使用JavaScript进行多次左手分配

Question

var var1 = 1,
    var2 = 1,
    var3 = 1;

这相当于:

var var1 = var2 = var3 = 1;

我很确定这是变量定义的顺序:var3,var2,var1,它等价于:

var var3 = 1, var2 = var3, var1 = var2;

有没有办法在JavaScript中确认这一点？可能使用一些分析器？

Answer 1

其实,

var var1 = 1, var2 = 1, var3 = 1;

是不是等同于:

var var1 = var2 = var3 = 1;

区别在于范围界定:

function good() {
  var var1 = 1, var2 = 1, var3 = 1;
}

function bad() {
  var var1 = var2 = var3 = 1;
}

good();
console.log(window.var2); // undefined

bad();
console.log(window.var2); // 1. Aggh!

实际上这表明赋值是正确的关联.该bad示例相当于:

var var1 = (window.var2 = (window.var3 = 1));

Answer 2

javascript中的赋值从右到左工作.var var1 = var2 = var3 = 1;.

如果这些变量的值是1该语句之后,那么在逻辑上它必须从右边开始,否则价值或var1和var2将是不确定的.

您可以将其视为等同于var var1 = (var2 = (var3 = 1));首先评估最内部括号集的位置.

Answer 3

a = (b = 'string is truthy'); // b gets string; a gets b, which is a primitive (copy)
a = (b = { c: 'yes' }); // they point to the same object; a === b (not a copy)

(a && b)在逻辑上(a ? b : a)和行为像乘法(例如!!a * !!b)

(a || b)在逻辑上(a ? a : b)和行为像添加(例如!!a + !!b)

(a = 0, b)如果a是真实的,那就是没有关心的短暂,隐含地回归b

a = (b = 0) && "nope, but a is 0 and b is 0"; // b is falsey + order of operations
a = (b = "b is this string") && "a gets this string"; // b is truthy + order of ops

JavaScript运算符优先级(运算顺序)

请注意,逗号运算符实际上是权限最小的运算符,但括号是最有特权的,并且在构造单行表达式时它们是相辅相成的.

最终,你可能需要'thunks'而不是硬编码的值,对我来说,thunk既是函数又是结果值(相同的'东西').

const windowInnerHeight = () => 0.8 * window.innerHeight; // a thunk

windowInnerHeight(); // a thunk

Answer 4

var var1 = 1,var2 = 1,var3 = 1;

在这种情况下, var关键字适用于所有三个变量.

var var1 = 1,
    var2 = 1,
    var3 = 1;

这不等于这个:

var var1 = var2 = var3 = 1;

在这种情况下,屏幕var关键字后面仅适用于var1由于变量提升而且表达式的其余部分正常评估,因此变量var2, var3变为全局变量

Javascript按此顺序处理此代码:

/*
var 1 is local to the particular scope because of var keyword
var2 and var3 will become globals because they've used without var keyword
*/

var var1;   //only variable declarations will be hoisted.

var1= var2= var3 = 1;