使用“&&"有什么区别?和'||'在三元运算符('?'和':')上? [英] What is the difference between using '&&' and '||' over a ternary operator ('?' and ':')?
问题描述
在JavaScript中,使用
In JavaScript, what is the difference between using
true ? 'Hello' : 'Goodbye' //Evaluates to "Hello"
false ? 'Hello' : 'Goodbye' //Evaluates to "Goodbye"
和
true && 'Hello' || 'Goodbye' //Evaluates to "Hello"
false && 'Hello' || 'Goodbye' //Evaluates to "Goodbye"
推荐答案
这是两个不同的概念,碰巧会为您提供相同的答案.
These are two different concepts that happen to give you the same answer.
第一个示例使用三元运算符和其结果仅取决于第一个操作数(在您的示例中为 true
/ false
):
The first example uses the ternary operator and its result depends only on the first operand (in your example true
/false
):
true ? 'Hello' : 'Goodbye' //Evaluates to "Hello"
false ? 'Hello' : 'Goodbye' //Evaluates to "Goodbye"
它是 if
/ else
的简写形式.如果第一个操作数为真,则返回第二个操作数( Hello
).如果第一个操作数为假,则返回第三个操作数( Goodbye
).
It is a shorthand form of an if
/else
. If the first operand is truthy, return the second operand (Hello
). If the first operand is falsy, return the third operand (Goodbye
).
第一个示例的第一个表达式可以重写为:
The first expression of your first example can be rewritten as:
if (true)
return 'Hello';
else
return 'Goodbye';
第二个示例使用逻辑运算符和其结果取决于第一个和第二个操作数 .
The second example makes use of logical operators and its result depends on both the first and the second operand.
true && 'Hello' || 'Goodbye' //Evaluates to "Hello"
false && 'Hello' || 'Goodbye' //Evaluates to "Goodbye"
如果 firstOperand&&secondOperand
的计算结果为真实值,返回 secondOperand
.如果他们认为错误,请返回 thirdOperand
(再见
).
If firstOperand && secondOperand
evaluates to a truthy value, return secondOperand
. If they evaluate to something falsy, return thirdOperand
(Goodbye
).
在您的示例中,由于非空字符串是真实的,因此 true&&"Hello"
的计算结果为 Hello
,而 false&&"Hello"
的计算结果为 false
.
In your examples, since a non-empty string is truthy, true && 'Hello'
evaluates to Hello
, and false && 'Hello'
evaluates to false
.
因此您的第二个示例变为:
So your second example turns into:
'Hello' || 'Goodbye'
false || 'Goodbye'
由于 ||
起作用,恰好输出您第一个示例输出的内容,但是它们是不同的概念.
Because of how ||
works, that happens to output what your first example outputs, but they're different concepts.
请注意,在第一个示例中,我们如何知道在评估第一个操作数之后将返回什么.在第二个示例中,我们必须先评估前两个 操作数,然后才能知道返回什么.
Notice how in the first example, we know what to return after evaluating the first operand. In the second example, we have to evaluate the first two operands before we know what to return.
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