javascript: Callback function code considered as string

Am a newbie in javascript, please help me understand below case where the callback function code is being considered as a string and passed as argument instead of passing the value of the callback function as an argument:

//Case3: Create Callback function in the argument section of calling statement.

function child3(callback,arg2=3) {
     return console.log("Case3: callback function - parent function out:", callback+arg2,"\n");
}

child3(parent3=()=>{
   let a=1;
   let b=1;
   return a+b;
},2);

Output:

Case3: callback function - parent function out: ()=>{
   let a=1;
   let b=1;
   return a+b;
}2

>Solution :

When you use +, there are two possibilities:

• If both operands are numbers or BigInts, they are added together
• Otherwise, both operands are concatenated together into a string

If you do

callback+arg2

and callback isn’t a number, the result will be a concatenation of it and arg2. In your code, callback is not a number; it’s a function. You probably wanted to call the callback instead of concatenating it – eg, callback().

Another issue is that

child3(parent3=()=>{

should almost certainly be

child3(()=>{

unless you deliberately wanted to both create a new global function named parent3 and pass that to child3.

function child3(callback, arg2 = 3) {
  console.log(callback() + arg2);
}

child3(() => {
  let a = 1;
  let b = 1;
  return a + b;
}, 2);