我最近开始保持别人的JavaScript代码,我正在修复错误,添加功能,也试图更新代码并使其更加一致。
以前的开发人员使用了两种方式来宣布功能,我无法解决是否有原因。
兩種方式是:
var functionOne = function() {
// Some code
};
function functionTwo() {
// Some code
}
使用这两种不同的方法的原因是什么?每个方法的优点和缺点是什么?可以用一种方法做些什么,不能用另一种方法做些什么?
当前回答
鉴于“名称函数在轨道上出现”的论点,现代的JavaScript引擎实际上非常能够代表匿名函数。
至于这篇文章,V8,SpiderMonkey,Chakra和Nitro总是提到他们的名字命名的函数,他们几乎总是提到一个匿名函数,它的识别器,如果它有一个。
SpiderMonkey 可以从另一个函数返回的匿名函数的名称。
如果你真的,真的想要你的 iterator 和成功的呼叫回复出现在轨道上,你可以命名那些也。
[].forEach(function iterator() {});
但是,大多数情况下,它不值得强调。
哈里斯(Fiddle)
'use strict';
var a = function () {
throw new Error();
},
b = function b() {
throw new Error();
},
c = function d() {
throw new Error();
},
e = {
f: a,
g: b,
h: c,
i: function () {
throw new Error();
},
j: function j() {
throw new Error();
},
k: function l() {
throw new Error();
}
},
m = (function () {
return function () {
throw new Error();
};
}()),
n = (function () {
return function n() {
throw new Error();
};
}()),
o = (function () {
return function p() {
throw new Error();
};
}());
console.log([a, b, c].concat(Object.keys(e).reduce(function (values, key) {
return values.concat(e[key]);
}, [])).concat([m, n, o]).reduce(function (logs, func) {
try {
func();
} catch (error) {
return logs.concat('func.name: ' + func.name + '\n' +
'Trace:\n' +
error.stack);
// Need to manually log the error object in Nitro.
}
}, []).join('\n\n'));
V8
func.name:
Trace:
Error
at a (http://localhost:8000/test.js:4:11)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: b
Trace:
Error
at b (http://localhost:8000/test.js:7:15)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: d
Trace:
Error
at d (http://localhost:8000/test.js:10:15)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name:
Trace:
Error
at a (http://localhost:8000/test.js:4:11)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: b
Trace:
Error
at b (http://localhost:8000/test.js:7:15)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: d
Trace:
Error
at d (http://localhost:8000/test.js:10:15)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name:
Trace:
Error
at e.i (http://localhost:8000/test.js:17:19)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: j
Trace:
Error
at j (http://localhost:8000/test.js:20:19)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: l
Trace:
Error
at l (http://localhost:8000/test.js:23:19)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name:
Trace:
Error
at http://localhost:8000/test.js:28:19
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: n
Trace:
Error
at n (http://localhost:8000/test.js:33:19)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27
func.name: p
Trace:
Error
at p (http://localhost:8000/test.js:38:19)
at http://localhost:8000/test.js:47:9
at Array.reduce (native)
at http://localhost:8000/test.js:44:27 test.js:42
蜘蛛侠
func.name:
Trace:
a@http://localhost:8000/test.js:4:5
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: b
Trace:
b@http://localhost:8000/test.js:7:9
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: d
Trace:
d@http://localhost:8000/test.js:10:9
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name:
Trace:
a@http://localhost:8000/test.js:4:5
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: b
Trace:
b@http://localhost:8000/test.js:7:9
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: d
Trace:
d@http://localhost:8000/test.js:10:9
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name:
Trace:
e.i@http://localhost:8000/test.js:17:13
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: j
Trace:
j@http://localhost:8000/test.js:20:13
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: l
Trace:
l@http://localhost:8000/test.js:23:13
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name:
Trace:
m</<@http://localhost:8000/test.js:28:13
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: n
Trace:
n@http://localhost:8000/test.js:33:13
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
func.name: p
Trace:
p@http://localhost:8000/test.js:38:13
@http://localhost:8000/test.js:47:9
@http://localhost:8000/test.js:54:1
查克拉
func.name: undefined
Trace:
Error
at a (http://localhost:8000/test.js:4:5)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at b (http://localhost:8000/test.js:7:9)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at d (http://localhost:8000/test.js:10:9)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at a (http://localhost:8000/test.js:4:5)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at b (http://localhost:8000/test.js:7:9)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at d (http://localhost:8000/test.js:10:9)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at e.i (http://localhost:8000/test.js:17:13)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at j (http://localhost:8000/test.js:20:13)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at l (http://localhost:8000/test.js:23:13)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at Anonymous function (http://localhost:8000/test.js:28:13)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at n (http://localhost:8000/test.js:33:13)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
func.name: undefined
Trace:
Error
at p (http://localhost:8000/test.js:38:13)
at Anonymous function (http://localhost:8000/test.js:47:9)
at Global code (http://localhost:8000/test.js:42:1)
尼特
func.name:
Trace:
a@http://localhost:8000/test.js:4:22
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: b
Trace:
b@http://localhost:8000/test.js:7:26
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: d
Trace:
d@http://localhost:8000/test.js:10:26
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name:
Trace:
a@http://localhost:8000/test.js:4:22
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: b
Trace:
b@http://localhost:8000/test.js:7:26
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: d
Trace:
d@http://localhost:8000/test.js:10:26
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name:
Trace:
i@http://localhost:8000/test.js:17:30
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: j
Trace:
j@http://localhost:8000/test.js:20:30
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: l
Trace:
l@http://localhost:8000/test.js:23:30
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name:
Trace:
http://localhost:8000/test.js:28:30
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: n
Trace:
n@http://localhost:8000/test.js:33:30
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
func.name: p
Trace:
p@http://localhost:8000/test.js:38:30
http://localhost:8000/test.js:47:13
reduce@[native code]
global code@http://localhost:8000/test.js:44:33
其他回答
在 Debugger/DevTools 中,如果您在控制台中使用格式函数Name() {} 函数时,您不能随后在控制台中使用函数Name() (即“未定义”),而在 var 函数Name = 函数() {} 后,您可以使用函数。
看这个问题。
一个描述什么时候优先于第一种方法,而不是第二种方法,是当你需要避免超越一个函数的以前定义时。
与
if (condition){
function myfunction(){
// Some code
}
}
,这个定义的微功能将超越任何以前的定义,因为它将在时间段进行。
虽然
if (condition){
var myfunction = function (){
// Some code
}
}
正确的定义功能工作只有在条件满足时。
兩個函數之間的另一個區別是函數One 可以用作可持有多個函數的變量,而函數Two 持有某些代碼區塊,在呼叫時全部執行。
var functionOne = (function() {
return {
sayHello: function(){
console.log('say hello')
},
redirectPage:function(_url){
window.location.href = _url;
}
}
})();
您有一个选项,该函数将被称为. e.g 函数One.sayHello 或函数One. redirectPage. 如果您呼叫函数Two 那么整个代码块将被执行。
在其他答案中没有提到的另一个区别是,如果您使用匿名函数
var functionOne = function() {
// Some code
};
用它作为一个建筑师
var one = new functionOne();
Function.name 是非标准的,但由 Firefox、Chrome、其他 Webkit 衍生浏览器和 IE 9+ 支持。
与
function functionTwo() {
// Some code
}
two = new functionTwo();
可以以 two.constructor.name 的字符串获取建筑师的名称。
a = 10
output : 10
(1 + 3)
output = 4
声明/声明:不返回值的东西 示例:
if (1 > 2) {
// do something.
}
此分類上一篇
同样,我们有函数声明/声明 vs 函数表达 请举一个例子:
// test.js
var a = 10;
// function expression
var fun_expression = function() {
console.log("Running function Expression");
}
// funciton expression
function fun_declaration() {
console.log("Running function Statement");
}
重要:在JavaScript引擎运行上面的JS文件时会发生什么。
现在,假设我们更新JS到。
// test.js
console.log(a) //output: udefined (No error)
console.log(fun_expression) // output: undefined (No error)
console.log(fun_expression()) // output: Error. As we trying to invoke undefined.
console.log(fun_declaration()) // output: running function statement (As fun_declaration is already hoisted in the memory).
var a = 10;
// function expression
var fun_expression = function() {
console.log('Running function expression')
}
// function declaration
function fun_declaration() {
console.log('running function declaration')
}
console.log(a) // output: 10
console.log(fun_expression()) //output: Running function expression
console.log(fun_declaration()) //output: running function declaration
在评论中提到的结果,应该有助于理解函数表达与函数声明/声明之间的区别。
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