Improve accuracy of _.throttle

[dcorb]

Repeated throttled executions are not as accurate as they could be.
For example, when

var throttled = _.throttle(doSomething,400)
and calling repeatedly throttled(); every 200ms.

I expect the executions happen once every 400ms, but sometimes there are delays of more than 100ms until the next execution.

This pull request fixes the situation at trailing:true, leading:false and `trailing:true, leading:true``

There is no way of achieving a 400ms rate of execution for trailing:false, leading:true in this example I put, because by definition, is impossible to foresee the future. We need to wait for the next throttled()

I discover this lack of accuracy with this example:

$('.clickme')
    .on('click', _.throttle(function (){  
     t1 = performance.now(); 
     console.log('Time from last throttle: ', (t1 - t2).toFixed(4), 'ms ');
     t2 = performance.now();
  }, 400, {trailing:true, leading:false}))
  .on('click', function(){
    t0 = performance.now(); 
    console.log('Time from last click:', (t0-t4).toFixed(4), 'ms ');
    t4 = performance.now();
  })

[jdalton]

Wow. Thank you for taking the time to dig into this. The throttle and debounce code is always so hard to reason about without a visual or example (timing is hard).

You're a big help here.
Have you signed our cla?

[dcorb]

Thank you, It's quite tough to wrap your head around it, indeed!.
I hope this change doesn't result in any side-effect, but I'm pretty sure it's safe.
I just signed the CLA (submitted a CLA on 2016-02-07 01:05:32)

I will keep you updated when the article about debounce/throttle gets published, with a revamped visualisation tool

[lock]

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