definitely

Find undefined property bugs close to the source.

Installation

$ npm install --save-dev definitely

Usage and rationale

JavaScript allows you to access nonexistent properties on an object and simply evaluates the expression to undefined. Usually when this happens it's a bug, but since no error is thrown such bugs can be a difficult to debug as the problem tends to rear its ugly head far from whence it came. For example,

const obj = {
  foo: 'bar'
}
 
someFunction(obj.bar)

obj has no property bar, so we're passing undefined to someFunction, which may pass it to another function, put it onto a queue, etc. It could be much later and in a completely different area of the code when someone tries to do something they shouldn't with that undefined value, at which point we'll begin the laborious process of trying to figure out where things went awry.

That's where definitely comes in:

import definitely from 'definitely'
 
const definiteObj = definitely({
  foo: 'bar'
})
 
someFunction(definiteObj.bar) // throws "Error: attempted to access nonexistent property `bar`"

Now you've caught the bug right at the source!

Note

The magic that enables this library is an ES2015 feature called Proxy, which allows intercepting arbitrary property access attempts and deciding what to do with them (in our case we throw an exception if the underlying object is missing the requisite property). Proxy is not yet supported in all browsers (e.g. IE and Safari) and in Node prior to version 6 requires a special flag (--harmony-proxies) and shim. If Proxy is not found in the global scope, this library becomes a no-op.

See also

definitely-loader, a Webpack loader which verifies that imported names exist within the target module.