在 JavaScript 中，为什么是“0"?等于假，但是当通过“如果"测试时，它本身不是假的? [英] In JavaScript, why is "0" equal to false, but when tested by 'if' it is not false by itself?

问题描述

以下说明 "0" 在 Javascript 中为 false:

The following shows that "0" is false in Javascript:

>>> "0" == false
true

>>> false == "0"
true

那么为什么下面会打印出"ha"?

So why does the following print "ha"?

>>> if ("0") console.log("ha")
ha

推荐答案

原因是因为当你显式做 "0" == false 时，两边都被转换成数字，而 然后进行比较.

The reason is because when you explicitly do "0" == false, both sides are being converted to numbers, and then the comparison is performed.

当你这样做时:if ("0") console.log("ha")，字符串值正在被测试.任何非空字符串都是true，而空字符串是false.

When you do: if ("0") console.log("ha"), the string value is being tested. Any non-empty string is true, while an empty string is false.

相等 (==)

如果两个操作数不属于同一类型，JavaScript 会转换操作数，然后应用严格比较.如果任一操作数是数字或布尔值，则尽可能将操作数转换为数字；否则，如果任一操作数是字符串，则如果可能，另一个操作数将转换为字符串.如果两个操作数都是对象，那么当操作数引用内存中的同一个对象时，JavaScript 会比较相等的内部引用.

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible. If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.

(来自 Mozilla 开发者网络中的 比较运算符)

(From Comparison Operators in Mozilla Developer Network)

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