Javascript - If 语句 - 条件中的字符串&&布尔值
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Javascript - If statement - string && boolean in condition
提问by Justin
If I am testing a boolean by itself I know I don't have to put == true.
如果我自己测试一个布尔值，我知道我不必输入 == true。
if (bool) {
foo();
}
I have an if condition that tests for the truthfulness of both a sting's value and a boolean, can I replace:
我有一个 if 条件来测试 sting 值和布尔值的真实性，我可以替换：
if ( string === "hello" && bool == true ) {
foo();
}
with:
和：
if ( string === "hello" && bool ) {
foo();
}
?
?
Also, would a bool value ever use a triple equal sign? So far what I've seen on bool tests are double equals only.
此外，布尔值是否会使用三重等号？到目前为止，我在 bool 测试中看到的只是双重等于。
Thanks.
谢谢。
回答by contactmatt
Use the triple equal sign, it's preferred.
使用三重等号，它是首选。
if(bool == true), if(bool === true) and if(bool)are all different expressions. The first is saying that the value bool must be of the primitive JavaScript type true, or the object representation of the Boolean (i.e. new Boolean()) object with a value of true. The second is saying that the value "bool" mustbe of the primitive type bool(the new Boolean() object, even with a value of true, will fail). The third expression is only false if bool is
if(bool == true),if(bool === true) and if(bool)都是不同的表达方式。第一个是说 bool 值必须是原始 JavaScript 类型 true，或值为 true 的 Boolean（即 new Boolean()）对象的对象表示。第二个是说值“bool”必须是原始类型 bool（新的 Boolean() 对象，即使值为 true，也会失败）。只有当 bool 为时，第三个表达式才为假
- false
- null
- undefined
- empty string ''
- The number 0
- The number NaN
- 错误的
- 空值
- 不明确的
- 空字符串''
- 数字 0
- 数字 NaN
Meaning, the second expression will evalauate to true if you pass in an empty object ({}), for example.
这意味着，例如，如果您传入一个空对象 ({})，则第二个表达式将评估为真。
Some Examples:
一些例子：
var bool = true;
var boolObj = new Boolean(true);
var boolEmptyObj = {};
if (bool === true) { alert("This will alert"); }
if (boolObj === true) { alert("Nope, this wont alert"); }
if (boolEmptyObj) { alert("This will alert"); }
回答by MattDiamant
Yes. If bool is truly a boolean, it will evaluate to true or false, just the same as any expression would. You can also make sure that bool is boolean by using:
是的。如果 bool 是真正的布尔值，它将评估为真或假，就像任何表达式一样。您还可以使用以下方法确保 bool 是布尔值：
!!bool
回答by Jefferson Henrique C. Soares
Yes, but when you use only boolthe value is falsefor these values: null, undefined, 0and "" (empty string). You need to be careful.
是的，但是当你只使用bool值是false这些值：null，undefined，0和"" (empty string)。你需要小心。
