Boolean

Baseline

20157

(Boolean) 2 true false

[ NOT (!)] Array.isArray() && ||

if...else while (? :) Array.prototype.filter()

JavaScript JavaScript if (condition) if (!condition) if (condition === true) if (condition === false)

// :
// 
const isObject = (obj) => !!obj && typeof obj === "object";

// :
const isObject = (obj) => Boolean(obj) && typeof obj === "object";

// :
const isObject = (obj) => obj !== null && typeof obj === "object";

// :
//  false 
const isObject = (obj) => obj && typeof obj === "object";

Boolean

Boolean Boolean() new

const good = Boolean(expression);
const good2 = !!expression;

const bad = new Boolean(expression); //

Boolean false true

if (new Boolean(true)) {
  console.log("");
}

if (new Boolean(false)) {
  console.log("");
}

const myFalse = new Boolean(false); // myFalse  Boolean  false 
const g = Boolean(myFalse); // g  true
const myString = new String("Hello"); // myString  String 
const s = Boolean(myString); // s  true

: Boolean

undefined false
null false
0, -0, NaN false true
0n false BigInt true
Symbol true
true

: document.all false

false (falsy) (truthy)

JavaScript 2

: !!x x 2 x
Boolean() Boolean(x) x

true false

if ([]) {
  console.log("[] ");
}
if ([] == false) {
  console.log("[] == false");
}
// [] 
// [] == false

[] false [] false [] Array.prototype.toString() "" 0 [] == false true == false

NaN, undefined, null false
"0" "" 0 false
false

true true false

Boolean(): Boolean

Boolean.prototype Boolean

Boolean.prototype.constructor: Boolean Boolean

Boolean.prototype.toString(): true false Object.prototype.toString()
Boolean.prototype.valueOf(): Boolean Object.prototype.valueOf()

false

const bNoParam = Boolean();
const bZero = Boolean(0);
const bNull = Boolean(null);
const bEmptyString = Boolean("");
const bfalse = Boolean(false);

true

const btrue = Boolean(true);
const btrueString = Boolean("true");
const bfalseString = Boolean("false");
const bSuLin = Boolean("Su Lin");
const bArrayProto = Boolean([]);
const bObjProto = Boolean({});


ECMAScript 2027 LanguageSpecification # sec-boolean-objects

Boolean

MDN

2025830 MDN

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