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. .

, ?

(callable) ' . .

name

name .

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

alert(sayHi.name); // sayHi

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

alert(sayHi.name); // sayHi (   !)

function f(sayHi = function() {}) {
  alert(sayHi.name); // sayHi ( !)
}

f();

'contextual name . .

name .

let user = {

  sayHi() {
    // ...
  },

  sayBye: function() {
    // ...
  }

}

alert(user.sayHi.name); // sayHi
alert(user.sayBye.name); // sayBye

. , name . .

//    
let arr = [function() {}];

alert( arr[0].name ); // < >
//      name

length

length . .

function f1(a) {}
function f2(a, b) {}
function many(a, b, ...more) {}

alert(f1.length); // 1
alert(f2.length); // 2
alert(many.length); // 2

, length (type introspection) .

question handler ask .

ask . ask .

, OK
, OK Cancel

handler.length handler .

, ask handler.length .

function ask(question, ...handlers) {
  let isYes = confirm(question);

  for(let handler of handlers) {
    if (handler.length == 0) {
      if (isYes) handler();
    } else {
      handler(isYes);
    }
  }

}

//  OK  ,     
//  Cancel  ,    
ask(" ?", () => alert('OK .'), result => alert(result));

( length ) (polymorphism) . .

counter .

function sayHi() {
  alert("Hi");

  //     .
  sayHi.counter++;
}
sayHi.counter = 0; // 

sayHi(); // Hi
sayHi(); // Hi

alert( ` : ${sayHi.counter}` ); //  : 2

sayHi.counter = 0 counter . counter let counter .

, . . .

. counter .

function makeCounter() {

  // let count = 0   () 

  function counter() {
    return counter.count++;
  };

  counter.count = 0;

  return counter;
}

let counter = makeCounter();
alert( counter() ); // 0
alert( counter() ); // 1

count .

count . count . count . count .

function makeCounter() {

  function counter() {
    return counter.count++;
  };

  counter.count = 0;

  return counter;
}

let counter = makeCounter();

counter.count = 10;
alert( counter() ); // 10

(Named Function Expression, NFE) .

, .

          let sayHi = function(who) {
  alert(`Hello, ${who}`);
};
        

          let sayHi = function func(who) {
  alert(`Hello, ${who}`);
};
        

? "func" ?

. function "func" .

sayHi() .

let sayHi = function func(who) {
  alert(`Hello, ${who}`);
};

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

func . .

sayHi . sayHi who , "Guest" .

let sayHi = function func(who) {
  if (who) {
    alert(`Hello, ${who}`);
  } else {
    func("Guest"); // func   .
  }
};

sayHi(); // Hello, Guest

//    func   .
func(); // Error, func is not defined (        .)

sayHi func ?

          let sayHi = function(who) {
  if (who) {
    alert(`Hello, ${who}`);
  } else {
    sayHi("Guest");
  }
};
        

sayHi . , null .

let sayHi = function(who) {
  if (who) {
    alert(`Hello, ${who}`);
  } else {
    sayHi("Guest"); // TypeError: sayHi is not a function
  }
};

let welcome = sayHi;
sayHi = null;

welcome(); //  sayHi    !

sayHi . (local) sayHi sayHi , sayHi null .

let sayHi = function func(who) {
  if (who) {
    alert(`Hello, ${who}`);
  } else {
    func("Guest"); //    .
  }
};

let welcome = sayHi;
sayHi = null;

welcome(); // Hello, Guest (   )

"func" (function-local) . . .

sayHi welcome func ' .

' , . .

. .

name . , ( ) .
length .

. .

. jQuery $ . lodash _ _.clone, _.keyBy . lodash . . .

, .

counter

makeCounter() .

counter() .
counter.set(value) counter value .
counter.decrease() counter 1 .

sandbox , .

: counter . .

count , counter . counter count .

          function makeCounter() {
  let count = 0;

  function counter() {
    return count++;
  }

  counter.set = value => count = value;

  counter.decrease = () => count--;

  return counter;
}
        

sum .

          sum(1)(2) == 3; // 1 + 2
sum(1)(2)(3) == 6; // 1 + 2 + 3
sum(5)(-1)(2) == 6
sum(6)(-1)(-2)(-3) == 0
sum(0)(1)(2)(3)(4)(5) == 15
        

: - .

sum .
sum .
== . , - , - .

function sum(a) {

  let currentSum = a;

  function f(b) {
    currentSum += b;
    return f;
  }

  f.toString = function() {
    return currentSum;
  };

  return f;
}

alert( sum(1)(2) ); // 3
alert( sum(5)(-1)(2) ); // 6
alert( sum(6)(-1)(-2)(-3) ); // 0
alert( sum(0)(1)(2)(3)(4)(5) ); // 15

sum . sum f .

f currentSum .

f .

          function f(b) {
  currentSum += b;
  return f(); // <--  
}
        

f .

          function f(b) {
  currentSum += b;
  return f; // <--     .
}
        

f , . toString currentSum () . Symbol.toPrimitive valueOf .

          function sum(a) {

  let currentSum = a;

  function f(b) {
    currentSum += b;
    return f;
  }

  f.toString = function() {
    return currentSum;
  };

  return f;
}
        