Why 0 && true echo 0 in js?

Question

As title

As I know

(if this part is true) && (this part will execute)

if(condition){
   (this part will execute)
}

0 is false, so why not echo false but 0?

Answer 1

Because operator && return first falsey element otherwise they return last element

1 && 0 && false // 0
1 && 2 && 3     // 3

Answer 2

From MDN:

expr1 && expr2 -- Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.

expr1 || expr2 -- Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true.

!expr -- Returns false if its single operand can be converted to true; otherwise, returns true.

Some expressions that can be converted to false are:

• null
• NaN
• 0
• empty string("" or '' or ``)
• undefined

Short-circuit evaluation

As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:

• false && (anything) is short-circuit evaluated to false.
• true || (anything) is short-circuit evaluated to true.

Answer 3

The JavaScript documentation of logical operators explains:

Logical operators are typically used with Boolean (logical) values. When they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value.