真值與假值：
在 Javascript 中，真值和假值用來決定布林值中的值是 true 或 false
context.這個概念對於使用 if 語句等條件控製程式流程至關重要。
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
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