jQuery assertion support / defensive programming?

Question

Is there any built in support in jQuery for basic assertion checking, primarily of things like \'expected number of returned elements\'.

For instance I may have a simple

Answer 1

I created a plugin for that: jquery-assert. You could use it like this:

Make sure, one or more elements were found, before calling the next function:

$('#element01').assertFound().val('x')

Assert that 1 element was selected:

$('#element01').assertOne(); // same as assertFound(1)

Make sure a given number of elements were found before calling further functions:

$('.item').assertFound(10).find('p').assertFound(10).data(...)

Answer 2

Enhancements to Tomalak’s answer: throw the error to halt execution, include the offending selector and add the assertSingle variant.

$.fn.assertSize = function(size) {
    if (this.length != size) { 
        throw "Expected " + size + " elements, but selector '" + this.selector + "' found " + this.length + ".";
    }
    return this;
};
$.fn.assertSingle = function() { return this.assertSize(1); };

Answer 3

It doesn't look like there is anything built-in. But writing an extension isn't too hard:

$.fn.assertSize = function(size) { 
  if (this.length != size) { 
    alert("Expected " + size + " elements, but got " + this.length + ".");
    // or whatever, maybe use console.log to be more unobtrusive
  }
  return this;
};

Usage is exactly as you propose in your question.

$("#btnSignup").assertSize(1).click(function() {
  return validateForm();
);

Note that in its current form the function returns successfully even if the assertion fails. Anything you have chained will still be executed. Use return false; instead of return this; to stop further execution of the chain.

Answer 4

You probably want http://docs.jquery.com/Traversing/eq#index:

Reduce the set of matched elements to a single element.

If your selector returns no objects then jQuery simply won't run the chained code as it has no elements to run them on