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A List Apart: Articles: Getting Out of Binding Situations in ...

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Saved by 11 people (2 private), first by anonymouse user on 2008-07-02


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In short, for most OOP languages, binding is implicit. This is true in Java, C#, Ruby, Delphi, and C++

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PHP and JavaScript do require you to explicitly state which object you’re accessing, even if it’s the current one

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This is the single most important issue with JavaScript binding—something I’ll refer to as “binding loss.” It happens whenever you’re accessing a method through a reference instead of directly through its owner object

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The method loses its implicit binding, and this stops referencing its owner object and goes back to its default value, which in this case is window

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Binding-sensitive code patterns

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Every JavaScript function is equipped with an apply method that allows you to call that function with specific binding (a specific this, if you will). It takes two arguments: the binding object, and an array of the arguments to be passed to the function

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When you do know exactly which arguments you want to pass, call may feel nicer, as it takes the arguments themselves, not an array of them

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The only way to achieve this requires us to wrap our original method in another one, that will perform the apply call

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That function, when called, will take our original method and invoke apply on it, passing:
  1. the original object’s binding (the variable named object), and
  2. whatever arguments were provided at call time, as an array

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JavaScript frameworks do it

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var that = this

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that.

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This code uses a language feature called “lexical closure.” In short, closures let code at point A access identifiers declared in scopes surrounding A

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  • Any member access must be qualified with the object it pertains to, even when it is this.
  • Any sort of function reference (assigning as a value, passing as an argument) loses the function’s original binding.
  • JavaScript provides two equivalent ways of explicitly specifying a function’s binding when calling it: apply and call.
  • Creating a “bound method reference” requires an anonymous wrapper function, and a calling cost. In specific situations, leveraging closures may be a better alternative.

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