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To the user, they appear the same as any typical property. However, internally a custom function you provide is used to determine the value when the property is accessed (the getter), and to preform any necessary changes when the property is assigned (the setter).")]),t._v(" "),a("p",[t._v("In a "),a("code",[t._v("class")]),t._v(" definition, a getter is written like a no-argument method prefixed by the "),a("code",[t._v("get")]),t._v(" keyword. A setter is similar, except that it accepts one argument (the new value being assigned) and the "),a("code",[t._v("set")]),t._v(" keyword is used instead.")]),t._v(" "),a("p",[t._v("Here's an example class which provides a getter and setter for its "),a("code",[t._v(".name")]),t._v(" property. Each time it's assigned, we'll record the new name in an internal "),a("code",[t._v(".names_")]),t._v(" array. 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most common obstacles using classes is finding the proper approach to handle private states.\nThere are 4 common solutions for handling private states:")]),t._v(" "),a("h3",{attrs:{id:"using-symbols"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#using-symbols"}},[t._v("#")]),t._v(" Using Symbols")]),t._v(" "),a("p",[t._v("Symbols are new primitive type introduced on in ES2015,\nas defined at "),a("a",{attrs:{href:"https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Symbol",target:"_blank",rel:"noopener noreferrer"}},[t._v("MDN"),a("OutboundLink")],1)]),t._v(" "),a("blockquote"),t._v(" "),a("p",[t._v("A symbol is a unique and immutable data type that may be used as an identifier for object properties.")]),t._v(" "),a("p",[t._v("When using symbol as a property key, it is not enumerable.")]),t._v(" "),a("p",[t._v("As such, they won't be revealed using "),a("code",[t._v("for var in")]),t._v(" or "),a("code",[t._v("Object.keys")]),t._v(".")]),t._v(" "),a("p",[t._v("Thus we can use symbols to store private data.")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" topSecret "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("Symbol")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v("'topSecret'")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// our private key; will only be accessible on the scope of the module file")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("export")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("SecretAgent")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("constructor")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("secret")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("this")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("topSecret"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" secret"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// we have access to the symbol key (closure)")]),t._v("\n  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private property.")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("import")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("SecretAgent"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("from")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token string"}},[t._v("'SecretAgent.js'")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" agent "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("SecretAgent")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v("'steal all the ice 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