JavaScript (JS )

? JavaScript , . , . JavaScript , , .

. JavaScript 1995 , Netscape. Netscape 2 1996 . LiveScript, , Java Sun Microsystem . .

Microsoft JScript, IE3. Netscape Ecma International, , ECMAScript 1997. ECMAScript edition 3 1999, . , - . ECMAScript 5 ( 2009) ECMAScript 6 ( 2015).

, JavaScript (input) (output). , , . , JavaScript Adobe Acrobat, Photoshop, Yahoo!'s Widget engine, , node.js.


`a.toString()`	, .
`a.toLocaleString()`	.
`a.concat(item1[, item2[, ...[, itemN]]])`	.
`a.join(sep)`	, `sep`
`a.pop()`	.
`a.push(item1, ..., itemN)`	.
`a.reverse()`	.
`a.shift()`	.
`a.slice(start[, end])`	.
`a.sort([cmpfn])`	.
`a.splice(start, delcount[, item1[, ...[, itemN]]])`	.
`a.unshift(item1[, item2[, ...[, itemN]]])`	.

JavaScript. :

function add(x, y) {
  var total = x + y;
  return total;
}

, . JavaScript . , . return . return ( , ), JavaScript undefined.

, . , undefined:

add(); // NaN
//    undefined  undefined

, :

add(2, 3, 4); // 5
//     , "4"

, "" arguments, , . , :

function add() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum;
}

add(2, 3, 4, 5); // 14

function avg() {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum / arguments.length;
}
avg(2, 3, 4, 5); // 3.5

, . . , . : ...variable , . for for...of , .

function avg(...args) {
  var sum = 0;
  for (let value of args) {
    sum += value;
  }
  return sum / args.length;
}

avg(2, 3, 4, 5); // 3.5

JavaScript :

var avg = function () {
  var sum = 0;
  for (var i = 0, j = arguments.length; i < j; i++) {
    sum += arguments[i];
  }
  return sum / arguments.length;
};

function avg(). . , "" :

var a = 1;
var b = 2;
(function () {
  var b = 3;
  a += b;
})();
a; // 4
b; // 2

JavaScript . () ( , DOM).

function countChars(elm) {
  if (elm.nodeType == 3) {
    // TEXT_NODE
    return elm.nodeValue.length;
  }
  var count = 0;
  for (var i = 0, child; (child = elm.childNodes[i]); i++) {
    count += countChars(child);
  }
  return count;
}

: , ? JavaScript IIFEs (Immediately Invoked Function Expressions). :

var charsInBody = (function counter(elm) {
  if (elm.nodeType == 3) {
    // TEXT_NODE
    return elm.nodeValue.length;
  }
  var count = 0;
  for (var i = 0, child; (child = elm.childNodes[i]); i++) {
    count += counter(child);
  }
  return count;
})(document.body);

. .

- () , . JavaScript - , , , , C++ Java. ( , , .) JavaScript . , . : " " ", ". :

function makePerson(first, last) {
  return {
    first: first,
    last: last,
  };
}

function personFullName(person) {
  return person.first + " " + person.last;
}

function personFullNameReversed(person) {
  return person.last + ", " + person.first;
}

s = makePerson("Simon", "Willison");
personFullName(s); // Simon Willison
personFullNameReversed(s); // Willison, Simon

, . , . , . , :

function makePerson(first, last) {
  return {
    first: first,
    last: last,
    fullName: function () {
      return this.first + " " + this.last;
    },
    fullNameReversed: function () {
      return this.last + ", " + this.first;
    },
  };
}
s = makePerson("Simon", "Willison");
s.fullName(); // Simon Willison
s.fullNameReversed(); // Willison, Simon

- : this. this , . . , this . this . . :

s = makePerson("Simon", "Willison");
var fullName = s.fullName;
fullName(); // undefined undefined

fullName(), this . first last, undefined.

this, makePerson:

function Person(first, last) {
  this.first = first;
  this.last = last;
  this.fullName = function () {
    return this.first + " " + this.last;
  };
  this.fullNameReversed = function () {
    return this.last + ", " + this.first;
  };
}
var s = new Person("Simon", "Willison");

: new. this. , , this . , new . , - .

, fullName().

, , . :

function personFullName() {
  return this.first + " " + this.last;
}
function personFullNameReversed() {
  return this.last + ", " + this.first;
}
function Person(first, last) {
  this.first = first;
  this.last = last;
  this.fullName = personFullName;
  this.fullNameReversed = personFullNameReversed;
}

: - , - . ? :

function Person(first, last) {
  this.first = first;
  this.last = last;
}
Person.prototype.fullName = function fullName() {
  return this.first + " " + this.last;
};
Person.prototype.fullNameReversed = function fullNameReversed() {
  return this.last + ", " + this.first;
};

Person.prototype , Person. . , Person, JavaScript , Person.prototype. , Person.prototype, this .

. JavaScript , , :

s = new Person("Simon", "Willison");
s.firstNameCaps();
// TypeError on line 1: s.firstNameCaps is not a function

Person.prototype.firstNameCaps = function () {
  return this.first.toUpperCase();
};
s.firstNameCaps(); // "SIMON"

, JavaScript. reversed String, :

var s = "Simon";
s.reversed(); // TypeError on line 1: s.reversed is not a function

String.prototype.reversed = function reversed() {
  var r = "";
  for (var i = this.length - 1; i >= 0; i--) {
    r += this[i];
  }
  return r;
};
s.reversed(); // "nomiS"

"This can now be reversed".reversed();
// desrever eb won nac sihT

, prototype . Object.prototype, toString() , , . Person:

var s = new Person("Simon", "Willison");
s.toString(); // [object Object]

Person.prototype.toString = function () {
  return "<Person: " + this.fullName() + ">";
};

s.toString(); // "<Person: Simon Willison>"

, avg.apply() null? : , apply() , this. new:

function trivialNew(constructor, ...args) {
  var o = {}; //   
  constructor.apply(o, args);
  return o;
}

new, ( ). apply() , . , ...args ( ) , .

var bill = trivialNew(Person, "William", "Orange");

var bill = new Person("William", "Orange");

JavaScript apply() call(), this, , .

function lastNameCaps() {
  return this.last.toUpperCase();
}
var s = new Person("Simon", "Willison");
lastNameCaps.call(s);
//  :
s.lastNameCaps = lastNameCaps;
s.lastNameCaps(); // WILLISON

. , makePerson(). , , -:

function parentFunc() {
  var a = 1;

  function nestedFunc() {
    var b = 4; // parentFunc can't use this
    return a + b;
  }
  return nestedFunc(); // 5
}

, . , , . , .

. , . , . -, ( ), .

(Closures)

JavaScript. .

function makeAdder(a) {
  return function (b) {
    return a + b;
  };
}

var x = makeAdder(5);
var y = makeAdder(20);
x(6); // ?
y(7); // ?

makeAdder , , .

, , . . , . . "" makeAdder, ( , - 5, - 20). :

x(6); //  11
y(7); //  27

: JavaScript , 'scope', , . , . 'Scope' , , : 'scope' , , 'scope' . .

makeAdder 'scope' : a, , . makeAdder . ' ' scope, . scope , .

scope , JavaScript.

scope, . .

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JavaScript (JS )

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