Summary of using && and || in Javascript_javascript tips

&& and || in ordinary cases are relatively simple and will not be discussed here.

Prepare two objects for the following discussion.

In javascript, && can not only be used for boolean type, nor can it only return Boolean type results.
l If the first operand is of type Boolean and the value is false, then return false directly.
l If the first operand is of type Boolean and the value is true, and the other operand is of type object, then this object will be returned.
l If both operands are of object type, return the second object.
l If any operand is null, then null is returned.
l If either operand is NaN, return NaN.
l If any operand is undefined, return undefined.


alert(false && alice); // false
alert(true && alice); // alice

alert(alice && smith); // smith
alert(smith && alice); // alice

alert(null && alice); // null
alert(NaN && alice); // NaN
alert(undefined && alice); // undefined
alert(alice && undefined); // undefined

For ||, it is not only used for Boolean type, nor only returns Boolean type results.
In fact, null, undefined, and NaN will be treated as false. And the object is treated as true.

l If the first operand is of type boolean and the value is true, then return true directly.
l If the first operand is of type Boolean and the value is false and the second operand is object, then the object object is returned.
l If both operands are of type object, return the first object.
l If both operands are null, then null is returned.
l If both operands are NaN, return NaN.
l If both operands are undefined, then undefined is returned.
alert(false || alice); // alice

alert(true || alice); // true

alert(alice || smith); // alice

alert(smith || alice); // smith

alert(null || alice); // alice

alert(alice || null); // alice

alert(null || null); // null

alert(NaN || alice); // alice

alert(alice || NaN); // alice

alert(NaN || NaN);                               // NaN

alert(undefined || alice); // alice

alert(alice || undefined); // alice

alert(undefined || undefined); // undefined

You don’t need to make it so complicated. I recommend you read this part of the explanation.
a && b: Convert a and b to Boolean types, and then perform logical AND. True returns b, false returns a
a || b: Convert a, b to Boolean type, and then perform logical OR, true returns a, false returns b
Conversion rules:
Object is true
Non-zero number is true
Non-empty strings are true
Others are false

Related articles can refer to the following articles to summarize
js AND or operator || && Magical Uses
JS uses AND or operator priority to implement if else conditional judgment expression
Alternative usage skills of javascript && and || algorithms