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JavaScript

          function sayHi() {
  alert( "Hello" );
}
        

          let sayHi = function() {
  alert( "Hello" );
};
        

sayHi function() { alert("Hello"); }

=

function

sayHi

alert

function sayHi() {
  alert( "Hello" );
}

alert( sayHi ); //

sayHi JavaScript

JavaScript

sayHi()

function sayHi() {   // (1) 
  alert( "Hello" );
}

let func = sayHi;    // (2) 

func(); // Hello     // (3) 
sayHi(); // Hello    //

(1) sayHi
(2) sayHi funcsayHi func = sayHi() sayHi() func sayHi
sayHi() func()

sayHi

          let sayHi = function() { // (1) 
  alert( "Hello" );
};

let func = sayHi;
// ...
        

;

          function sayHi() {
  // ...
}

let sayHi = function() {
  // ...
};
        

let sayHi = ...; function() {} ;

let sayHi = 5;

ask(question, yes, no)

question
yes: Yes
no: No

question yes() no()

function ask(question, yes, no) {
  if (confirm(question)) yes()
  else no();
}

function showOk() {
  alert( "You agreed." );
}

function showCancel() {
  alert( "You canceled the execution." );
}

//  showOk  showCancel  ask
ask("Do you agree?", showOk, showCancel);

confirm

ask showOk showCancel

showOk yes showCancel no

function ask(question, yes, no) {
  if (confirm(question)) yes()
  else no();
}

ask(
  "Do you agree?",
  function() { alert("You agreed."); },
  function() { alert("You canceled the execution."); }
);

ask(...) ask

JavaScript

action

vs

          // 
function sum(a, b) {
  return a + b;
}
        

=

          // 
let sum = function(a, b) {
  return a + b;
};
        

JavaScript

let sum = function

JavaScript

sayHi("John"); // Hello, John

function sayHi(name) {
  alert( `Hello, ${name}` );
}

sayHi JavaScript

sayHi("John"); // error!

let sayHi = function(name) {  // (*) no magic any more
  alert( `Hello, ${name}` );
};

(*)

age welcome()

let age = prompt("What is your age?", 18);

// 
if (age < 18) {

  function welcome() {
    alert("Hello!");
  }

} else {

  function welcome() {
    alert("Greetings!");
  }

}

// 
welcome(); // Error: welcome is not defined

let age = 16; //  16 

if (age < 18) {
  welcome();               // \   ()
                           //  |
  function welcome() {     //  |
    alert("Hello!");       //  |  
  }                        //  |
                           //  |
  welcome();               // /   ()

} else {

  function welcome() {
    alert("Greetings!");
  }
}

// 


welcome(); // Error: welcome is not defined

welcome if

let age = prompt("What is your age?", 18);

let welcome;

if (age < 18) {

  welcome = function() {
    alert("Hello!");
  };

} else {

  welcome = function() {
    alert("Greetings!");
  };

}

welcome(); //

?

let age = prompt("What is your age?", 18);

let welcome = (age < 18) ?
  function() { alert("Hello!"); } :
  function() { alert("Greetings!"); };

welcome(); //

function f() {} let f = function() {}