Why setter with set keyword and function expressed in ES5 throws syntax error?

Question

Why defining a setter throws syntax error when the function is expressed the ES5 way but works when expressed in ES6.

Setter with ES5 syntax(Doesn't work)

var person = {
  firstName: "John",
  lastName : "Doe",
  language : "",
  set setLanguage: function(val) {
     this.language = val    
  },
  get getLanguage() {
     return this.language   
  }     
}

person.setLanguage = 'EN'
person.getLanguage
`Uncaught SyntaxError: Unexpected token :`

Setter with ES6 syntax (Works)

var person = {
  firstName: "John",
  lastName : "Doe",
  language : "",
  set setLanguage(val) {
     this.language = val    
  },
  get getLanguage() {
     return this.language   
  }     
}

person.setLanguage = 'EN'
person.getLanguage

Thanks

Answer 1

Your second example is not ES6 specific, it's valid ES5 syntax:

set setLanguage(val) {
    this.language = val;    
},

From Object Initialiser's specification (ES5)

PropertyAssignment :
 PropertyName : AssignmentExpression
 get PropertyName ( ) { FunctionBody }
 set PropertyName ( PropertySetParameterList ) { FunctionBody }

As you can see, there's a : after PropertyName, but there's no : or function in getter or setter

Update:

You propably confused concepts of computed property names and getters/setters

ES6 introduced computed property names, so you can do this (this snippet propably won't work in older browsers, like IE):

const person = {
  language: "",
  setLanguage(val){
    this.language = val;
  },
  getLanguage(){
    return this.language;
  }
}

person.setLanguage("EN");

console.log("person.language", person.language);
console.log("person.getLanguage()",person.getLanguage())

Note lack of set or get keywords and that you have to call person.setLanguage("EN"); instead of person.setLanguage = "EN";

In ES5 you'll have to (change const to var and) use:

setLanguage: function(val) {
    this.language = val;    
},
getLanguage: function() {
    return this.language;    
},