|| and && usage in JavaScript (detailed explanation)

&& and || are particularly widely used in JQuery source code. I found some examples on the Internet as a reference and studied their usage:

##&&

function a(){
    alert("a");
    return true;
}
function b(){
    alert("b");
    return true;
}
var c=a()&&b();
alert(c);

a() && b(): If true is returned after executing a(), b() is executed and the value of b is returned; if false is returned after a() is executed, the entire expression is returned The value of a(), b() is not executed;

||

function a(){
    alert("a");
    return true;
}
function b(){
    alert("b");
    return false;
}
var c=a()||b();
alert(c);

a() || b(): if executed If true is returned after a(), the entire expression returns the value of a(), and b() is not executed; if false is returned after executing a(), b() is executed and the value of b() is returned;

&& has a higher priority than ||

alert((1 && 3 || 0) && 4); //Result 4 ①

alert(1 && 3 || 0 && 4); //Result 3 ②
alert(0 && 3 || 1 && 4); //Result 4 ③

Analysis:

Statement ①: 1&&3 Return 3 => 3 || 0 Returns 3 => 3&&4 Returns 4
Statement ②: Execute 1&&3 first and return 3, then execute 0&&4 and return 0, and finally compare the execution results with 3||0 Return 3
Statement ③: First Executing 0&&3 returns 0, executing 1&&4 returns 4, and the final execution result is compared with 0||4 and returns 4

Note: All non-0 integers are true, and undefined, null and empty string "" are false.

In javascript, && can not only be used for boolean types, 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 type object, then the second object is returned.
l If any operand is null, then null is returned.
l If any operand is NaN, return NaN.
l If any operand is undefined, then undefined is returned.

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. result.

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 true is returned 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, then the first object is returned.
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

No need to make it so complicated. I recommend you read this part of the explanation

a && b: Convert a and b to Boolean type, then perform logical AND, true returns b, false returns a
a || b: Convert a, b to Boolean type, then perform logical OR, true returns a, false returns b

Conversion rules:
Object is true
Non-zero number is true
Non-empty string is true
Others are false

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