javascript-falsy-values

Falsy values in JavaScript

JavaScript has the following falsy values. When used in a boolean context, they will be coerced to false.

Value	Type	Description
null	Null	The keyword null — the absence of any value.
undefined	Undefined	undefined — the primitive value.
false	Boolean	The keyword false.
NaN	Number	NaN — not a number.
0	Number	The Number zero, also including 0.0, 0x0, etc.
-0	Number	The Number negative zero, also including -0.0, -0x0, etc.
0n	BigInt	The BigInt zero, also including 0x0n, etc. Note that there is no BigInt negative zero — the negation of 0n is 0n.
“”	String	Empty string value, also including '' and ``.
document.all	Object	The only falsy object in JavaScript is the built-in document.all.

注意：falsyvalue在if(...)中返回false。

Truthy values

除了上面的Falsy values，剩下的都是truthy values.

Truthy values actually ==false

注意，有些值虽然是truthy values，但是它们竟然==fasle，所以，永远不要在JS中使用==!.
以下这些比较结果都是true。至于原因嘛，请看这篇

console.log('0' == false); // true
console.log(new Number(0) == false); // true
console.log(new Boolean(false) == false); // true
console.log(new String('0') == false); // true
console.log([] == false); // true
console.log([[]] == false); // true
console.log([0] == false); // true

Reference: https://developer.mozilla.org/en-US/docs/Glossary/Falsy