带比较运算符的布尔表达式的typeof [英] typeof of boolean expression with comparison operator

问题描述

if (typeof foo !== 'undefined') {
    // Now we know that foo is defined, we are good to go.
}

typeof 评估根据变量 foo 是否为 true 或 false 定义与否。

The typeof evaluates to true or false based on whether the variable foo is defined or not.

但是，如果 foo！=='undefined'评估为 true ，然后 typeof true 应评估为'布尔'。
为什么它评估为 true 或 false ？

But, say if foo !== 'undefined' evaluates to true, then typeof of true should evaluate to 'boolean'. Why does it evaluate to true or false?

推荐答案

因为 typeof 和不等式运算符的优先规则定义该表达式被解析为

Because precedence rules for the typeof and inquality operators define that that expression is parsed as

(typeof foo) !== 'undefined'

有关详细信息，请参阅 MDN页面关于运营商优先级。 typeof 优先级为16; inquality优先级10. typeof 的优先级越高意味着它绑定到它的操作数更紧密。

For more information, see the MDN page on operator precedence. typeof is precedence 16; inquality is precedence 10. The higher precedence of typeof means that it "binds" more tightly to its operand.

顺便说一下，为什么你的代码中有未定义的变量？

By the way, why do you have undefined variables in your code?

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