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This repository was archived by the owner on Mar 21, 2024. It is now read-only.

JavaScript Style Guide

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A mostly reasonable approach to JavaScript. Based on AirBnB Style Guide and adapted to ComparaOnline practices.

Table of Contents

  1. Types
  2. Objects
  3. Arrays
  4. Strings
  5. Functions
  6. Properties
  7. Variables
  8. Hoisting
  9. Comparison Operators & Equality
  10. Blocks
  11. Comments
  12. Whitespace
  13. Commas
  14. Semicolons
  15. Type Casting & Coercion
  16. Naming Conventions
  17. Accessors
  18. Constructors
  19. Classes
  20. Events
  21. Modules
  22. jQuery
  23. ECMAScript 5 Compatibility
  24. Testing
  25. Performance
  26. Resources
  27. In the Wild
  28. Translation
  29. The JavaScript Style Guide Guide
  30. Contributors
  31. License

Types

  • Primitives: When you access a primitive type you work directly on its value.

    • string
    • number
    • boolean
    • null
    • undefined
    varfoo=1;varbar=foo;bar=9;console.log(foo,bar);// => 1, 9
  • Complex: When you access a complex type you work on a reference to its value.

    • object
    • array
    • function
    varfoo=[1,2];varbar=foo;bar[0]=9;console.log(foo[0],bar[0]);// => 9, 9

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Objects

  • Use the literal syntax for object creation.

    // badvaritem=newObject();// goodvaritem={};
  • Don't usereserved words as keys. It won't work in IE8.More info.

    // badvarsuperman={default:{clark:'kent'},private:true};// goodvarsuperman={defaults:{clark:'kent'},hidden:true};
  • Use readable synonyms in place of reserved words.

    // badvarsuperman={class:'alien'};// badvarsuperman={klass:'alien'};// goodvarsuperman={type:'alien'};

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Arrays

  • Use the literal syntax for array creation.

    // badvaritems=newArray();// goodvaritems=[];
  • Use Array#push instead of direct assignment to add items to an array.

    varsomeStack=[];// badsomeStack[someStack.length]='abracadabra';// goodsomeStack.push('abracadabra');
  • When you need to copy an array use Array#slice.jsPerf

    varlen=items.length;varitemsCopy=[];vari;// badfor(i=0;i<len;i++){itemsCopy[i]=items[i];}// gooditemsCopy=items.slice();
  • To convert an array-like object to an array, use Array#slice.

    functiontrigger(){varargs=Array.prototype.slice.call(arguments);  ...}

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Strings

  • Use single quotes'' for strings.

    // badvarname="Bob Parr";// goodvarname='Bob Parr';// badvarfullName="Bob "+this.lastName;// goodvarfullName='Bob '+this.lastName;
  • Strings longer than 100 characters should be written across multiple lines using string concatenation.

  • Note: If overused, long strings with concatenation could impact performance.jsPerf &Discussion.

    // badvarerrorMessage='This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';// badvarerrorMessage='This is a super long error that was thrown because \of Batman. When you stop to think about how Batman had anything to do \with this, you would get nowhere \fast.';// goodvarerrorMessage='This is a super long error that was thrown because '+'of Batman. When you stop to think about how Batman had anything to do '+'with this, you would get nowhere fast.';
  • When programmatically building up a string, use Array#join instead of string concatenation. Mostly for IE:jsPerf.

    varitems;varmessages;varlength;vari;messages=[{state:'success',message:'This one worked.'},{state:'success',message:'This one worked as well.'},{state:'error',message:'This one did not work.'}];length=messages.length;// badfunctioninbox(messages){items='<ul>';for(i=0;i<length;i++){items+='<li>'+messages[i].message+'</li>';}returnitems+'</ul>';}// goodfunctioninbox(messages){items=[];for(i=0;i<length;i++){// use direct assignment in this case because we're micro-optimizing.items[i]='<li>'+messages[i].message+'</li>';}return'<ul>'+items.join('')+'</ul>';}

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Functions

  • Function expressions:

    // anonymous function expressionvaranonymous=function(){returntrue;};// named function expressionvarnamed=functionnamed(){returntrue;};// immediately-invoked function expression (IIFE)(function(){console.log('Welcome to the Internet. Please follow me.');})();
  • Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.

  • Note: ECMA-262 defines ablock as a list of statements. A function declaration is not a statement.Read ECMA-262's note on this issue.

    // badif(currentUser){functiontest(){console.log('Nope.');}}// goodvartest;if(currentUser){test=functiontest(){console.log('Yup.');};}
  • Never name a parameterarguments. This will take precedence over thearguments object that is given to every function scope.

    // badfunctionnope(name,options,arguments){// ...stuff...}// goodfunctionyup(name,options,args){// ...stuff...}

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Properties

  • Use dot notation when accessing properties.

    varluke={jedi:true,age:28};// badvarisJedi=luke['jedi'];// goodvarisJedi=luke.jedi;
  • Use subscript notation[] when accessing properties with a variable.

    varluke={jedi:true,age:28};functiongetProp(prop){returnluke[prop];}varisJedi=getProp('jedi');

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Variables

  • Always usevar to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that.

    // badsuperPower=newSuperPower();// goodvarsuperPower=newSuperPower();
  • Use onevar declaration per variable.It's easier to add new variable declarations this way, and you never haveto worry about swapping out a; for a, or introducing punctuation-onlydiffs.

    // badvaritems=getItems(),goSportsTeam=true,dragonball='z';// bad// (compare to above, and try to spot the mistake)varitems=getItems(),goSportsTeam=true;dragonball='z';// goodvaritems=getItems();vargoSportsTeam=true;vardragonball='z';
  • Declare unassigned variables last. This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

    // badvari,len,dragonball,items=getItems(),goSportsTeam=true;// badvari;varitems=getItems();vardragonball;vargoSportsTeam=true;varlen;// goodvaritems=getItems();vargoSportsTeam=true;vardragonball;varlength;vari;
  • Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.

    // badfunction(){test();console.log('doing stuff..');//..other stuff..varname=getName();if(name==='test'){returnfalse;}returnname;}// goodfunction(){varname=getName();test();console.log('doing stuff..');//..other stuff..if(name==='test'){returnfalse;}returnname;}// bad - unnecessary function callfunction(){varname=getName();if(!arguments.length){returnfalse;}this.setFirstName(name);returntrue;}// goodfunction(){varname;if(!arguments.length){returnfalse;}name=getName();this.setFirstName(name);returntrue;}

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Hoisting

  • Variable declarations get hoisted to the top of their scope, but their assignment does not.

    // we know this wouldn't work (assuming there// is no notDefined global variable)functionexample(){console.log(notDefined);// => throws a ReferenceError}// creating a variable declaration after you// reference the variable will work due to// variable hoisting. Note: the assignment// value of `true` is not hoisted.functionexample(){console.log(declaredButNotAssigned);// => undefinedvardeclaredButNotAssigned=true;}// The interpreter is hoisting the variable// declaration to the top of the scope,// which means our example could be rewritten as:functionexample(){vardeclaredButNotAssigned;console.log(declaredButNotAssigned);// => undefineddeclaredButNotAssigned=true;}
  • Anonymous function expressions hoist their variable name, but not the function assignment.

    functionexample(){console.log(anonymous);// => undefinedanonymous();// => TypeError anonymous is not a functionvaranonymous=function(){console.log('anonymous function expression');};}
  • Named function expressions hoist the variable name, not the function name or the function body.

    functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionsuperPower();// => ReferenceError superPower is not definedvarnamed=functionsuperPower(){console.log('Flying');};}// the same is true when the function name// is the same as the variable name.functionexample(){console.log(named);// => undefinednamed();// => TypeError named is not a functionvarnamed=functionnamed(){console.log('named');}}
  • Function declarations hoist their name and the function body.

    functionexample(){superPower();// => FlyingfunctionsuperPower(){console.log('Flying');}}
  • For more information refer toJavaScript Scoping & Hoisting byBen Cherry.

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Comparison Operators & Equality

  • Use=== and!== over== and!=.

  • Conditional statements such as theif statement evaluate their expression using coercion with theToBoolean abstract method and always follow these simple rules:

    • Objects evaluate totrue
    • Undefined evaluates tofalse
    • Null evaluates tofalse
    • Booleans evaluate tothe value of the boolean
    • Numbers evaluate tofalse if+0, -0, or NaN, otherwisetrue
    • Strings evaluate tofalse if an empty string'', otherwisetrue
    if([0]){// true// An array is an object, objects evaluate to true}
  • Use shortcuts.

    // badif(name!==''){// ...stuff...}// goodif(name){// ...stuff...}// badif(collection.length>0){// ...stuff...}// goodif(collection.length){// ...stuff...}
  • For more information seeTruth Equality and JavaScript by Angus Croll.

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Blocks

  • Use braces with all multi-line blocks.

    // badif(test)returnfalse;// goodif(test)returnfalse;// goodif(test){returnfalse;}// badfunction(){returnfalse;}// goodfunction(){returnfalse;}
  • If you're using multi-line blocks withif andelse, putelse on the same line as yourif block's closing brace.

    // badif(test){thing1();thing2();}else{thing3();}// goodif(test){thing1();thing2();}else{thing3();}

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Comments

  • Use/** ... */ for multi-line comments. Include a description, specify types and values for all parameters and return values.

    // bad// make() returns a new element// based on the passed in tag name////@param {String} tag//@return {Element} elementfunctionmake(tag){// ...stuff...returnelement;}// good/** * make() returns a new element * based on the passed in tag name * *@param {String} tag *@return {Element} element */functionmake(tag){// ...stuff...returnelement;}
  • Use// for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment.

    // badvaractive=true;// is current tab// good// is current tabvaractive=true;// badfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'vartype=this._type||'no type';returntype;}// goodfunctiongetType(){console.log('fetching type...');// set the default type to 'no type'vartype=this._type||'no type';returntype;}
  • Prefixing your comments withFIXME orTODO helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions areFIXME -- need to figure this out orTODO -- need to implement.

  • Use// FIXME: to annotate problems.

    functionCalculator(){// FIXME: shouldn't use a global heretotal=0;returnthis;}
  • Use// TODO: to annotate solutions to problems.

    functionCalculator(){// TODO: total should be configurable by an options paramthis.total=0;returnthis;}

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Whitespace

  • Use soft tabs set to 2 spaces.

    // badfunction(){∙∙∙∙varname;}// badfunction(){∙varname;}// goodfunction(){∙∙varname;}
  • Place 1 space before the leading brace.

    // badfunctiontest(){console.log('test');}// goodfunctiontest(){console.log('test');}// baddog.set('attr',{age:'1 year',breed:'Bernese Mountain Dog'});// gooddog.set('attr',{age:'1 year',breed:'Bernese Mountain Dog'});
  • Place 1 space before the opening parenthesis in control statements (if,while etc.). Place no space before the argument list in function calls and declarations.

    // badif(isJedi){fight();}// goodif(isJedi){fight();}// badfunctionfight(){console.log('Swooosh!');}// goodfunctionfight(){console.log('Swooosh!');}
  • Set off operators with spaces.

    // badvarx=y+5;// goodvarx=y+5;
  • End files with a single newline character.

    // bad(function(global){// ...stuff...})(this);
    // bad(function(global){// ...stuff...})(this);
    // good(function(global){// ...stuff...})(this);
  • Use indentation when making long method chains. Use a leading dot, whichemphasizes that the line is a method call, not a new statement.

    // bad$('#items').find('.selected').highlight().end().find('.open').updateCount();// bad$('#items').find('.selected').highlight().end().find('.open').updateCount();// good$('#items').find('.selected').highlight().end().find('.open').updateCount();// badvarleds=stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led',true).attr('width',(radius+margin)*2).append('svg:g').attr('transform','translate('+(radius+margin)+','+(radius+margin)+')').call(tron.led);// goodvarleds=stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led',true).attr('width',(radius+margin)*2).append('svg:g').attr('transform','translate('+(radius+margin)+','+(radius+margin)+')').call(tron.led);
  • Leave a blank line after blocks and before the next statement

    // badif(foo){returnbar;}returnbaz;// goodif(foo){returnbar;}returnbaz;// badvarobj={foo:function(){},bar:function(){}};returnobj;// goodvarobj={foo:function(){},bar:function(){}};returnobj;

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Commas

  • Leading commas:Nope.

    // badvarstory=[once,upon,aTime];// goodvarstory=[once,upon,aTime];// badvarhero={firstName:'Bob',lastName:'Parr',heroName:'Mr. Incredible',superPower:'strength'};// goodvarhero={firstName:'Bob',lastName:'Parr',heroName:'Mr. Incredible',superPower:'strength'};
  • Additional trailing comma:Nope. This can cause problems with IE6/7 and IE9 if it's in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 (source):

Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.

```javascript// badvar hero = {  firstName: 'Kevin',  lastName: 'Flynn',};var heroes = [  'Batman',  'Superman',];// goodvar hero = {  firstName: 'Kevin',  lastName: 'Flynn'};var heroes = [  'Batman',  'Superman'];```

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Semicolons

  • Yup.

    // bad(function(){varname='Skywalker'returnname})()// good(function(){varname='Skywalker';returnname;})();// good (guards against the function becoming an argument when two files with IIFEs are concatenated);(function(){varname='Skywalker';returnname;})();

    Read more.

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Type Casting & Coercion

  • Perform type coercion at the beginning of the statement.

  • Strings:

    //  => this.reviewScore = 9;// badvartotalScore=this.reviewScore+'';// goodvartotalScore=''+this.reviewScore;// badvartotalScore=''+this.reviewScore+' total score';// goodvartotalScore=this.reviewScore+' total score';
  • UseparseInt for Numbers and always with a radix for type casting.

    varinputValue='4';// badvarval=newNumber(inputValue);// badvarval=+inputValue;// badvarval=inputValue>>0;// badvarval=parseInt(inputValue);// goodvarval=Number(inputValue);// goodvarval=parseInt(inputValue,10);
  • If for whatever reason you are doing something wild andparseInt is your bottleneck and need to use Bitshift forperformance reasons, leave a comment explaining why and what you're doing.

    // good/** * parseInt was the reason my code was slow. * Bitshifting the String to coerce it to a * Number made it a lot faster. */varval=inputValue>>0;
  • Note: Be careful when using bitshift operations. Numbers are represented as64-bit values, but Bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits.Discussion. Largest signed 32-bit Int is 2,147,483,647:

    2147483647>>0//=> 21474836472147483648>>0//=> -21474836482147483649>>0//=> -2147483647
  • Booleans:

    varage=0;// badvarhasAge=newBoolean(age);// goodvarhasAge=Boolean(age);// goodvarhasAge=!!age;

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Naming Conventions

  • Avoid single letter names. Be descriptive with your naming.

    // badfunctionq(){// ...stuff...}// goodfunctionquery(){// ..stuff..}
  • Use camelCase when naming objects, functions, and instances.

    // badvarOBJEcttsssss={};varthis_is_my_object={};varo={};functionc(){}// goodvarthisIsMyObject={};functionthisIsMyFunction(){}
  • Use PascalCase when naming constructors or classes.

    // badfunctionuser(options){this.name=options.name;}varbad=newuser({name:'nope'});// goodfunctionUser(options){this.name=options.name;}vargood=newUser({name:'yup'});
  • Use a leading underscore_ when naming private properties.

    // badthis.__firstName__='Panda';this.firstName_='Panda';// goodthis._firstName='Panda';
  • When saving a reference tothis use_this.

    // badfunction(){varself=this;returnfunction(){console.log(self);};}// badfunction(){varthat=this;returnfunction(){console.log(that);};}// goodfunction(){var_this=this;returnfunction(){console.log(_this);};}
  • Name your functions. This is helpful for stack traces.

    // badvarlog=function(msg){console.log(msg);};// goodvarlog=functionlog(msg){console.log(msg);};
  • Note: IE8 and below exhibit some quirks with named function expressions. Seehttp://kangax.github.io/nfe/ for more info.

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Accessors

  • Accessor functions for properties are not required.

  • If you do make accessor functions use getVal() and setVal('hello').

    // baddragon.age();// gooddragon.getAge();// baddragon.age(25);// gooddragon.setAge(25);
  • If the property is a boolean, use isVal() or hasVal().

    // badif(!dragon.age()){returnfalse;}// goodif(!dragon.hasAge()){returnfalse;}
  • It's okay to create get() and set() functions, but be consistent.

    functionJedi(options){options||(options={});varlightsaber=options.lightsaber||'blue';this.set('lightsaber',lightsaber);}Jedi.prototype.set=function(key,val){this[key]=val;};Jedi.prototype.get=function(key){returnthis[key];};

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Constructors

  • Assign methods to the prototype object, instead of overwriting the prototype with a new object. Overwriting the prototype makes inheritance impossible: by resetting the prototype you'll overwrite the base!

    functionJedi(){console.log('new jedi');}// badJedi.prototype={fight:functionfight(){console.log('fighting');},block:functionblock(){console.log('blocking');}};// goodJedi.prototype.fight=functionfight(){console.log('fighting');};Jedi.prototype.block=functionblock(){console.log('blocking');};
  • Methods can returnthis to help with method chaining.

    // badJedi.prototype.jump=function(){this.jumping=true;returntrue;};Jedi.prototype.setHeight=function(height){this.height=height;};varluke=newJedi();luke.jump();// => trueluke.setHeight(20);// => undefined// goodJedi.prototype.jump=function(){this.jumping=true;returnthis;};Jedi.prototype.setHeight=function(height){this.height=height;returnthis;};varluke=newJedi();luke.jump().setHeight(20);
  • It's okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

    functionJedi(options){options||(options={});this.name=options.name||'no name';}Jedi.prototype.getName=functiongetName(){returnthis.name;};Jedi.prototype.toString=functiontoString(){return'Jedi - '+this.getName();};

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Classes

  • Simple classes should be defined using the anonymous function syntax:

    varClassName=(function(){// body here ...});
  • Variables should be defined in the lowest scope possible:

    // badvarClassName=(function(){varvarTotal=$('#some-id').data('total');varsquareTotal=functionsquareTotal(){returnvarTotal*varTotal;};});// goodvarClassName=(function(){varsquareTotal=functionsquareTotal(){varvarTotal=$('#some-id').data('total');returnvarTotal*varTotal;};});// badvarClassName=(function(){vardecrTotal=functiondecrTotal(){varvarTotal=$('#some-id').data('total');returnvarTotal-1;};varincrTotal=functionincrTotal(){varvarTotal=$('#some-id').data('total');returnvarTotal+1;};});// goodvarClassName=(function(){varvarTotal=$('#some-id').data('total');vardecrTotal=functiondecrTotal(){returnvarTotal-1;};varincrTotal=functionincrTotal(){returnvarTotal+1;};});

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Events

  • When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass a hash instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

    // bad$(this).trigger('listingUpdated',listing.id);...$(this).on('listingUpdated',function(e,listingId){// do something with listingId});

    prefer:

    // good$(this).trigger('listingUpdated',{listingId :listing.id});...$(this).on('listingUpdated',function(e,data){// do something with data.listingId});

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Modules

  • The module should start with a!. This ensures that if a malformed module forgets to include a final semicolon there aren't errors in production when the scripts get concatenated.Explanation

  • The file should be named with camelCase, live in a folder with the same name, and match the name of the single export.

  • Add a method callednoConflict() that sets the exported module to the previous version and returns this one.

  • Always declare'use strict'; at the top of the module.

    // fancyInput/fancyInput.js!function(global){'use strict';varpreviousFancyInput=global.FancyInput;functionFancyInput(options){this.options=options||{};}FancyInput.noConflict=functionnoConflict(){global.FancyInput=previousFancyInput;returnFancyInput;};global.FancyInput=FancyInput;}(this);

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jQuery

  • Prefix jQuery object variables with a$.

    // badvarsidebar=$('.sidebar');// goodvar$sidebar=$('.sidebar');
  • Cache jQuery lookups.

    // badfunctionsetSidebar(){$('.sidebar').hide();// ...stuff...$('.sidebar').css({'background-color':'pink'});}// goodfunctionsetSidebar(){var$sidebar=$('.sidebar');$sidebar.hide();// ...stuff...$sidebar.css({'background-color':'pink'});}
  • For DOM queries use Cascading$('.sidebar ul') or parent > child$('.sidebar > ul').jsPerf

  • Usefind with scoped jQuery object queries.

    // bad$('ul','.sidebar').hide();// bad$('.sidebar').find('ul').hide();// good$('.sidebar ul').hide();// good$('.sidebar > ul').hide();// good$sidebar.find('ul').hide();

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ECMAScript 5 Compatibility

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Testing

  • Yup.

    function(){returntrue;}

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Performance

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Resources

Read This

Tools

Other Style Guides

Other Styles

Further Reading

Books

Blogs

Podcasts

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In the Wild

This is a list of organizations that are using this style guide. Send us a pull request or open an issue and we'll add you to the list.

Translation

This style guide is also available in other languages:

The JavaScript Style Guide Guide

Contributors

License

(The MIT License)

Copyright (c) 2014 Airbnb

Permission is hereby granted, free of charge, to any person obtaininga copy of this software and associated documentation files (the'Software'), to deal in the Software without restriction, includingwithout limitation the rights to use, copy, modify, merge, publish,distribute, sublicense, and/or sell copies of the Software, and topermit persons to whom the Software is furnished to do so, subject tothe following conditions:

The above copyright notice and this permission notice shall beincluded in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OFMERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANYCLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THESOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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};

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