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
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。
其他回答
让它很有趣,因为它允许我们做这样的事情:
(() => {
var count = 0;
for (let i = 0; i < 2; ++i) {
for (let i = 0; i < 2; ++i) {
for (let i = 0; i < 2; ++i) {
console.log(count++);
}
}
}
})();
其次,有数以数以数。
在哪里
(() => {
var count = 0;
for (var i = 0; i < 2; ++i) {
for (var i = 0; i < 2; ++i) {
for (var i = 0; i < 2; ++i) {
console.log(count++);
}
}
}
})();
只有数数(0,1 )。
在 MDN 中查看此链接
let x = 1;
if (x === 1) {
let x = 2;
console.log(x);
// expected output: 2
}
console.log(x);
// expected output: 1
下面表明“让我们”和“存在”在范围内是如何不同的:
let gfoo = 123;
if (true) {
let gfoo = 456;
}
console.log(gfoo); // 123
var hfoo = 123;
if (true) {
var hfoo = 456;
}
console.log(hfoo); // 456
gfoo,定义为 let 最初是在全球范围内,当我们宣布 gfoo 再次在如果条款的范围改变,当一个新的值被分配到变量在该范围内,它不会影响全球范围。
虽然 hfoo,由 var 定义,起初在全球范围内,但再次当我们在 if 条款内宣布它时,它考虑到全球范围 hfoo,尽管 var 已再次被用来宣布它。
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。
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