Logical operators in JavaScript — how do you use them?

问题

I don't understand how &&, ||, and ! work... both with bools and other data types. How do you use them?

回答1:

All values in Javascript are either "truthy" or "falsy".

• a && b evaluates to the first falsy operand
• a || b evaluates to the first truthy operand

Both operators will not evaluate any operands after the one the return.
If all operands don't match, it will evaluate to the last one.

• !a evaluates to true if a is falsy and false if a is truthy.

---

All values are truthy except the following, which are falsy:

• false
• +0
• -0
• NaN
• ""
• null
• undefined
• document.all

回答2:

If you want to test that both of two conditions are truthy then use &&:

if (isX && isY)
{
     // Do something.
}

If you want to test that one or both of two conditions are truthy then use ||:

if (isX || isY)
{
     // Do something.
}

The ! inverts a boolean (a truthy value becomes false and vice versa).

来源：https://stackoverflow.com/questions/4535647/logical-operators-in-javascript-how-do-you-use-them