Demonstrates v8 optimization for reassigning vs. not reassigning `arguments` object

optimization-status.js

// reassigning an argument
function argReassign1(a) {
    a = a || {};
}

// not reassigning an argument
function argReassign2(a) {
    if (a === void 0) var a = {};
}


function printStatus(fn) {
    switch(%GetOptimizationStatus(fn)) {
        case 1: console.log("Function is optimized"); break;
        case 2: console.log("Function is not optimized"); break;
        case 3: console.log("Function is always optimized"); break;
        case 4: console.log("Function is never optimized"); break;
        case 6: console.log("Function is maybe deoptimized"); break;
        case 7: console.log("Function is optimized by TurboFan"); break;
        default: console.log("Unknown optimization status"); break;
    }
}

// 2 calls each are needed to go from uninitialized -> pre-monomorphic -> monomorphic
argReassign1({});
argReassign2({});
argReassign1({});
argReassign2({});

%OptimizeFunctionOnNextCall(argReassign1);

//The next call
argReassign1()

console.log('argReassign1:');
//Check
printStatus(argReassign1);



%OptimizeFunctionOnNextCall(argReassign2);

//The next call
argReassign2()

console.log('argReassign2:');
//Check
printStatus(argReassign2);

To run this you will have to use the allow_natives_syntax flag
e.g.

node --allow_natives_syntax optimization-status.js

Dug in a little further with --trace-opt, but doesn't seem like there's any bailout reason specifically for argReassign1.

[optimizing 0x37b63088099 <JS Function argReassign1 (SharedFunctionInfo 0x337c8530e461)> - took 0.022, 0.058, 0.014 ms]
[evicting entry from optimizing code map (notify deoptimized) for 0x337c8530e461 <SharedFunctionInfo argReassign1>]