[natorion]

Try/catch/finally is handled by TurboFan (one of V8's optimizing compilers) - see https://groups.google.com/forum/#!topic/v8-users/maH60gh_a8s for more detail. There is a certain set of features which go through TurboFan instead of CrankShaft (the other optimizing compiler).

Disclaimer: I work on the V8 team.

[pygy_]

Still, using `try` seems much slower (in Chrome 52):

    var times = [[],[]]
    var sums = [0, 0]
    var funcs = [
        function(){     sums[0]+=1            },
        function(){try{ sums[1]+=1 }catch(e){}}
    ]

    function bench(n){
        var t0 = performance.now()
        for(var i = 0; i< 30000; i++) funcs[n]()
        times[n].push(performance.now()-t0)
    }

    for(var j = 0; j< 10000; j++) bench(j%2)

    function avg(ary){return ary.reduce(function(acc, v){return acc+v}, 0)/ary.length}

    console.log(times.map(avg))

    // results: 
    //         [bare             , wrapped in try{}  ]
    // VM89:18 [0.347546000033617, 0.7464809999451041]

Let's do some more work inside the function / try block:

    var times = [[],[]]
    var sums = [0, 0]
    var funcs = [
        function(){     for (var i = 0; i < 1000; i++) sums[0]+=1            },
        function(){try{ for (var i = 0; i < 1000; i++) sums[1]+=1 }catch(e){}}
    ]

    function bench(n){
        var t0 = performance.now()
        for(var i = 0; i< 30; i++) funcs[n]()
        times[n].push(performance.now()-t0)
    }

    for(var j = 0; j< 10000; j++) bench(j%2)

    function avg(ary){return ary.reduce(function(acc, v){return acc+v}, 0)/ary.length}

    console.log(times.map(avg))

    // results: 
    //         [bare               , wrapped in try{}  ]
    // VM90:18 [0.09457900004982948, 0.5069960000008344]

I'm sure I'm doing many things wrong since I'm not a JS perf expert, but still, the difference here is goes from 2 to 50 times slower with the `try` block...

Edit: for the sake of completeness, on the same machine:

Firefox gives [10.2, 10.3] and [5.30, 5.45]

Safari gives [5.56, 5.60] and [4.91, 4.94]