How does Function.prototype.bind work under the hood? Can you implement a polyfill for it?

Q: How does Function.prototype.bind work under the hood? Can you implement a polyfill for it?

if (!Function.prototype.bind) { Function.prototype.bind = function(thisArg, ...boundArgs) { const fn = this; if (typeof fn !== 'function') { throw new TypeError('Bind must be called on a function'); } return function(...callArgs) { // If called with 'new', use the original function's prototype chain if (this instanceof fn) { return new fn(...boundArgs, ...callArgs); } return fn.apply(thisArg, boundArgs.concat(callArgs)); }; }; } Under the hood, `bind` creates a closure that stores the original function (`fn`), the `thisArg`, and any pre-applied arguments (`boundArgs`). When the bound function is called, it uses `apply` to invoke the original function with the combined arguments. The polyfill must also handle the case where the bound function is used as a constructor with `new`, in which case the original `this` (the new object) should be preserved rather than the bound `thisArg`.

Difficulty: 6/10

this binding, function prototype, polyfill implementation

`Function.prototype.bind` returns a new function that, when called, has its `this` set to the provided value, with any arguments partially applied. A polyfill must handle `new` calls and chaining.

Simple bind polyfill

Under the hood, bind creates a closure that stores the original function (fn), the thisArg, and any pre-applied arguments (boundArgs). When the bound function is called, it uses apply to invoke the original function with the combined arguments. The polyfill must also handle the case where the bound function is used as a constructor with new, in which case the original this (the new object) should be preserved rather than the bound thisArg.

Scenario Questions

0-2 years experience

2-5 years experience

1Our legacy module creates many bound functions and we’re seeing a memory leak. How would you investigate the cause and what alternatives could you consider?

Follow-up Questions

Difficulty: 6/10

this binding, function prototype, polyfill implementation

`Function.prototype.bind` returns a new function that, when called, has its `this` set to the provided value, with any arguments partially applied. A polyfill must handle `new` calls and chaining.

Simple bind polyfill

Scenario Questions

0-2 years experience

2-5 years experience

1Our legacy module creates many bound functions and we’re seeing a memory leak. How would you investigate the cause and what alternatives could you consider?

How does Function.prototype.bind work under the hood? Can you implement a polyfill for it?

Function.prototype.bind returns a new function that, when called, has its this set to the provided value, with any arguments partially applied. A polyfill must handle new calls and chaining.

Scenario Questions

Follow-up Questions

How does Function.prototype.bind work under the hood? Can you implement a polyfill for it?

Function.prototype.bind returns a new function that, when called, has its this set to the provided value, with any arguments partially applied. A polyfill must handle new calls and chaining.

Scenario Questions

Follow-up Questions

`Function.prototype.bind` returns a new function that, when called, has its `this` set to the provided value, with any arguments partially applied. A polyfill must handle `new` calls and chaining.

`Function.prototype.bind` returns a new function that, when called, has its `this` set to the provided value, with any arguments partially applied. A polyfill must handle `new` calls and chaining.