logical expression of js
&& ,||, !
operand:
the 3 operator all accept any values as operand,
return value:
&& and || return one of the 2 operand as return value,
! always return true or false as return value,
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truthy & falsy value
falsy value:
false, null, undefined, 0, -0, '', NaN,
truthy value:
any other value,
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&&
logic:
when left value is falsy, return left value,
when left value is truthy, return right value,
short circuit:
if left value is falsy, then the right value will not be evaluated,
e.g.
var a=1;
var b=2;
var c=0;
var d=false;
console.log(a && b); // => 2
console.log(a && c); // => 0
console.log(a && d); // => false
console.log(c && b); // => 0
console.log(c && d); // => 0
console.log(d && c); // => false
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||
logic:
when left value is truthy, return left value,
when left value is falsy, return right value,
short circuit:
if left value is truthy, then the right value will not be evaluated,
e.g.
var a=1;
var b=2;
var c=0;
var d=false;
console.log(a || b); // => 1
console.log(a || c); // => 1
console.log(a || d); // => 1
console.log(c || b); // => 2
console.log(c || d); // => false
console.log(d || c); // => 0
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!
logic:
if value is truthy, return false,
if value is falsy, return true,
e.g.
var a=1;
var b=2;
var c=0;
var d=false;
console.log(!a); // => false
console.log(!b); // => false
console.log(!c); // => true
console.log(!d); // => true
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