javascript 返回&&
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returning with &&
提问by steve
What does it mean to return a value with &&?
用&&返回一个值是什么意思？
else if (document.defaultView && document.defaultView.getComputedStyle) {
// It uses the traditional ' text-align' style of rule writing,
// instead of textAlign
name = name.replace(/([A-Z]) /g, " -" );
name = name.toLowerCase();
// Get the style object and get the value of the property (if it exists)
var s = document.defaultView.getComputedStyle(elem, " ") ;
return s && s.getPropertyValue(name) ;
回答by dheerosaur
return a && bmeans "return a if a is falsy, return b if a is truthy".
return a && b意思是“如果 a 为假则返回 a，如果 a 为真则返回 b”。
It is equivalent to
它相当于
if (a) return b;
else return a;
回答by Spiny Norman
The logical AND operator, &&, works similarly. If the first object is falsy, it returns that object. If it is truthy, it returns the second object. (from https://www.nfriedly.com/techblog/2009/07/advanced-javascript-operators-and-truthy-falsy/).
逻辑 AND 运算符 && 的工作原理类似。如果第一个对象为假，则返回该对象。如果为真，则返回第二个对象。（来自https://www.nfriedly.com/techblog/2009/07/advanced-javascript-operators-and-truthy-falsy/）。
Interesting stuff!
有趣的东西！
EDIT:
So, in your case, if document.defaultView.getComputedStyle(elem, " ")does not return a meaningful ("truthy") value, that value is returned. Otherwise, it returns s.getPropertyValue(name).
编辑：因此，在您的情况下，如果document.defaultView.getComputedStyle(elem, " ")没有返回有意义的（“真实”）值，则返回该值。否则，它返回s.getPropertyValue(name)。
回答by S7_0
The AND && operator does the following:
AND && 运算符执行以下操作：
- Evaluate operands from left to right.
- For each operand, convert it to a boolean. If the resultis false, stop and return the original value of that result.
- If all other operands have been assessed (i.e. all were truthy), return the last operand.
- 从左到右计算操作数。
- 对于每个操作数，将其转换为布尔值。如果结果为假，则停止并返回该结果的原始值。
- 如果所有其他操作数都已评估（即全部为真），则返回最后一个操作数。
As I said, each operand is convert to a boolean, if it's 0 it's falsyand every other value different than 0 (1, 56, -2, etc etc) are truthy
正如我所说，每个操作数都被转换为一个布尔值，如果它是 0 它是假的，并且每个其他不同于 0 的值（1、56、-2 等）都是真
In other words, AND returns the first falsy value or the last value if none were found.
换句话说，如果没有找到，AND 返回第一个假值或最后一个值。
// if the first operand is truthy,
// AND returns the second operand:
return 1 && 0 // 0
return 1 && 5 // 5
// if the first operand is falsy,
// AND returns it. The second operand is ignored
return null && 5 // null
return 0 && "no matter what" // 0
We can also pass several values in a row. See how the first falsy one is returned:
我们还可以连续传递多个值。看看第一个假是如何返回的：
return 1 && 2 && null && 3 // null
When all values are truthy, the last value is returned:
当所有值都为真时，返回最后一个值：
return 1 && 2 && 3 // 3, the last one
You can learn more about the logical operator here https://javascript.info/logical-operators
您可以在此处了解有关逻辑运算符的更多信息https://javascript.info/logical-operators
