javascript - 为什么绑定(bind)比闭包慢？

Question

以前的发帖人问 Function.bind vs Closure in Javascript : how to choose?

并部分收到了这个答案，这似乎表明绑定(bind)应该比闭包更快:

Scope traversal means, when you are reaching to grab a value (variable,object) that exists in a different scope, therefore additional overhead is added (code becomes slower to execute).

Using bind, you 're calling a function with an existing scope, so that scope traversal does not take place.

两个 jsperfs 表明 bind 实际上比 closure 慢得多。 .

这是作为对上述内容的评论发布的

而且，我决定写 my own jsperf

那么为什么绑定(bind)这么慢(70+% 在 Chrome 上)？

既然它不是更快，而且闭包可以达到同样的目的，应该避免绑定(bind)吗？

Answer 1

Chrome 59 更新:正如我在下面的答案中预测的那样，使用新的优化编译器绑定(bind)不再慢。这是带有详细信息的代码:https://codereview.chromium.org/2916063002/

大多数时候都没有关系。

除非您正在创建一个应用程序，其中 .bind 是我不会打扰的瓶颈。在大多数情况下，可读性比纯粹的性能更重要。我认为使用原生 .bind 通常会提供更易读和可维护的代码——这是一个很大的优势。

但是，是的，当它重要时 - .bind 更慢

是的，.bind 比闭包要慢得多 - 至少在 Chrome 中，至少在 v8 中的当前实现方式。由于性能问题，我个人不得不切换到 Node.JS(更一般地说，在性能密集型情况下，闭包有点慢)。

为什么？因为 .bind 算法比用另一个函数包装一个函数并使用 .call 或 .apply 复杂得多。 (有趣的是，它还返回一个 toString 设置为 [native function] 的函数)。

有两种方式来看待这个问题，从规范的 Angular 来看，从实现的 Angular 来看。让我们观察两者。

首先，让我们 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.

看起来很复杂，不仅仅是一个包装。

第二，我们看how it's implemented in Chrome .

我们来看看FunctionBind在 v8(chrome JavaScript 引擎)源代码中:

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;

我们可以在实现中看到一堆昂贵的东西。即%_IsConstructCall()。这当然需要遵守规范 - 但在许多情况下它也比简单的包装慢。

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另一方面，调用 .bind 也略有不同，规范注释“使用 Function.prototype.bind 创建的函数对象没有原型(prototype)属性或 [[Code]]，[ [FormalParameters]] 和 [[Scope]] 内部属性"