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Don't reassign to arguments; it deoptimizes engines.
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@ljharb
ljharb committed Apr 13, 2015
1 parent c9e03bb commit f702085
Showing 1 changed file with 41 additions and 40 deletions.
81 changes: 41 additions & 40 deletions es6-shim.js
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Original file line number Diff line number Diff line change
Expand Up @@ -206,13 +206,13 @@
$iterator$ = '@@iterator';

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@paulmillr

paulmillr Apr 21, 2015

Owner

Does it deoptimize only V8 or other engines too?

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@ljharb

ljharb Apr 21, 2015

Author Collaborator

I've only seen evidence of it in v8, but I wouldn't be surprised if it had a similar effect in others too. Since v8 backs 2 browsers and node/iojs, it's enough evidence for me :-)
}
var addIterator = function (prototype, impl) {
if (!impl) { impl = function iterator() { return this; }; }
var implementation = impl || function iterator() { return this; };
var o = {};
o[$iterator$] = impl;
o[$iterator$] = implementation;
defineProperties(prototype, o);
if (!prototype[$iterator$] && Type.symbol($iterator$)) {
// implementations are buggy when $iterator$ is a Symbol
prototype[$iterator$] = impl;
prototype[$iterator$] = implementation;
}
};

Expand Down Expand Up @@ -372,17 +372,18 @@

var emulateES6construct = function (o) {
if (!ES.TypeIsObject(o)) { throw new TypeError('bad object'); }
var object = o;
// es5 approximation to es6 subclass semantics: in es6, 'new Foo'
// would invoke Foo.@@species to allocation/initialize the new object.
// In es5 we just get the plain object. So if we detect an
// uninitialized object, invoke o.constructor.@@species
if (!o._es6construct) {
if (o.constructor && ES.IsCallable(o.constructor[symbolSpecies])) {
o = o.constructor[symbolSpecies](o);
if (!object._es6construct) {
if (object.constructor && ES.IsCallable(object.constructor[symbolSpecies])) {
object = object.constructor[symbolSpecies](object);
}
defineProperties(o, { _es6construct: true });
defineProperties(object, { _es6construct: true });
}
return o;
return object;
};

// Firefox 31 reports this function's length as 0
Expand Down Expand Up @@ -460,32 +461,32 @@
repeat: function repeat(times) {
ES.RequireObjectCoercible(this);
var thisStr = String(this);
times = ES.ToInteger(times);
if (times < 0 || times >= stringMaxLength) {
var numTimes = ES.ToInteger(times);
if (numTimes < 0 || numTimes >= stringMaxLength) {
throw new RangeError('repeat count must be less than infinity and not overflow maximum string size');
}
return stringRepeat(thisStr, times);
return stringRepeat(thisStr, numTimes);
},

startsWith: function startsWith(searchStr) {
startsWith: function startsWith(searchString) {
ES.RequireObjectCoercible(this);
var thisStr = String(this);
if (Type.regex(searchStr)) {
if (Type.regex(searchString)) {
throw new TypeError('Cannot call method "startsWith" with a regex');
}
searchStr = String(searchStr);
var searchStr = String(searchString);
var startArg = arguments.length > 1 ? arguments[1] : void 0;
var start = Math.max(ES.ToInteger(startArg), 0);
return thisStr.slice(start, start + searchStr.length) === searchStr;
},

endsWith: function endsWith(searchStr) {
endsWith: function endsWith(searchString) {
ES.RequireObjectCoercible(this);
var thisStr = String(this);
if (Type.regex(searchStr)) {
if (Type.regex(searchString)) {
throw new TypeError('Cannot call method "endsWith" with a regex');
}
searchStr = String(searchStr);
var searchStr = String(searchString);
var thisLen = thisStr.length;
var posArg = arguments.length > 1 ? arguments[1] : void 0;
var pos = typeof posArg === 'undefined' ? thisLen : ES.ToInteger(posArg);
Expand Down Expand Up @@ -750,10 +751,10 @@
var end = arguments[2]; // copyWithin.length must be 2
var o = ES.ToObject(this);
var len = ES.ToLength(o.length);
target = ES.ToInteger(target);
start = ES.ToInteger(start);
var to = target < 0 ? Math.max(len + target, 0) : Math.min(target, len);
var from = start < 0 ? Math.max(len + start, 0) : Math.min(start, len);
var relativeTarget = ES.ToInteger(target);
var relativeStart = ES.ToInteger(start);
var to = relativeTarget < 0 ? Math.max(len + relativeTarget, 0) : Math.min(relativeTarget, len);
var from = relativeStart < 0 ? Math.max(len + relativeStart, 0) : Math.min(relativeStart, len);
end = typeof end === 'undefined' ? len : ES.ToInteger(end);
var fin = end < 0 ? Math.max(len + end, 0) : Math.min(end, len);
var count = Math.min(fin - from, len - to);
Expand Down Expand Up @@ -1053,8 +1054,8 @@
return result === FAKENULL ? null : result;
};
Object.setPrototypeOf = function (o, p) {
if (p === null) { p = FAKENULL; }
return spo(o, p);
var proto = p === null ? FAKENULL : p;
return spo(o, proto);
};
Object.setPrototypeOf.polyfill = false;
}());
Expand Down Expand Up @@ -1279,7 +1280,7 @@
} else {
result = Math.exp(Math.log(x) / 3);
// from http://en.wikipedia.org/wiki/Cube_root#Numerical_methods
result = (x / (result * result) + 2 * result) / 3;
result = (x / (result * result) + (2 * result)) / 3;
}
return negate ? -result : result;
},
Expand Down Expand Up @@ -1408,12 +1409,12 @@

imul: function imul(x, y) {
// taken from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/imul
x = ES.ToUint32(x);
y = ES.ToUint32(y);
var ah = (x >>> 16) & 0xffff;
var al = x & 0xffff;
var bh = (y >>> 16) & 0xffff;
var bl = y & 0xffff;
var a = ES.ToUint32(x);
var b = ES.ToUint32(y);
var ah = (a >>> 16) & 0xffff;
var al = a & 0xffff;
var bh = (b >>> 16) & 0xffff;
var bl = b & 0xffff;
// the shift by 0 fixes the sign on the high part
// the final |0 converts the unsigned value into a signed value
return ((al * bl) + (((ah * bl + al * bh) << 16) >>> 0) | 0);
Expand Down Expand Up @@ -1696,16 +1697,16 @@
// The `obj` parameter is a hack we use for es5
// compatibility.
var prototype = constructor.prototype || Promise$prototype;
obj = obj || create(prototype);
defineProperties(obj, {
var object = obj || create(prototype);
defineProperties(object, {
_status: void 0,
_result: void 0,
_resolveReactions: void 0,
_rejectReactions: void 0,
_promiseConstructor: void 0
});
obj._promiseConstructor = constructor;
return obj;
object._promiseConstructor = constructor;
return object;
});
defineProperties(Promise, {
all: function all(iterable) {
Expand Down Expand Up @@ -1991,9 +1992,9 @@
defineProperty(Map, symbolSpecies, function (obj) {
var constructor = this;
var prototype = constructor.prototype || Map$prototype;
obj = obj || create(prototype);
defineProperties(obj, { _es6map: true });
return obj;
var object = obj || create(prototype);
defineProperties(object, { _es6map: true });
return object;
});

Value.getter(Map.prototype, 'size', function () {
Expand Down Expand Up @@ -2173,9 +2174,9 @@
defineProperty(SetShim, symbolSpecies, function (obj) {
var constructor = this;
var prototype = constructor.prototype || Set$prototype;
obj = obj || create(prototype);
defineProperties(obj, { _es6set: true });
return obj;
var object = obj || create(prototype);
defineProperties(object, { _es6set: true });
return object;
});

// Switch from the object backing storage to a full Map.
Expand Down

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