ECMAScript 6 引入了许可声明。
我听说它被描述为一个当地变量,但我仍然不确定它是如何行为不同于 var 关键词。
什么是差异?什么时候应该被允许使用而不是 var?
ECMAScript 6 引入了许可声明。
我听说它被描述为一个当地变量,但我仍然不确定它是如何行为不同于 var 关键词。
什么是差异?什么时候应该被允许使用而不是 var?
当前回答
显然,至少在Visual Studio 2015中,TypeScript 1.5,“var”允许一个区块中的相同变量名称的多个声明,而“Let”不。
这不会导致编译错误:
var x = 1;
var x = 2;
这将是:
let x = 1;
let x = 2;
其他回答
使用 var 声明定义的变量在其定义的整个函数中已知,从函数的开始。 (*) 使用 let 声明定义的变量仅在其定义的区块中已知,从其定义的时刻开始。
// i IS NOT known here
// j IS NOT known here
// k IS known here, but undefined
// l IS NOT known here
function loop(arr) {
// i IS known here, but undefined
// j IS NOT known here
// k IS known here, but has a value only the second time loop is called
// l IS NOT known here
for( var i = 0; i < arr.length; i++ ) {
// i IS known here, and has a value
// j IS NOT known here
// k IS known here, but has a value only the second time loop is called
// l IS NOT known here
};
// i IS known here, and has a value
// j IS NOT known here
// k IS known here, but has a value only the second time loop is called
// l IS NOT known here
for( let j = 0; j < arr.length; j++ ) {
// i IS known here, and has a value
// j IS known here, and has a value
// k IS known here, but has a value only the second time loop is called
// l IS NOT known here
};
// i IS known here, and has a value
// j IS NOT known here
// k IS known here, but has a value only the second time loop is called
// l IS NOT known here
}
loop([1,2,3,4]);
for( var k = 0; k < arr.length; k++ ) {
// i IS NOT known here
// j IS NOT known here
// k IS known here, and has a value
// l IS NOT known here
};
for( let l = 0; l < arr.length; l++ ) {
// i IS NOT known here
// j IS NOT known here
// k IS known here, and has a value
// l IS known here, and has a value
};
loop([1,2,3,4]);
// i IS NOT known here
// j IS NOT known here
// k IS known here, and has a value
// l IS NOT known here
今天使用安全吗?
有些人会说,在未来,我们只会使用让陈述,而这些陈述会变得过时。JavaScript老师Kyle Simpson写了一篇非常复杂的文章,他认为为什么不会这样。
事实上,我们实际上需要问自己是否安全使用放弃声明,这个问题的答案取决于你的环境:
此分類上一篇
如何跟踪浏览器支持
下面是让关键词的解释,有几个例子。
主要区别在于,一个变量的范围是整个关闭功能。
此图在维基百科上显示哪些浏览器支持JavaScript 1.7.
请注意,只有 Mozilla 和 Chrome 浏览器支持它. IE、Safari 和其他可能不支持它。
如上所述:
差异是分解. var 被分解到最接近的函数区块,然后被分解到最接近的封锁区块,这可能比函数区块小。
例子1:
在我的两个例子中,我有一个功能 myfunc. myfunc 包含一个变量 myvar 等于 10. 在我的第一个例子中,我检查 myvar 等于 10 (myvar==10). 如果是的话,我 agian 宣布一个变量 myvar (现在我有两个 myvar 变量) 使用 var 关键词并将其分配到一个新的值(20)。
此分類上一篇
例2:在我的第二個例子中,而不是在我的條件區域中使用 var 關鍵字,我宣告 myvar 使用 let 關鍵字. 現在,當我呼叫 myfunc 我得到兩個不同的出口: myvar = 20 和 myvar = 10.
因此,差异很简单,即它的范围。
差异在于与每一个声明的变量范围。
让变量仅在其最接近的封锁区({... })中可见。让变量仅在变量被宣布后发生的代码线上可用。让变量不被随后的变量或变量重新宣布。让变量不被全球变量添加到全球窗口对象中。让变量易于使用与封锁(它们不会导致赛车条件)。
{
let x = 1;
}
console.log(`x is ${x}`); // ReferenceError during parsing: "x is not defined".
{
x = x + 1; // ReferenceError during parsing: "x is not defined".
let x;
console.log(`x is ${x}`); // Never runs.
}
没有重新宣言:下列代码表明,被允许宣言的变量不能随后重新宣言:
let x = 1;
let x = 2; // SyntaxError: Identifier 'x' has already been declared
var button = "I cause accidents because my name is too common.";
let link = "Though my name is common, I am harder to access from other JS files.";
console.log(link); // OK
console.log(window.link); // undefined (GOOD!)
console.log(window.button); // OK
5. 易于使用与关闭: 与 var 表示的变量与关闭内部的关闭工作不太好. 这里是一个简单的旋转,结果的值序列,变量 i 有在不同的时间点:
for (let i = 0; i < 5; i++) {
console.log(`i is ${i}`), 125/*ms*/);
}
具体而言,这些输出:
i is 0
i is 1
i is 2
i is 3
i is 4
在JavaScript中,我们经常使用变量比它们创建时更晚的时间。当我们通过延迟输出以关闭转移到设置Timeout时:
for (let i = 0; i < 5; i++) {
setTimeout(_ => console.log(`i is ${i}`), 125/*ms*/);
}
for (var i = 0; i < 5; i++) {
setTimeout(_ => console.log(`i is ${i}`), 125/*ms*/);
}
i is 5
i is 5
i is 5
i is 5
i is 5
显然,至少在Visual Studio 2015中,TypeScript 1.5,“var”允许一个区块中的相同变量名称的多个声明,而“Let”不。
这不会导致编译错误:
var x = 1;
var x = 2;
这将是:
let x = 1;
let x = 2;