I am practicing Javascript object function. Supposed I have firstName and lastName as two arguments of my function.I want to display like this {"firstName":"tim","lastName":doe} . Here is my code but it printed out undefined. Any idea? Thank you!
function myFunction(firstName, lastName) {
this.name1 = firstName;
this.name2 = lastName;
}
var obj = new myFunction();
console.log(myFunction('tim', 'doe'));
Try this:
console.log(new myFunction('tim', 'doe'));
Or this:
console.log(obj);
You can try this
function myFunction(firstName, lastName) {
this.name1 = firstName;
this.name2 = lastName;
}
var obj = new myFunction('tim', 'doe');
console.log(obj);
You can see this documentation JavaScript Constructors
myFunction -> mold obj -> instance So you can manages is the object, but not the mold this is just a definitionThis kind of function called constructor, and you shouldn't call it directly. You have to use it with new.
console.log(new myFunction('tim', 'doe'));
This will print the result as you expect.
To distinguish the constructors from normal functions, it's better to name it begin with capital letter, like this:
function MyFunction(...) {...}
the undefined you receive is from the function not having a return value, see this post regarding that:Simple function returning 'undefined' value
to get the result you want...
function myFunction(firstName, lastName) {
this.name1 = firstName;
this.name2 = lastName;
}
var obj = new myFunction('tim', 'doe');
console.log(obj);
Let's explore what this line does: console.log(myFunction('tim', 'doe'));
This part: myFunction('tim', 'doe') executes myFunction as a function. Since myFunction does not have a return operator, it's return value is 'undefined' which is javascript's way of saying it doesn't exist. Thus, the word 'undefined' is printed on the console.
Additional tips:
Try adding this line: console.log(typeof myFunction);
This should print 'function'. (May the 'typeof' operator become your best friend)
Try adding a return line as the last line of myFunctions such as:
return 'First name: ' + firstName + " Last name: " + lastName;
However, at this point the 'var obj = new myFunction();' line is unused.
Try adding another line:
console.log(typeof obj);
This should print 'object' which means that 'obj' is just that - an object.
Here is a complete example you can play with:
function myFunction(firstName, lastName) {
this.name1 = firstName;
this.name2 = lastName;
this.getNames = function() {
return 'First name: ' + firstName + " Last name: " + lastName;
}
console.log("This executes once upon instatiation (the line with var obj = new ...)");
return "Return value";
}
var obj = new myFunction('tim', 'doe');
console.log(typeof myFunction);
console.log(typeof obj);
console.log(obj.getNames());
Let me know if any of the above needs clarification. Good luck ...
BTW, this is what the output should look like on the console:
This executes once upon instatiation (the line with var obj = new ...)
script.js:14 function
script.js:15 object
script.js:16 First name: tim Last name: doe
objwhich is the instance of your objectstrictmode, it will throw an exception. Because you didn't callmyFunction()using thenewoperator, in strict mode,thiswill be set toundefinedandthis.name1 = firstName;will throw an exception. Non-strict mode doesn't throw an error becausethiswill point to thewindowobject, but it does not do what you want it to do - thus why we havestrictmode in the first place to keep you from writing and executing bad/wrong code.