From 218934fa2ed1c702a6d3923d2aa2cc6b43c48684 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:43:23 -0500 Subject: initial commit --- .../reference/functions/arrow_functions/index.html | 359 +++++++++++++ .../web/javascript/reference/functions/index.html | 596 +++++++++++++++++++++ 2 files changed, 955 insertions(+) create mode 100644 files/tr/web/javascript/reference/functions/arrow_functions/index.html create mode 100644 files/tr/web/javascript/reference/functions/index.html (limited to 'files/tr/web/javascript/reference/functions') diff --git a/files/tr/web/javascript/reference/functions/arrow_functions/index.html b/files/tr/web/javascript/reference/functions/arrow_functions/index.html new file mode 100644 index 0000000000..68d6fb3cc7 --- /dev/null +++ b/files/tr/web/javascript/reference/functions/arrow_functions/index.html @@ -0,0 +1,359 @@ +--- +title: Arrow functions +slug: Web/JavaScript/Reference/Functions/Arrow_functions +translation_of: Web/JavaScript/Reference/Functions/Arrow_functions +--- +
{{jsSidebar("Functions")}}
+ +
Arrow fonksiyonlar normal fonksiyonların kısa yoldan yazılabilen türüdür ve kendi içerisinde this, arguments, super, ya da new.target erişimine sahip değildir. Bu fonksiyon tanımlaması özellikle methodsuz fonksiyonlar için çok uygundur. Constructor olarak kullanılamaz. 
+ +

Söz Dizimi

+ +

Temel Söz Dizimi

+ +
(param1, param2, …, paramN) => { statements }
+(param1, param2, …, paramN) => expression
+// buna eşittir: (param1, param2, …, paramN) => { return expression; }
+
+// Eğer tek parametre var ise parantezsiz kullanılabilir:
+(singleParam) => { statements }
+singleParam => { statements }
+singleParam => expression
+
+
+// Parametre beklenmeyen durumlarda parantez çifti kullanılmalıdır
+() => { statements }
+
+ +

İleri Düzey Söz Dizimi

+ +
// Parantez çifti kullanılarak obje tipi veri dönüşü yapılır.
+params => ({foo: bar})
+
+// Rest parameters ve default parameters desteklenmektedir
+(param1, param2, ...rest) => { statements }
+(param1 = defaultValue1, param2, …, paramN = defaultValueN) => { statements }
+
+// Destructuring parametre listesi içinde de desteklenir
+let f = ([a, b] = [1, 2], {x: c} = {x: a + b}) => a + b + c;
+f();
+// 6
+
+ +

Description

+ +

Şunu da inceleyin "ES6 In Depth: Arrow functions" on hacks.mozilla.org.

+ +

Arrow fonksiyonlarının iki dikkat çekici özelliği vardır: daha kısa fonksiyonlara ihtiyaç ve this anahtar kelimesinin kullanımı.

+ +

Kısa fonksiyonlar

+ +
var materials = [
+  'Hydrogen',
+  'Helium',
+  'Lithium',
+  'Beryllium'
+];
+
+materials.map(function(material) {
+  return material.length;
+}); // [8, 6, 7, 9]
+
+materials.map((material) => {
+  return material.length;
+}); // [8, 6, 7, 9]
+
+materials.map(({length}) => length); // [8, 6, 7, 9]
+
+ +

No separate this

+ +

Arrow fonksiyonlara kadar, her yeni fonksiyon nasıl çağrıldığına bağlı olarak kendi this değerini belirlerdi:

+ +

Constructor durumunda yeni bir nesne.
+ Strict mode fonksiyon çağrılması durumunda undefined.
+ Eğer fonksiyon "nesne metodu" olarak çağrılmışsa temel nesne.
+ etc.
+
+ Bu kullanım nesne-yönelimli programla içerisinde ideal kullanım değildi.

+ +
function Person() {
+  // The Person() fonksiyonu kendini temsil eden this değerini oluşturuyor
+  this.age = 0;
+
+  setInterval(function growUp() {
+    // non-strict modda, growUp() fonksiyonuda her fonksiyon gibi
+    // kendi this değerini tanımlar
+    // bu sayede bir üstteki this değerine artık ulaşamıyor oluruz
+    this.age++; //bu işlem Person() fonksiyonundaki age değerine işlemez.
+  }, 1000);
+}
+
+var p = new Person();
+ +

ECMAScript 3/5'te bu this sorunu this değerinin başka bir değişkene atanarak aşılabilmekteydi.

+ +
function Person() {
+  var that = this;
+  that.age = 0;
+
+  setInterval(function growUp() {
+    // 'that' bir üstteki this değerine etki eder.
+    that.age++;
+  }, 1000);
+}
+ +

Alternatif olarak, bound function tanımlaması yaparak önceden atanmış this değeri growUp() fonksiyonuna bu işlevi kazandırabilir.

+ +

Fakat arrow fonksiyonlar kendi this değerine sahip değildir;  kapsayıcı yürütme fonksiyonunun this değeri kullanılır. Böylelikle aşağıdaki örnekte olduğu gibi setInterval'e atanmış arrow fonksiyon kendi this değeri olmadığı için Person() fonksiyonunun this değerine etki eder.

+ +
function Person(){
+  this.age = 0;
+
+  setInterval(() => {
+    this.age++; // |this| person objesine atıfta bulunur
+  }, 1000);
+}
+
+var p = new Person();
+ +

Strict mode ile ilişkisi

+ +

Kapsayıcı sözcüksel bağlamından gelen this değeri, strict mode kuralları uygulandığında görmezden gelinir

+ +
var f = () => { 'use strict'; return this; };
+f() === window; // or the global object
+ +

Diğer strict mode kuralları normal olarak geçerlidir.

+ +

Invoked through call or apply

+ +

Arrow fonksiyonların this değeri olmadığı için,  call() ve apply() methotları sadece parametre verilebilir. thisArg görmezden gelinir.

+ +
var adder = {
+  base: 1,
+
+  add: function(a) {
+    var f = v => v + this.base;
+    return f(a);
+  },
+
+  addThruCall: function(a) {
+    var f = v => v + this.base;
+    var b = {
+      base: 2
+    };
+
+    return f.call(b, a);
+  }
+};
+
+console.log(adder.add(1));         // This would log to 2
+console.log(adder.addThruCall(1)); // This would log to 2 still
+ +

No binding of arguments

+ +

Arrow functions do not have their own arguments object. Thus, in this example, arguments is simply a reference to the the arguments of the enclosing scope:

+ +
var arguments = [1, 2, 3];
+var arr = () => arguments[0];
+
+arr(); // 1
+
+function foo(n) {
+  var f = () => arguments[0] + n; // foo's implicit arguments binding. arguments[0] is n
+  return f(10);
+}
+
+foo(1); // 2
+ +

Çoğu durumda rest parametreleri, arguments nesnesinin iyi bir alternatifidir.

+ +
function foo(n) {
+  var f = (...args) => args[0] + n;
+  return f(10);
+}
+
+foo(1); // 11
+ +

Method olarak kullanılan Arrow fonksiyonları

+ +

As stated previously, arrow function expressions are best suited for non-method functions. Let's see what happens when we try to use them as methods:

+ +
'use strict';
+var obj = {
+  i: 10,
+  b: () => console.log(this.i, this),
+  c: function() {
+    console.log(this.i, this);
+  }
+}
+obj.b(); // prints undefined, Window {...} (or the global object)
+obj.c(); // prints 10, Object {...}
+ +

Arrow functions do not have their own this. Another example involving {{jsxref("Object.defineProperty()")}}:

+ +
'use strict';
+var obj = {
+  a: 10
+};
+
+Object.defineProperty(obj, 'b', {
+  get: () => {
+    console.log(this.a, typeof this.a, this);
+    return this.a + 10; // represents global object 'Window', therefore 'this.a' returns 'undefined'
+  }
+});
+
+ +

new operatörü kullanımı

+ +

Arrow functions cannot be used as constructors and will throw an error when used with new.

+ +
var Foo = () => {};
+var foo = new Foo(); // TypeError: Foo is not a constructor
+ +

prototype özelliği kullanımı

+ +

Arrow fonksiyonlarının prototype özelliği yoktur.

+ +
var Foo = () => {};
+console.log(Foo.prototype); // undefined
+
+ +

yield anahtarının kullanımı

+ +

The yield keyword may not be used in an arrow function's body (except when permitted within functions further nested within it). As a consequence, arrow functions cannot be used as generators.

+ +

Function body

+ +

Arrow functions can have either a "concise body" or the usual "block body".

+ +

In a concise body, only an expression is specified, which becomes the explicit return value. In a block body, you must use an explicit return statement.

+ +
var func = x => x * x;
+// concise body syntax, implied "return"
+
+var func = (x, y) => { return x + y; };
+// with block body, explicit "return" needed
+
+ +

Returning object literals

+ +

Keep in mind that returning object literals using the concise body syntax params => {object:literal} will not work as expected.

+ +
var func = () => { foo: 1 };
+// Calling func() returns undefined!
+
+var func = () => { foo: function() {} };
+// SyntaxError: function statement requires a name
+ +

This is because the code inside braces ({}) is parsed as a sequence of statements (i.e. foo is treated like a label, not a key in an object literal).

+ +

Remember to wrap the object literal in parentheses.

+ +
var func = () => ({foo: 1});
+ +

Line breaks

+ +

An arrow function cannot contain a line break between its parameters and its arrow.

+ +
var func = ()
+           => 1;
+// SyntaxError: expected expression, got '=>'
+ +

Parsing order

+ +

Although the arrow in an arrow function is not an operator, arrow functions have special parsing rules that interact differently with operator precedence compared to regular functions.

+ +
let callback;
+
+callback = callback || function() {}; // ok
+
+callback = callback || () => {};
+// SyntaxError: invalid arrow-function arguments
+
+callback = callback || (() => {});    // ok
+
+ +

More examples

+ +
// An empty arrow function returns undefined
+let empty = () => {};
+
+(() => 'foobar')();
+// Returns "foobar"
+// (this is an Immediately Invoked Function Expression
+// see 'IIFE' in glossary)
+
+var simple = a => a > 15 ? 15 : a;
+simple(16); // 15
+simple(10); // 10
+
+let max = (a, b) => a > b ? a : b;
+
+// Easy array filtering, mapping, ...
+
+var arr = [5, 6, 13, 0, 1, 18, 23];
+
+var sum = arr.reduce((a, b) => a + b);
+// 66
+
+var even = arr.filter(v => v % 2 == 0);
+// [6, 0, 18]
+
+var double = arr.map(v => v * 2);
+// [10, 12, 26, 0, 2, 36, 46]
+
+// More concise promise chains
+promise.then(a => {
+  // ...
+}).then(b => {
+  // ...
+});
+
+// Parameterless arrow functions that are visually easier to parse
+setTimeout( () => {
+  console.log('I happen sooner');
+  setTimeout( () => {
+    // deeper code
+    console.log('I happen later');
+  }, 1);
+}, 1);
+
+ +

Specifications

+ + + + + + + + + + + + + + + + + + + +
SpecificationStatusComment
{{SpecName('ES2015', '#sec-arrow-function-definitions', 'Arrow Function Definitions')}}{{Spec2('ES2015')}}Initial definition.
{{SpecName('ESDraft', '#sec-arrow-function-definitions', 'Arrow Function Definitions')}}{{Spec2('ESDraft')}}
+ +

Browser compatibility

+ +
+ + +

{{Compat("javascript.functions.arrow_functions")}}

+
+ +

See also

+ + diff --git a/files/tr/web/javascript/reference/functions/index.html b/files/tr/web/javascript/reference/functions/index.html new file mode 100644 index 0000000000..0b67f9aa30 --- /dev/null +++ b/files/tr/web/javascript/reference/functions/index.html @@ -0,0 +1,596 @@ +--- +title: Functions +slug: Web/JavaScript/Reference/Functions +tags: + - Constructor + - Function + - Functions + - JavaScript + - NeedsTranslation + - Parameter + - TopicStub + - parameters +translation_of: Web/JavaScript/Reference/Functions +--- +
{{jsSidebar("Functions")}}
+ +

Generally speaking, a function is a "subprogram" that can be called by code external (or internal in the case of recursion) to the function. Like the program itself, a function is composed of a sequence of statements called the function body. Values can be passed to a function, and the function will return a value.

+ +

In JavaScript, functions are first-class objects, because they can have properties and methods just like any other object. What distinguishes them from other objects is that functions can be called. In brief, they are Function objects.

+ +

For more examples and explanations, see also the JavaScript guide about functions.

+ +

Description

+ +

Every function in JavaScript is a Function object. See {{jsxref("Function")}} for information on properties and methods of Function objects.

+ +

To return a value other than the default, a function must have a return statement that specifies the value to return. A function without a return statement will return a default value. In the case of a constructor called with the new keyword, the default value is the value of its this parameter. For all other functions, the default return value is {{jsxref("undefined")}}.

+ +

The parameters of a function call are the function's arguments. Arguments are passed to functions by value. If the function changes the value of an argument, this change is not reflected globally or in the calling function. However, object references are values, too, and they are special: if the function changes the referred object's properties, that change is visible outside the function, as shown in the following example:

+ +
/* Declare the function 'myFunc' */
+function myFunc(theObject) {
+  theObject.brand = "Toyota";
+}
+
+/*
+ * Declare variable 'mycar';
+ * create and initialize a new Object;
+ * assign reference to it to 'mycar'
+ */
+var mycar = {
+  brand: "Honda",
+  model: "Accord",
+  year: 1998
+};
+
+/* Logs 'Honda' */
+console.log(mycar.brand);
+
+/* Pass object reference to the function */
+myFunc(mycar);
+
+/*
+ * Logs 'Toyota' as the value of the 'brand' property
+ * of the object, as changed to by the function.
+ */
+console.log(mycar.brand);
+
+ +

The this keyword does not refer to the currently executing function, so you must refer to Function objects by name, even within the function body.

+ +

Defining functions

+ +

There are several ways to define functions:

+ +

The function declaration (function statement)

+ +

There is a special syntax for declaring functions (see function statement for details):

+ +
function name([param[, param[, ... param]]]) {
+   statements
+}
+
+ +
+
name
+
The function name.
+
+ +
+
param
+
The name of an argument to be passed to the function. A function can have up to 255 arguments.
+
+ +
+
statements
+
The statements comprising the body of the function.
+
+ +

The function expression (function expression)

+ +

A function expression is similar to and has the same syntax as a function declaration (see function expression for details). A function expression may be a part of a larger expression. One can define "named" function expressions (where the name of the expression might be used in the call stack for example) or "anonymous" function expressions. Function expressions are not hoisted onto the beginning of the scope, therefore they cannot be used before they appear in the code.

+ +
function [name]([param[, param[, ... param]]]) {
+   statements
+}
+
+ +
+
name
+
The function name. Can be omitted, in which case the function becomes known as an anonymous function.
+
+ +
+
param
+
The name of an argument to be passed to the function. A function can have up to 255 arguments.
+
statements
+
The statements comprising the body of the function.
+
+ +

Here is an example of an anonymous function expression (the name is not used):

+ +
var myFunction = function() {
+    statements
+}
+ +

It is also possible to provide a name inside the definition in order to create a named function expression:

+ +
var myFunction = function namedFunction(){
+    statements
+}
+
+ +

One of the benefit of creating a named function expression is that in case we encounted an error, the stack trace will contain the name of the function, making it easier to find the origin of the error.

+ +

As we can see, both examples do not start with the function keyword. Statements involving functions which do not start with function are function expressions.

+ +

When functions are used only once, a common pattern is an IIFE (Immediately Invokable Function Expression).

+ +
(function() {
+    statements
+})();
+ +

IIFE are function expressions that are invoked as soon as the function is declared.

+ +

The generator function declaration (function* statement)

+ +

There is a special syntax for generator function declarations (see {{jsxref('Statements/function*', 'function* statement')}} for details):

+ +
function* name([param[, param[, ... param]]]) {
+   statements
+}
+
+ +
+
name
+
The function name.
+
+ +
+
param
+
The name of an argument to be passed to the function. A function can have up to 255 arguments.
+
+ +
+
statements
+
The statements comprising the body of the function.
+
+ +

The generator function expression (function* expression)

+ +

A generator function expression is similar to and has the same syntax as a generator function declaration (see {{jsxref('Operators/function*', 'function* expression')}} for details):

+ +
function* [name]([param[, param[, ... param]]]) {
+   statements
+}
+
+ +
+
name
+
The function name. Can be omitted, in which case the function becomes known as an anonymous function.
+
+ +
+
param
+
The name of an argument to be passed to the function. A function can have up to 255 arguments.
+
statements
+
The statements comprising the body of the function.
+
+ +

The arrow function expression (=>)

+ +

An arrow function expression has a shorter syntax and lexically binds its this value (see arrow functions for details):

+ +
([param[, param]]) => {
+   statements
+}
+
+param => expression
+
+ +
+
param
+
The name of an argument. Zero arguments need to be indicated with ().  For only one argument, the parentheses are not required. (like foo => 1)
+
statements or expression
+
Multiple statements need to be enclosed in brackets. A single expression requires no brackets. The expression is also the implicit return value of the function.
+
+ +

The Function constructor

+ +
+

Note: Using the Function constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.

+
+ +

As all other objects, {{jsxref("Function")}} objects can be created using the new operator:

+ +
new Function (arg1, arg2, ... argN, functionBody)
+
+ +
+
arg1, arg2, ... argN
+
Zero or more names to be used by the function as formal parameters. Each must be a proper JavaScript identifier.
+
+ +
+
functionBody
+
A string containing the JavaScript statements comprising the function body.
+
+ +

Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

+ +

The GeneratorFunction constructor

+ +
+

Note: GeneratorFunction is not a global object, but could be obtained from generator function instance (see {{jsxref("GeneratorFunction")}} for more detail).

+
+ +
+

Note: Using the GeneratorFunction constructor to create functions is not recommended since it needs the function body as a string which may prevent some JS engine optimizations and can also cause other problems.

+
+ +

As all other objects, {{jsxref("GeneratorFunction")}} objects can be created using the new operator:

+ +
new GeneratorFunction (arg1, arg2, ... argN, functionBody)
+
+ +
+
arg1, arg2, ... argN
+
Zero or more names to be used by the function as formal argument names. Each must be a string that conforms to the rules for a valid JavaScript identifier or a list of such strings separated with a comma; for example "x", "theValue", or "a,b".
+
+ +
+
functionBody
+
A string containing the JavaScript statements comprising the function definition.
+
+ +

Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

+ +

Function parameters

+ +

Default parameters

+ +

Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed. For more details, see default parameters.

+ +

Rest parameters

+ +

The rest parameter syntax allows to represent an indefinite number of arguments as an array. For more details, see rest parameters.

+ +

The arguments object

+ +

You can refer to a function's arguments within the function by using the arguments object. See arguments.

+ + + +

Defining method functions

+ +

Getter and setter functions

+ +

You can define getters (accessor methods) and setters (mutator methods) on any standard built-in object or user-defined object that supports the addition of new properties. The syntax for defining getters and setters uses the object literal syntax.

+ +
+
get
+
+

Binds an object property to a function that will be called when that property is looked up.

+
+
set
+
Binds an object property to a function to be called when there is an attempt to set that property.
+
+ +

Method definition syntax

+ +

Starting with ECMAScript 2015, you are able to define own methods in a shorter syntax, similar to the getters and setters. See method definitions for more information.

+ +
var obj = {
+  foo() {},
+  bar() {}
+};
+ +

Constructor vs. declaration vs. expression

+ +

Compare the following:

+ +

A function defined with the Function constructor assigned to the variable multiply:

+ +
var multiply = new Function('x', 'y', 'return x * y');
+ +

A function declaration of a function named multiply:

+ +
function multiply(x, y) {
+   return x * y;
+} // there is no semicolon here
+
+ +

A function expression of an anonymous function assigned to the variable multiply:

+ +
var multiply = function(x, y) {
+   return x * y;
+};
+
+ +

A function expression of a function named func_name assigned to the variable multiply:

+ +
var multiply = function func_name(x, y) {
+   return x * y;
+};
+
+ +

Differences

+ +

All do approximately the same thing, with a few subtle differences:

+ +

There is a distinction between the function name and the variable the function is assigned to. The function name cannot be changed, while the variable the function is assigned to can be reassigned. The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or undefined if the function name was previously declared via a var statement). For example:

+ +
var y = function x() {};
+alert(x); // throws an error
+
+ +

The function name also appears when the function is serialized via Function's toString method.

+ +

On the other hand, the variable the function is assigned to is limited only by its scope, which is guaranteed to include the scope in which the function is declared.

+ +

As the 4th example shows, the function name can be different from the variable the function is assigned to. They have no relation to each other. A function declaration also creates a variable with the same name as the function name. Thus, unlike those defined by function expressions, functions defined by function declarations can be accessed by their name in the scope they were defined in:

+ +

A function defined by 'new Function' does not have a function name. However, in the SpiderMonkey JavaScript engine, the serialized form of the function shows as if it has the name "anonymous". For example, alert(new Function()) outputs:

+ +
function anonymous() {
+}
+
+ +

Since the function actually does not have a name, anonymous is not a variable that can be accessed within the function. For example, the following would result in an error:

+ +
var foo = new Function("alert(anonymous);");
+foo();
+
+ +

Unlike functions defined by function expressions or by the Function constructor, a function defined by a function declaration can be used before the function declaration itself. For example:

+ +
foo(); // alerts FOO!
+function foo() {
+   alert('FOO!');
+}
+
+ +

A function defined by a function expression or by a function declaration inherits the current scope. That is, the function forms a closure. On the other hand, a function defined by a Function constructor does not inherit any scope other than the global scope (which all functions inherit).

+ +
/*
+ * Declare and initialize a variable 'p' (global)
+ * and a function 'myFunc' (to change the scope) inside which
+ * declare a varible with same name 'p' (current) and
+ * define three functions using three different ways:-
+ *     1. function declaration
+ *     2. function expression
+ *     3. function constructor
+ * each of which will log 'p'
+ */
+var p = 5;
+function myFunc() {
+    var p = 9;
+
+    function decl() {
+        console.log(p);
+    }
+    var expr = function() {
+        console.log(p);
+    };
+    var cons = new Function('\tconsole.log(p);');
+
+    decl();
+    expr();
+    cons();
+}
+myFunc();
+
+/*
+ * Logs:-
+ * 9  - for 'decl' by function declaration (current scope)
+ * 9  - for 'expr' by function expression (current scope)
+ * 5  - for 'cons' by Function constructor (global scope)
+ */
+
+ +

Functions defined by function expressions and function declarations are parsed only once, while those defined by the Function constructor are not. That is, the function body string passed to the Function constructor must be parsed each and every time the constructor is called. Although a function expression creates a closure every time, the function body is not reparsed, so function expressions are still faster than "new Function(...)". Therefore the Function constructor should generally be avoided whenever possible.

+ +

It should be noted, however, that function expressions and function declarations nested within the function generated by parsing a Function constructor 's string aren't parsed repeatedly. For example:

+ +
var foo = (new Function("var bar = \'FOO!\';\nreturn(function() {\n\talert(bar);\n});"))();
+foo(); // The segment "function() {\n\talert(bar);\n}" of the function body string is not re-parsed.
+ +

A function declaration is very easily (and often unintentionally) turned into a function expression. A function declaration ceases to be one when it either:

+ + + +
var x = 0;               // source element
+if (x === 0) {           // source element
+   x = 10;               // not a source element
+   function boo() {}     // not a source element
+}
+function foo() {         // source element
+   var y = 20;           // source element
+   function bar() {}     // source element
+   while (y === 10) {    // source element
+      function blah() {} // not a source element
+      y++;               // not a source element
+   }
+}
+
+ +

Examples

+ +
// function declaration
+function foo() {}
+
+// function expression
+(function bar() {})
+
+// function expression
+x = function hello() {}
+
+
+if (x) {
+   // function expression
+   function world() {}
+}
+
+
+// function declaration
+function a() {
+   // function declaration
+   function b() {}
+   if (0) {
+      // function expression
+      function c() {}
+   }
+}
+
+ +

Block-level functions

+ +

In strict mode, starting with ES2015, functions inside blocks are now scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.

+ +
'use strict';
+
+function f() {
+  return 1;
+}
+
+{
+  function f() {
+    return 2;
+  }
+}
+
+f() === 1; // true
+
+// f() === 2 in non-strict mode
+
+ +

Block-level functions in non-strict code

+ +

In a word: Don't.

+ +

In non-strict code, function declarations inside blocks behave strangely. For example:

+ +
if (shouldDefineZero) {
+   function zero() {     // DANGER: compatibility risk
+      console.log("This is zero.");
+   }
+}
+
+ +

ES2015 says that if shouldDefineZero is false, then zero should never be defined, since the block never executes. However, it's a new part of the standard. Historically, this was left unspecified, and some browsers would define zero whether the block executed or not.

+ +

In strict mode, all browsers that support ES2015 handle this the same way: zero is defined only if shouldDefineZero is true, and only in the scope of the if-block.

+ +

A safer way to define functions conditionally is to assign a function expression to a variable:

+ +
var zero;
+if (shouldDefineZero) {
+   zero = function() {
+      console.log("This is zero.");
+   };
+}
+
+ +

Examples

+ +

Returning a formatted number

+ +

The following function returns a string containing the formatted representation of a number padded with leading zeros.

+ +
// This function returns a string padded with leading zeros
+function padZeros(num, totalLen) {
+   var numStr = num.toString();             // Initialize return value as string
+   var numZeros = totalLen - numStr.length; // Calculate no. of zeros
+   for (var i = 1; i <= numZeros; i++) {
+      numStr = "0" + numStr;
+   }
+   return numStr;
+}
+
+ +

The following statements call the padZeros function.

+ +
var result;
+result = padZeros(42,4); // returns "0042"
+result = padZeros(42,2); // returns "42"
+result = padZeros(5,4);  // returns "0005"
+
+ +

Determining whether a function exists

+ +

You can determine whether a function exists by using the typeof operator. In the following example, a test is performed to determine if the window object has a property called noFunc that is a function. If so, it is used; otherwise some other action is taken.

+ +
 if ('function' === typeof window.noFunc) {
+   // use noFunc()
+ } else {
+   // do something else
+ }
+
+ +

Note that in the if test, a reference to noFunc is used—there are no brackets "()" after the function name so the actual function is not called.

+ +

Specifications

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
SpecificationStatusComment
{{SpecName('ES1')}}{{Spec2('ES1')}}Initial definition. Implemented in JavaScript 1.0
{{SpecName('ES5.1', '#sec-13', 'Function Definition')}}{{Spec2('ES5.1')}} 
{{SpecName('ES6', '#sec-function-definitions', 'Function definitions')}}{{Spec2('ES6')}}New: Arrow functions, Generator functions, default parameters, rest parameters.
{{SpecName('ESDraft', '#sec-function-definitions', 'Function definitions')}}{{Spec2('ESDraft')}} 
+ +

Browser compatibility

+ + + +

{{Compat("javascript.functions")}}

+ +

See also

+ + -- cgit v1.2.3-54-g00ecf