JavaScript line with semicolon behaving strange

Question

I am facing a strange issue with this code. I can fix it easily but I would like to know why this is happening.

If I comment out child below I get undefined error in IFFY. The function is located in global scope and should work everywhere. Not sure why?

JSFiddle link

parent=function(){   
    parent.prototype.method1= function() {}
    parent.prototype.property = true;
}

child=function() {
    parent.call(this);    
    child.prototype = new parent(); 
}

child; // This is important, if I remove this, I get an undefined error in IFFY ?


(function(){
    var instance1 = new child();
    console.log( instance1 );  // Empty Object

    var instance2 = new child();
    console.log( instance2 ); // Object is not empty

}());

(there are also other issues with this code which I asked about in an extra question)

You probably meant to manipulate each "type"s `prototype` outside the function. It will probably solve your problems — Amit, Jun 28 '15 at 13:23
This is just automatic semicolon insertion messing with you. Watch those semicolons. — Benjamin Gruenbaum, Jun 28 '15 at 13:32
Are you aware that the `parent.prototype` property refers to the prototype used when instantiating a new object with `new parent()`, and NOT the actual prototype of the instance itself? In order to access the actual prototype you'd need to use Object.getPrototypeOf(), or the (non-standard) `__proto__` property. — Matt Browne, Jun 28 '15 at 13:34
@JS-JMS-WEB because the function itself is defining the Constructor. The prototype is defined based off of the Constructor. — Danny Blue, Jun 28 '15 at 13:36

score 4 · Answer 1 · answered Jun 28 '15 at 13:34

4

This is just automatic semicolon insertion messing with you. When you omit the child you get:

child=function() {
    parent.call(this);    
    child.prototype = new parent(); 
}

(function(){
    var instance1 = new child();
    console.log( instance1 );  // Empty Object

    var instance2 = new child();
    console.log( instance2 ); // Object is not empty

}());

Which is parsed as:

child=function() {
    parent.call(this);    
    child.prototype = new parent(); 
}(function(){ // FUNCTION CALL
    var instance1 = new child();
    console.log( instance1 );  // Empty Object

    var instance2 = new child();
    console.log( instance2 ); // Object is not empty
}());

answered Jun 28 '15 at 13:34

Benjamin Gruenbaum

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But I get the similar result when I do `var parent=function(){ parent.prototype.method1= function() {} parent.prototype.property = true; }, child=function() { child.prototype = new parent(); };new child(); // normal new child(); //parent's prototype` – Amit Joki Jun 28 '15 at 13:37
"if I remove this, I get an undefined error in IFFY ?" my answer solves _that part_. – Benjamin Gruenbaum Jun 28 '15 at 13:41

JavaScript line with semicolon behaving strange

1 Answers1