Why is bind slower than a closure?

Chrome 59 update: As I predicted in the answer below bind is no longer slower with the new optimizing compiler. Here’s the code with details: https://codereview.chromium.org/2916063002/

Most of the time it does not matter.

Unless you’re creating an application where .bind is the bottleneck I wouldn’t bother. Readability is much more important than sheer performance in most cases. I think that using native .bind usually provides for more readable and maintainable code – which is a big plus.

However yes, when it matters – .bind is slower

Yes, .bind is considerably slower than a closure – at least in Chrome, at least in the current way it’s implemented in v8. I’ve personally had to switch in Node.JS for performance issues some times (more generally, closures are kind of slow in performance intensive situations).

Why? Because the .bind algorithm is a lot more complicated than wrapping a function with another function and using .call or .apply. (Fun fact, it also returns a function with toString set to [native function]).

There are two ways to look at this, from the specification point of view, and from the implementation point of view. Let’s observe both.

First, let’s look at the bind algorithm defined in the specification:

1. Let Target be the this value.
2. If IsCallable(Target) is false, throw a TypeError exception.
3. Let A be a new (possibly empty) internal list of all of the argument values provided after thisArg (arg1, arg2 etc), in order.

…

(21. Call the [[DefineOwnProperty]] internal method of F with arguments “arguments”, PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.

(22. Return F.

Seems pretty complicated, a lot more than just a wrap.

Second , let’s see how it’s implemented in Chrome.

Let’s check FunctionBind in the v8 (chrome JavaScript engine) source code:

function FunctionBind(this_arg) { // Length is 1.
  if (!IS_SPEC_FUNCTION(this)) {
    throw new $TypeError('Bind must be called on a function');
  }
  var boundFunction = function () {
    // Poison .arguments and .caller, but is otherwise not detectable.
    "use strict";
    // This function must not use any object literals (Object, Array, RegExp),
    // since the literals-array is being used to store the bound data.
    if (%_IsConstructCall()) {
      return %NewObjectFromBound(boundFunction);
    }
    var bindings = %BoundFunctionGetBindings(boundFunction);

    var argc = %_ArgumentsLength();
    if (argc == 0) {
      return %Apply(bindings[0], bindings[1], bindings, 2, bindings.length - 2);
    }
    if (bindings.length === 2) {
      return %Apply(bindings[0], bindings[1], arguments, 0, argc);
    }
    var bound_argc = bindings.length - 2;
    var argv = new InternalArray(bound_argc + argc);
    for (var i = 0; i < bound_argc; i++) {
      argv[i] = bindings[i + 2];
    }
    for (var j = 0; j < argc; j++) {
      argv[i++] = %_Arguments(j);
    }
    return %Apply(bindings[0], bindings[1], argv, 0, bound_argc + argc);
  };

  %FunctionRemovePrototype(boundFunction);
  var new_length = 0;
  if (%_ClassOf(this) == "Function") {
    // Function or FunctionProxy.
    var old_length = this.length;
    // FunctionProxies might provide a non-UInt32 value. If so, ignore it.
    if ((typeof old_length === "number") &&
        ((old_length >>> 0) === old_length)) {
      var argc = %_ArgumentsLength();
      if (argc > 0) argc--;  // Don't count the thisArg as parameter.
      new_length = old_length - argc;
      if (new_length < 0) new_length = 0;
    }
  }
  // This runtime function finds any remaining arguments on the stack,
  // so we don't pass the arguments object.
  var result = %FunctionBindArguments(boundFunction, this,
                                      this_arg, new_length);

  // We already have caller and arguments properties on functions,
  // which are non-configurable. It therefore makes no sence to
  // try to redefine these as defined by the spec. The spec says
  // that bind should make these throw a TypeError if get or set
  // is called and make them non-enumerable and non-configurable.
  // To be consistent with our normal functions we leave this as it is.
  // TODO(lrn): Do set these to be thrower.
  return result;

We can see a bunch of expensive things here in the implementation. Namely %_IsConstructCall(). This is of course needed to abide to the specification – but it also makes it slower than a simple wrap in many cases.

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On another note, calling .bind is also slightly different, the spec notes “Function objects created using Function.prototype.bind do not have a prototype property or the [[Code]], [[FormalParameters]], and [[Scope]] internal properties”