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StrongMode &
SoundScript
@yosuke_furukawa
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StrictMode
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“use strict”
“use strict”;
function foo() {
abc = “def”; // throw TypeError when variables does not have
`var` declaration.
var obj1 = {};
Object.defineProperty(obj1, "x", { value: 42, writable: false });
obj1.x = 9; // throw TypeError when you substitute unwritable
variable
with (obj1) { // Restrict `with`
x;
}
}
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make your code strict
AND
those rules can prevent
some mistakes
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In TC39, Google v8
developers suggested more
stricter mode.
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Strong Mode
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StrongMode Motivation
• Usability especially `maintainability`
• Performance especially `predictability`
• embrace ES2015
• avoid bad legacy codes
• (AND types)
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“use strong”
“use strong”;
function foo() {
// v8 4.2 enabled (io.js v2.0 also)
var obj1 = ‘aaa’; // Restrict var (use let or const instead)
let args = arguments; // Restrict arguments (use Rest Params …args)
if (obj1 == ‘aaa’) {} // Restrict == (use strict equal ===)
if (obj1 === ‘aaa’); // Restrict empty if and for
for (let n in [1,2,3]) // Restrict for-in (use for-of instead)
delete obj1.key // Restrict delete operator (use Map/Set instead)
// v8 4.5 enabled TODO: try
let undefined = ‘aaa’; // Restrict undefined binding
let obj3 = { foo : ‘aaa’};
obj3.bar = ‘123’; // Restrict undefined property access
}
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Demo
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“use strong”
“use strong”;
function foo() {
// v8 4.2 enabled (io.js v2.0 also)
var obj1 = ‘aaa’; // Restrict var (use let or const instead)
let args = arguments; // Restrict arguments (use Rest Params …args)
if (obj1 == ‘aaa’) {} // Restrict == (use strict equal ===)
if (obj1 === ‘aaa’); // Restrict empty if and for
for (let n in [1,2,3]) // Restrict for-in (use for-of instead)
delete obj1.key // Restrict delete operator (use Map/Set instead)
// v8 4.5 enabled TODO: try
let undefined = ‘aaa’; // Restrict undefined binding
let obj3 = { foo : ‘aaa’};
obj3.bar = ‘123’; // Restrict undefined property access
}
6TBCJMJUZ&4
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“use strong”
“use strong”;
function foo() {
// v8 4.2 enabled (io.js v2.x also)
var obj1 = ‘aaa’; // Restrict var (use let or const instead)
let args = arguments; // Restrict arguments (use Rest Params …args)
if (obj1 == ‘aaa’) {} // Restrict == (use strict equal ===)
if (obj1 === ‘aaa’); // Restrict empty if and for
for (let n in [1,2,3]) // Restrict for-in (use for-of instead)
delete obj1.key // Restrict delete operator (use Map/Set instead)
// v8 4.5 enabled TODO: try
let undefined = ‘aaa’; // Restrict undefined binding
let obj3 = { foo : ‘aaa’};
obj3.bar = ‘123’; // Restrict undefined property access
}
6TBCJMJUZ&4 1FSG1SFEJDUBCJMJUZ
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Predictability
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Optimizer Killer -
property assign/delete
function Point(x, y) {
this.x = x;
this.y = y;
}
let p1 = new Point(1, 2);
let p2 = new Point(3, 4);
p2.name = “test”; // <—- Optimizer Killer(eliminate hidden
class) !!!!!!!
delete p1.x // <—- Optimizer Killer !!!!!!
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Optimizer Killer - arguments
function assignToArgs() {
arguments = 3 // <— Optimizer Killer !!!!
return arguments;
}
function leaksArguments2() {
var args = [].slice.call(arguments); // <— Optimizer Killer !!!!
return args;
}
// Actually, arguments should not be assigned any params and should never use
arguments directly !!!!!
// arguments.lenght, arguments[i] are better.
// see: https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-
managing-arguments
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“use strong” kills
Optimizer killer
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Should we use “use strong”
at the moment???
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Not now.
please wait.
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Reason
• Experimental feature
• ES6 code is not so faster than ES5 code.
• v8 optimizer is not mature.
• ScalaJS author measures the performance between
ES5 and ES6 and StrongMode
• https://groups.google.com/forum/?hl=ja#!topic/
strengthen-js/Bfa_HPhVbiY
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SoundScript
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Not Implemented yet.
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SoundScript will be/have …
• Based on TypeScript
• interop with old code
• Aggressive optimisations
• Optional Types
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SoundScript
“use stricter+types”; // use stricter seals every class and instance
class Point {
constructor(x : int, y : int) { // aggressive optimize
this.x = x;
this.y = y;
}
move(dx : int, dy : int) : void {
···
}
}
function norm(p : Point) : double {
return Math.sqrt(p.x*p.x + p.y*p.y);
}
http://www.2ality.com/2015/02/soundscript.html
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Conclusion
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Conclusion
• StrongMode is more stricter than StrictMode.
• StrongMode is Optimizer friendly.
• SoundScript looks like TypeScript, flow type
• But SoundScript gives optimizer hints
• The Future Typed JavaScript looks bright….
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Reference
• https://esdiscuss.org/notes/2015-01/JSExperimentalDirections.pdf
• https://developers.google.com/v8/experiments
• https://github.com/iojs/io.js/pull/933#discussion_r26895522
• http://incaseofstairs.com/2015/06/es6-feature-performance/
• https://github.com/v8/v8-git-mirror/blob/
85dbfb9a389e7b21bd2a63862202ee97fc5d7982/src/messages.h
• https://github.com/petkaantonov/bluebird/wiki/Optimization-killers
• http://efcl.info/2014/09/13/v8-optimization-memo/
• http://mrale.ph/irhydra/2/