• A function with an empty return or without it returns undefined.
  • JavaScript assumes a semicolon after return so never add a newline between return and the value.
• There is another syntax for creating a function that is called a

// This approach of defining a function is called "**Function Expression".**
let sayHi = function() {
  console.log("Hello");
};
console.log(sayHi); // Shows the function code.
sayHi(); // Hello

• Function Expression have a semicolon at the end, but Function Declaration does not.
  • Function Expression is used inside a statement and it’s not a code block.
  • So the semicolon is not related to the Function Expression itself in any way, it just terminates the statement.
• A Function Expression is created when the execution reaches it and is usable from then on.
  • Once the execution flow passes to the right side of the assignment, the function is created and can be used.
• A Function Declaration is usable in the whole script/code block.
  • When JavaScript prepares to run the script or a code block, it first looks for Function Declarations in it and creates the functions.
  • After all of the Function Declarations are processed, the execution goes on.
  • As a result, a function declared as a Function Declaration can be called earlier than it is defined.
• When a Function Declaration is made within a code block, it is visible everywhere inside that block but not outside of it.

Local & Global Variables

• A variable declared inside a function is only visible inside that function.
• A function can access an outer variables.
  • The outer variable is only used if there’s no local one.

Parameters

• If a parameter is not provided, then its value becomes undefined.
• If we want to use a “default” text in this case, then we can specify it after for each argument.

function showMessage1(from, text) {
  // If text is falsy then text gets the "default" value.
  text = text || 'no text given';
}

function showMessage2(from, text = "no text given") {
  alert(from + ": " + text);
}

function showMessage3(from, text = anotherFunction()) {
  // anotherFunction() only executed if no text given its result becomes the value of text.
}

Arrow Functions

• Arrow functions do not have this.
  • If this is accessed, it is taken from the outside.
  • Not having this naturally means another limitation: arrow functions can’t be used as constructors.
  • They can’t be called with new.

let sayHi = () => alert("Hello!");
let sum = (a, b) => a + b;
let sum = (a, b) => {  // the curly brace opens a multiline function
  let result = a + b;
  return result; // if we use curly braces, use return to get results
};
let double = n => n * 2;

• There’s a subtle difference between an arrow function => and a regular function called with .bind(this):
  • While .bind(this) creates a “bound version” of the function, the arrow => doesn’t create any binding.
• Arrow functions also have no arguments variable.

Rest Parameters