callback method in javascript

Question

Write a function that accepts a callback method and returns a function that will execute only if all the parameters passed to it are defined, otherwise it returns nothing. Here's what I wrote why is not it working?

function sumof(checkof) {
 sum = 0
 for (a in this.myNumbers)
 if (checkof(a) == 1)
 sum += a
 else
 return null
 return sum
}
function check(a) {
 if (a == null)
 return 0
 else
 return 1
}
function init() {
 let myNumbers = [22, 1, 8, 4, 17];
 var x = myNumbers.sumof(check)
 alert(x)
}
init()

• AKX: You're failing to tell us what exactly is wrong – "why isn't it working" is never really enough.
• AKX: (But why would you imagine myNumbers.sumof(...) would work?)
• Itamar: change it to sumof(myNumbers)\
• isherwood: Your function is not a method of your array. You'll need to separate them in the function call.
• ADyson: @Itamar no, because the first argument to sumof is supposed to be a callback function.

Answer 1

myNumbers.sumof doesn't work because sumof is a standalone function, not part of Array.

this.myNumbers also makes no sense because myNumbers isn't global.

The simplest solution is to pass myNumbers into your sumof function:

function sumof(checkof, myNumbers) {
 sum = 0
 for (a in myNumbers)
 if (checkof(a) == 1)
 sum += a
 else
 return null
 return sum
}
function check(a) {
 if (a == null)
 return 0
 else
 return 1
}
function init() {
 let myNumbers = [22, 1, 8, 4, 17];
 var x = sumof(check, myNumbers)
 alert(x)
}
init()

Answer 2

you are calling it like a array property method. but you have defined that function as a standalone function. you should add your function as a array property method.

Array.prototype.sumof = function(checkof) {
 if ( ! this.length ) {
 return null;
 }
 sum = 0;
 for (i = 0; i < this.length; i++) {
 a = this[i];
 if (checkof(a) == 1) {
 sum += a;
 }
 }
 return sum;
};
function check(a) {
 if (a == null)
 return 0;
 else
 return 1;
}
function init() {
 let myNumbers = [22, 1, 8, 4, 17];
 var x = myNumbers.sumof(check);
 console.log(x);
 myNumbers = [22, 1, null, 4, 17];
 x = myNumbers.sumof(check);
 console.log(x);
}
init();

also if you are using Array property method, you should avoid using for..in. because when you looping through this, it will also have your new method too. use for with array length.