Boolean

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

Any object, 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 the Boolean() constructor with new to convert a non-boolean value to a boolean value — use Boolean as a function or a double NOT instead:

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

Many built-in operations that expect booleans first coerce their arguments to booleans. The operation can be summarized as follows:

• Booleans are returned as-is.
• undefined turns into false.
• null turns into false.
• 0, -0, and NaN turn into false; other numbers turn into true.
• 0n turns into false; other BigInts turn into true.
• The empty string "" turns into false; other strings turn into true.
• Symbols turn into true.
• All objects become true.

In other words, there are only a handful of values that get coerced to false — these are called falsy values. All other values are called truthy values. A value's truthiness is important when used with logical operators, conditional statements, or any boolean context.

There are two ways to achieve the same effect in JavaScript.

• Double NOT: !!x negates x twice, which converts x to a boolean using the same algorithm as above.
• The Boolean() function: Boolean(x) uses the same algorithm as above to convert x.

Note that truthiness is not the same as being loosely equal to true or false.

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

[] is truthy, but it's also loosely equal to false. It's truthy, because all objects are truthy. However, when comparing with false, which is a primitive, [] is also converted to a primitive, which is "" via Array.prototype.toString(). Comparing strings and booleans results in both being converted to numbers, and they both become 0, so [] == false is true. In general, falsiness and == false differ in the following cases:

• NaN, undefined, and null are falsy but not loosely equal to false.
• "0" (and other string literals that are not "" but get coerced to 0) is truthy but loosely equal to false.
• Objects are always truthy, but their primitive representation may be loosely equal to false.

Truthy values are even more unlikely to be loosely equal to true. All values are either truthy or falsy, but most values are loosely equal to neither true nor false.

Boolean()

Creates a new Boolean object.

These properties are defined on Boolean.prototype and shared by all Boolean instances.

Boolean.prototype.constructor

The constructor function that created the instance object. For Boolean instances, the initial value is the Boolean constructor.

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)