javascript prototype declaration -

there 2 pieces of codes.

why first 1 correct second 1 incorrect?

what's wrong this.prototype?

function person(name, age, job){      this.name = name;     this.age = age;     this.job = job;      if (typeof this.sayname != "function"){          person.prototype.sayname = function(){             alert(this.name);         };      } } 

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function person(name, age, job){      this.name = name;     this.age = age;     this.job = job;      if (typeof this.sayname != "function"){          this.prototype.sayname = function(){             alert(this.name);         };      } } 

both of them incorrect. more on later.

the second 1 incorrect because objects don't have prototype property. only functions have such property.

in second example, this object, this.prototype undefined. in first example setting person.prototype , person function, "good".

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why first example still wrong? because don't have reason extend prototype object inside constructor. constructor should contain instance specific code , prototype object should hold properties shared instances.

so above example correctly written as:

function person(name, age, job){     this.name = name;     this.age = age;     this.job = job; }  person.prototype.sayname = function(){     alert(this.name); };