• Binding in constructor (this.onClick = this.onClick.bind(this)) creates an extra function per instance eagerly.
• Using arrow properties (onClick = () => { ... }) also allocates per instance and hides the method from the prototype (costs memory, harder to spy/patch).

@bind offers a declarative, lazy alternative:

• Prototype still holds a single original function.
• First access through an instance installs a bound copy directly onto that instance.
• Subsequent calls are zero‑cost (direct property hit).

---

Quick Start

import {bind} from '@exadel/esl/modules/esl-utils/decorators';

class ButtonController {
  clicks = 0;

  @bind
  handleClick(evt: Event) {
    this.clicks++;
  }

  attach(el: HTMLElement) {
    el.addEventListener('click', this.handleClick); // always correctly bound
  }
}

---

API

function bind(target: object, key: string, descriptor: TypedPropertyDescriptor<Function>): TypedPropertyDescriptor<Function>;

Applied to: class prototype methods (not getters, not setters, not fields).

Result

The original descriptor is replaced with an accessor descriptor:

• get: returns original function when accessed through the prototype; otherwise installs & returns a bound version on the instance.
• set: supports reassignment; the new value is defined as a normal writable value property.

---

Behavior Details

1. Decoration phase: method value captured (originalFn).
2. A getter is defined. On first instance access:
   • Detects access via instance (not prototype).
   • Binds originalFn with this.
   • Defines the bound function directly on the instance (overwriting the accessor at instance level only).
3. Future accesses hit the already bound instance property (no more indirection).
4. Access using Class.prototype.method still returns the unbound original (useful for testing or chaining decorators).

---

Examples

Event Handler

class Modal {
  @bind onEsc(e: KeyboardEvent) { if (e.key === 'Escape') this.close(); }
  open() { document.addEventListener('keydown', this.onEsc); }
  close() { document.removeEventListener('keydown', this.onEsc); }
}

With Other Decorators

Order matters when stacking decorators that wrap execution (e.g. memoization). @bind only adjusts access / binding and does not change the call semantics of the wrapped function itself. Place @bind outermost (lowest in code) if the method result should reflect wrapper transforms before binding.

class Service {
  @bind
  @memoize()
  heavy() { /* ... */ }
}

(Here the memoized wrapper is what gets bound.)

---

Limitations

---

Typing Notes

The decorator preserves the original method signature. No return type widening occurs. Works with this typing as originally declared.

---

Best Practices

• Use for callback-style methods passed around frequently.
• Prefer over constructor binding to reduce startup cost in large component trees.
• Keep method logic independent of decoration (test original via Class.prototype.method).
• Combine with performance decorators (e.g. @memoize) by placing @bind last (closest to the method) if you want the bound version to be the final wrapped function.