Understanding function.prototype.bind() and function.length in JavaScript

As a JavaScript developer, you‘ve likely encountered situations where you need more control over a function‘s execution context (the ‘this‘ keyword) or want to pre-apply arguments to a function. The built-in function.prototype.bind() method is an invaluable tool for these scenarios. Additionally, checking a function‘s expected arity (number of arguments) via the function.length property can help write more robust and self-documenting code. In this article, we‘ll dive deep into both of these features.

What does function.prototype.bind() do?

The .bind() method is available on all JavaScript functions. It allows you to create a new function with a fixed ‘this‘ context and optionally pre-apply arguments, without immediately invoking the function. The new function can then be called later with any additional arguments.

The first argument passed to .bind() sets the ‘this‘ context for the new function. Subsequent arguments will be prepended to the arguments list when the bound function is called.

Here‘s a simple example:

const obj = { x: 42 };

function foo() {
  console.log(this.x);
}

const boundFoo = foo.bind(obj);
boundFoo(); // logs 42

In this case, .bind() is used to create boundFoo, a new function that wraps foo but with ‘this‘ permanently set to obj. So when boundFoo is called, this.x resolves to obj.x.

Pre-applying arguments with .bind()

.bind() also allows you to pre-apply arguments to a function. Any arguments passed after the first ‘this‘ argument will be prepended to the arguments list of the bound function:

function greet(greeting, name) {
  return `${greeting}, ${name}!`;
}

const sayHello = greet.bind(null, ‘Hello‘);

console.log(sayHello(‘John‘)); // Hello, John!
console.log(sayHello(‘Mary‘)); // Hello, Mary!

Here sayHello is a new function that wraps greet with the first argument already set to ‘Hello‘. So sayHello() only needs to be passed a name string, and it will prepend ‘Hello‘ to it.

This ability to pre-apply arguments is sometimes called partial application or currying. It lets you create more specialized functions out of more general ones.

The function.length property

Every JavaScript function has a .length property that returns the number of named arguments the function expects – its "arity". This refers to the number of arguments in the function declaration, and does not include rest parameters.

function foo(a, b, c) {}
console.log(foo.length); // 3

function bar(x, ...args) {} 
console.log(bar.length); // 1

In the example above, foo.length is 3 because foo is declared with three named arguments. bar.length is 1 because only the x argument counts towards its length, not the …args rest parameter.

Function length and ES2015 features

The introduction of default parameters and rest parameters in ES2015 affects how .length is calculated.

With default parameters, only arguments before the first default value are counted:

function baz(a, b = 2, c = 3) {}
console.log(baz.length); // 1

Even though baz lists three arguments, only the first one doesn‘t have a default value. So baz.length returns 1.

Similarly, rest parameters are not counted towards a function‘s length:

function qux(a, b, ...rest) {}
console.log(qux.length); // 2

Here, only a and b contribute to qux.length, while …rest is ignored. So qux.length is 2.

Practical uses of .bind() and .length

Understanding .bind() and .length isn‘t just academic – they enable useful real-world patterns for JavaScript developers.

Setting ‘this‘ with .bind() vs arrow functions

A common issue in JavaScript is losing track of ‘this‘ when a method is used as a callback or event handler. For example:

const counter = {
  count: 0,
  increment() {
    this.count++;
  }
};

document.querySelector(‘button‘)
  .addEventListener(‘click‘, counter.increment); // doesn‘t work!

Here, counter.increment is passed to addEventListener as a plain function reference. When called, its ‘this‘ will point to the DOM button element, not the counter object, resulting in an error.

One solution is to use .bind() to hard-bind the method‘s ‘this‘ to the counter object:

document.querySelector(‘button‘)
  .addEventListener(‘click‘, counter.increment.bind(counter)); // works

Alternatively, you can wrap the method call in an arrow function:

document.querySelector(‘button‘)
  .addEventListener(‘click‘, () => counter.increment()); // works

The arrow function version is more concise, but .bind() may perform slightly better as the binding only happens once. Use whichever version you find more readable.

Checking arity with .length

Checking a function‘s .length enables patterns like validation of passed arguments or conditional logic based on arity.

For example, let‘s say you have a function that requires at least 2 arguments. You could throw an error if that‘s not the case:

function requireTwo(a, b) {
  if (arguments.length < requireTwo.length) {
    throw new Error(`${requireTwo.name} requires at least ${requireTwo.length} arguments, but only ${arguments.length} were passed`);
  }
  // ...
}

Some libraries and frameworks use .length for things like automatic dependency injection based on a function‘s declared parameters.

Potential issues with .bind() and .length

While .bind() and .length are handy, they have some gotchas to watch out for.

One issue with .bind() is that it creates a new function every time it‘s called. If you‘re repeatedly binding the same function in a loop or hot code path, this can lead to performance issues or excess memory usage. In cases like these, consider using arrow functions or assigning the bound function to a variable.

Another potential pitfall is that .bind() produces a special "exotic bound function", which doesn‘t have a prototype property. This means bound functions cannot be used as constructors with new. If you need a bound function that‘s also a constructor, you may need to use an alternate approach like assigning the original function‘s prototype property.

With .length, remember that it only counts non-default named arguments before a rest parameter. This means it may not always match the actual number of arguments a function can accept. Always refer to a function‘s documentation or implementation for the most accurate arity info.

Conclusion

function.prototype.bind() and function.length are two powerful JavaScript features that give you more control over function invocation, argument handling, and arity checking. Understanding how they work and when to use them will make you a more effective JavaScript developer.

While .bind() is great for setting ‘this‘ context and pre-applying arguments, keep an eye out for performance issues if binding repeatedly. Arrow functions are a popular modern alternative for ‘this‘ binding.

Similarly, .length is useful for introspection and arity-based logic, but it has quirks with default and rest parameters that mean it‘s not always the full picture.

I hope this deep dive has solidified your understanding of these features and how they fit into your JavaScript toolkit. Happy coding!