No need to declare types in parameters in javascript?

Question

I noticed while learning javascript that it does not require you to declare types in the parameter of a function like java does. How does the compiler know what type is passed? is there any type checking? Lets say my function handles numbers instead of strings and I pass a string?

Also normally in javascript do you not need to specify in the parameters that you are passing a function? Again how does the compiler know?

function invokeAdd(a,b){
    return a()+b();
}

Answer 1

To target your specific code blocks, kindly take a look at the below code

function sumA()
{
  return 2;
}

function sumB()
{
  return 3;
}

function invokeAdd(a,b){
  return a()+b();
}

console.log(invokeAdd(sumA, sumB)); //5

The above code will resulted in 5. Let's break them into steps.

When we first invoke the function invokeAdd, we passed in 2 variable sumA to be first parameter which is a and sumB as second parameter which is b. Now invokeAdd becomes something as below:

return sumA() + sumB() 

and hence return 5.

Now let's take another example where we switch the paramter to numbers.

console.log(invokeAdd(1, 2));

We now invoke the function invokeAdd and passing in 2 parameters, 1 and 2 respectively and our invokeAdd has turned into below

return 1() + 2();

Because 1 is not a function and we have a parenthesis () beside it, JS engine will throw an error. () means execute the function

TypeError: a is not a function

If you wish to learn more about types in javascript, You-don't-know-JS is a very good book with detailed explainations