lecture-13.md

Episode 13 : First Class Functions ft. Anonymous Functions

Functions are heart ♥ of Javascript.

Q: What is Function statement?

Below way of creating function are function statement.

function a() {
  console.log("Hello");
}
a(); // Hello

Q: What is Function Expression?

Assigning a function to a variable. Function acts like a value.

var b = function() {
  console.log("Hello");
}
b();

Q: Difference between function statement and expression

The major difference between these two lies in Hoisting.

a(); // "Hello A"
b(); // TypeError
function a() {
  console.log("Hello A");
}
var b = function() {
  console.log("Hello B");
}
// Why? During mem creation phase a is created in memory and function assigned to a. But b is created like a variable (b:undefined) and until code reaches the function()  part, it is still undefined. So it cannot be called.

Q: What is Function Declaration?

Other name for function statement.

Q: What is Anonymous Function?

A function without a name.

function () {

}// this is going to throw Syntax Error - Function Statement requires function name.

• They don't have their own identity. So an anonymous function without code inside it results in an error.
• Anonymous functions are used when functions are used as values eg. the code sample for function expression above.

Q: What is Named Function Expression?

Same as Function Expression but function has a name instead of being anonymous.

var b = function xyz() {
  console.log("b called");
}
b(); // "b called"
xyz(); // Throws ReferenceError:xyz is not defined.
// xyz function is not created in global scope. So it can't be called.

Q: Parameters vs Arguments?

var b = function(param1, param2) { // labels/identifiers are parameters
  console.log("b called");
}
b(arg1, arg2); // arguments - values passed inside function call

Q: What is First Class Function aka First Class Citizens?

We can pass functions inside a function as arguments and /or return a function(HOF). These ability are altogether known as First class function. It is programming concept available in some other languages too.

var b = function(param1) {
  console.log(param1); // prints " f() {} "
}
b(function(){});

// Other way of doing the same thing:
var b = function(param1) {
  console.log(param1);
}
function xyz(){
}
b(xyz); // same thing as prev code

// we can return a function from a function:
var b = function(param1) {
  return function() {
  }  
}
console.log(b()); //we log the entire fun within b. 

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