真值和假值：
在 javascript 中，真值和假值用于确定布尔值中的值是 true 还是 false
context.这个概念对于使用 if 语句等条件控制程序流程至关重要。
falsy values: 0," ",null,nan,false,undefined
console.log(boolean(0)); //false
console.log(boolean(undefined)); //false
console.log(boolean(' ')); //empty false
console.log(boolean(null)); //false
console.log(boolean(nan)); //false not a number
console.log(boolean(false)); //false
真值：任何非假值：
console.log(boolean(1)); //true
console.log(boolean(1833933)); //true
console.log(boolean(-1)); //true
console.log(boolean("hello")); //true
console.log(boolean(1)); //true
console.log(boolean([])); //true empty array
console.log(boolean({})); //true empty object
console.log(function (){}); //true
example:
t f
let cash =255 //0 ;
//conditions false statement block not run
if (cash){
console.log("you can buy burger with drink");
}else{
console.log("you can buy burger");
}else{
console.log("you don't have money");
}
let a; console.log(a); //false
output: undefined
let a = 10; console.log(a); //true
let a = null; console.log(a); //false