The value passed as the first parameter is converted to a boolean value, if necessary. If the value is omitted or is 0, -0, null, false, NaN, undefined, or the empty string (""), the object has an initial value of false. All other values, including any object, an empty array ([]), or the string "false", create an object with an initial value of true.

Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object.

Any object of which the value is not undefined or null, including a Boolean object whose value is false, evaluates to true when passed to a conditional statement. For example, the condition in the following if statement evaluates to true:

const x = new Boolean(false);
if (x) {
  // this code is executed
}

This behavior does not apply to Boolean primitives. For example, the condition in the following if statement evaluates to false:

const x = false;
if (x) {
  // this code is not executed
}

Do not use a Boolean object to convert a non-boolean value to a boolean value. To perform this task, instead, use Boolean as a function, or a double NOT operator:

const good = Boolean(expression);    // use this
const good2 = !!(expression);        // or this
const bad = new Boolean(expression); // don't use this!

If you specify any object, including a Boolean object whose value is false, as the initial value of a Boolean object, the new Boolean object has a value of true.

const myFalse = new Boolean(false);   // initial value of false
const g = Boolean(myFalse);           // initial value of true
const myString = new String('Hello'); // string object
const s = Boolean(myString);          // initial value of true

Do not use a Boolean object in place of a Boolean primitive.

When using == to loosely compare an object to a boolean primitive, it's important to have a clear understanding of what's actually being compared. Consider the following example:

if ([]) { console.log("[] is truthy")}         // logs "[] is truthy"
if ([] == false) { console.log("[] == false")} // logs "[] == false"

The reason for [] == false even though [] is truthy is: the comparison [] == false compares the value of [] to false. And to get the value of [], the JavaScript engine first calls [].toString(). That results in "", and that is what's actually compared to false. In other words, [] == false is equivalent to "" == false. And "" is falsy — and so that's what explains the behavior in the example.

Boolean()

Creates a new Boolean object.

Boolean.prototype.toString()

Returns a string of either true or false depending upon the value of the object. Overrides the Object.prototype.toString() method.

Boolean.prototype.valueOf()

Returns the primitive value of the Boolean object. Overrides the Object.prototype.valueOf() method.

• Boolean
• Boolean primitives
• Boolean data type (Wikipedia)