我知道>=运算符意味着大于或等于，但我在一些源代码中看到过=>。这个运算符是什么意思?
代码如下:
promiseTargetFile(fpParams, aSkipPrompt, relatedURI).then(aDialogAccepted => {
if (!aDialogAccepted)
return;
saveAsType = fpParams.saveAsType;
file = fpParams.file;
continueSave();
}).then(null, Components.utils.reportError);
当前回答
正如所有其他答案已经说过的，它是ES2015箭头函数语法的一部分。更具体地说，它不是一个操作符，而是一个标点符号，将参数与正文分隔开来。例如(params) => {/* body */}。
其他回答
JavaScript箭头函数大致相当于python中的lambda函数或Ruby中的块。这些是具有自己特殊语法的匿名函数，并在其封闭作用域的上下文中操作。这意味着它们没有自己的“This”，而是从直接的封闭函数中访问one。
来自ECMA标准:
ArrowFunction不为参数定义本地绑定， 超级，这个，或者新的目标。任何对参数、super、this或new的引用。ArrowFunction中的target必须解析为 在词汇封闭的环境中绑定。通常这将是 直接封闭函数的函数环境。
通常你会读成“箭头函数表达式是传统函数表达式的紧凑替代”，这是不正确的。箭头函数不是传统函数的简写，它们的行为与传统函数不同。
语法
// Traditional Function
// Create their own scope inside the function
function (a){
return a + 100;
}
// Arrow Function
// Do NOT create their own scope
// (Each step along the way is a valid "arrow function")
// 1. Remove the word "function" and place arrow between the argument and opening body bracket
(a) => {
return a + 100;
}
// 2. Remove the body braces and word "return" -- the return is implied.
(a) => a + 100;
// 3. Remove the argument parentheses (only valid with exactly one argument)
a => a + 100;
正如其他人所述，常规(传统)函数使用调用该函数的对象的this(例如，被单击的按钮)。相反，箭头函数使用定义该函数的对象中的this。
考虑两个几乎相同的函数:
regular = function() {
' Identical Part Here;
}
arrow = () => {
' Identical Part Here;
}
下面的代码段演示了这对于每个函数所代表的内容之间的基本区别。常规函数输出[object HTMLButtonElement]，而箭头函数输出[object Window]。
<html> <button id="btn1">Regular: `this` comes from "this button"</button> <br><br> <button id="btn2">Arrow: `this` comes from object that defines the function</button> <p id="res"/> <script> regular = function() { document.getElementById("res").innerHTML = this; } arrow = () => { document.getElementById("res").innerHTML = this; } document.getElementById("btn1").addEventListener("click", regular); document.getElementById("btn2").addEventListener("click", arrow); </script> </html>
正如其他人所说，这是一种创建函数的新语法。
但这类函数与普通函数不同:
They bind the this value. As explained by the spec, An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 4 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the scope that is captured by the function object of the ArrowFunction. They are non-constructors. That means they have no [[Construct]] internal method, and thus can't be instantiated, e.g. var f = a => a; f(123); // 123 new f(); // TypeError: f is not a constructor
这被称为箭头函数，是ECMAScript 2015规范的一部分…
Var foo = ['a'， 'ab'， 'abc']; Var bar = foo。Map (f => f.length); console.log(栏);/ / 1、2、3
比前面的语法更短:
// < es6: Var foo = ['a'， 'ab'， 'abc']; Var bar = foo.map(函数(f) { 返回f.length; })； console.log(栏);/ / 1、2、3
DEMO
另一件很棒的事情是词汇上的。通常，你会这样做:
函数Foo() { This.name = name; 这一点。计数= 0; this.startCounting (); ｝ Foo.prototype.startCounting = function() { Var self = this; setInterval(函数(){ //这是窗口，而不是Foo{}，因为你可能期望 console.log(这个);//[对象窗口] //这就是为什么我们在setInterval()之前重新分配给self console.log (self.count); self.count + +; }, 1000) ｝ 新Foo ();
但可以用箭头这样重写:
函数Foo() { This.name = name; 这一点。计数= 0; this.startCounting (); ｝ Foo.prototype.startCounting = function() { setInterval(() => { console.log(这个);// [object对象] console.log (this.count);// 1, 2, 3 this.count + +; }, 1000) ｝ 新Foo ();
DEMO
中数 更多关于语法
这里有一个关于何时使用箭头函数的很好的答案。
使用Arrowfunction添加简单的CRUD示例
//Arrow Function
var customers = [
{
name: 'Dave',
contact:'9192631770'
},
{
name: 'Sarah',
contact:'9192631770'
},
{
name: 'Akhil',
contact:'9928462656'
}],
// No Param READ
getFirstCustomer = () => {
console.log(this);
return customers[0];
};
console.log("First Customer "+JSON.stringify(getFirstCustomer())); // 'Dave'
//1 Param SEARCH
getNthCustomer = index=>{
if( index>customers.length)
{
return "No such thing";
}
else{
return customers[index];
}
};
console.log("Nth Customer is " +JSON.stringify(getNthCustomer(1)));
//2params ADD
addCustomer = (name, contact)=> customers.push({
'name': name,
'contact':contact
});
addCustomer('Hitesh','8888813275');
console.log("Added Customer "+JSON.stringify(customers));
//2 param UPDATE
updateCustomerName = (index, newName)=>{customers[index].name= newName};
updateCustomerName(customers.length-1,"HiteshSahu");
console.log("Updated Customer "+JSON.stringify(customers));
//1 param DELETE
removeCustomer = (customerToRemove) => customers.pop(customerToRemove);
removeCustomer(getFirstCustomer());
console.log("Removed Customer "+JSON.stringify(customers));
