Question

I’ve been working in backbone.js and came across the following snippet of code.

    _(view.buttonViews).each(function(button) {
        button.render();
      });

Where view.buttonViews is an array. If I take away the _() and have

    view.buttonViews.each(function(button) {
        button.render();
      });

then I get an error that each is not a function. What does the _() add? Thanks!

Answer 1

I guess it is the Underscore.js library which provides the each method:

_.each(list, iterator, [context]) Alias: forEach
Iterates over a list of elements, yielding each in turn to an iterator function. The iterator is bound to the context object, if one is passed. Each invocation of iterator is called with three arguments: (element, index, list). If list is a JavaScript object, iterator’s arguments will be (value, key, list). Delegates to the native forEach function if it exists.

This way, _([...]).each(...), is just another way of calling it.

BTW, it is also described in Backbone’s documentation:

Backbone’s only hard dependency is Underscore.js.

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And FWIW, as @Jonathon already said, in general, _ is a valid variable name and in this case it contains a function. Adding parenthesis behind a function references calls that function and therefore, _() calls the function referred to by _. It is nothing special.

Besides that, parenthesis can occur as part of a function declaration or expression (function foo() {...}) or as grouping operator (var i = (20 + 1) * 2;).