125 lines
2.6 KiB
HTML
125 lines
2.6 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<meta http-equiv="X-UA-Compatible" content="IE=edge">
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<title>Document</title>
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<script>
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{
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let a = 1;
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var b = 2;
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a = 9;
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let a = "llp"
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}
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// console.log(a)
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// console.log(b)
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for (var i = 0; i < 5; i++) {
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console.log(i)
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}
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console.log(i) //5
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for (let m = 0; m < 5; m++) {
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console.log(m, "llk")
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}
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// console.log(m)
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var a = [];
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for (var i = 0; i < 10; i++) {
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a[i] = function () {
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console.log(i);
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};
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}
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a[6](); // 10
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var a = [];
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for (let i = 0; i < 10; i++) {
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a[i] = function () {
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console.log(i);
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};
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}
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a[6](); // 10
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const c = 10
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c = 9;
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console.log(d)
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var d = 7
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// let aa = 1;
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// let bb = 2;
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// let cc = 3;
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// |
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let [aa, bb, cc] = ["a", "b", "c"] //解构赋值
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let [foo, [[bar], baz]] = [1, [[2], 3]];
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// foo 1
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// bar 2
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// baz 3
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let [, , third] = ["foo", "bar", "baz"];
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third // "baz"
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let [x, , y] = [1, 2, 3];
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x //1
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y //3
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let [head, ...tail] = [1, 2, 3, 4];
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head // 1
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tail // [2,3,4]
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let [x, y, ...z] = ['a'];
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x // a
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y // undefined
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z // []
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// ...先生成一个数组，把后面所有的元素都塞进去
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let [a, [b], d] = [1, [2, 3], 4];
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a // 1
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b // 2
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d // 4
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// 不完全解构的时候 左边的变量名只能匹配到第一个值作为他的变量值
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// 报错
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// let [foo] = 1;
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// let [foo] = false;
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// let [foo] = NaN;
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// let [foo] = undefined;
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// let [foo] = null;
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// let [foo] = {};
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// 如果等号的右边不是数组（或者严格地说，不是可遍历的结构，参见《Iterator》一章），那么将会报错。
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</script>
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</head>
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<body>
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结论 用Let声明的变量，只在当前代码块中能够访问，
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用var声明的变量，在代码块内部外部都能访问
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在同一个代码块中，let 不能重复定义
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let 不存在var的变量提升
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<p>
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const 声明的是常量， 定义完成以后值不能被修改 所以在定义的时候需要给他赋初始值
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</p>
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</body>
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</html> |