18 September 2015

Helper Function .bindAll

Like most JavaScripters, I often find myself needing to manually control the this-binding of a function on invocation. Event handlers and callback functions are often re-bound to a different context than the caller — meaning that if those event handlers or callbacks reference this, they might be referencing the wrong object.

These context-switching functions sometimes require an explicit binding, as in this next snippet.

Above, an event is fired when the app is 'ready' — a callback function is registered to handle this 'ready' event. Assuming the event handler (this.startMainWindow) uses this internally, it needs to be bound explicitly in order to preserve the this-binding that is desired. We do this by passing our desired this-binding into .bind as the first argument.

Great, now imagine that we have these context-switching functions all over our codebase. Also, imagine that groups of these functions are all bound to the same object. Pretty soon, you have a stupid amount of someObj.someFunc.bind(ctxObj) in your codebase.

Are you now motivated you to find a helper function for binding a bunch of functions at once? I am!


Underscore.js provides a _.bindAll method that binds a list of methods to one object. However, Underscore is a 1,500 line of code dependency that is outside of our control. I’d prefer 15 lines of code over which we have control.

So, let’s implement our own .bindAll function!

Here is one example using the .bindAll helper (line 6):

Beautiful. We can simply pass .bindAll the object to which to bind, along with a list of method names in the current context which should be bound to the first argument.