JS: Boolean. true false

Literal true and false

true and false are builtin Primitive Values . And they are the only possible values of boolean type.

console.assert(
 (typeof true) === "boolean",
);

console.assert(
 (typeof false) === "boolean",
);

• JS: “typeof” operator
• JS: console.assert

Force convert any value to boolean

When a value x is evaluated in boolean context, such as in a if then else , the function Boolean(x) is called to force the value into true or false.

• JS: Boolean Constructor

What values evaluates to false by boolean constructor

• Number value zero 0.
• empty string ""
• undefined
• null
• NaN
• false

Anything else is true.

console.assert(Boolean(0) === false);
console.assert(Boolean(0.0) === false);
console.assert(Boolean(0.) === false);
console.assert(Boolean(.0) === false);

console.assert(Boolean("") === false);

console.assert(Boolean(null) === false);
console.assert(Boolean(undefined) === false);
console.assert(Boolean(NaN) === false);

// anything else is true
console.assert(Boolean(1));
console.assert(Boolean(-1));
console.assert(Boolean(Infinity));
console.assert(Boolean({}));
console.assert(Boolean([]));
console.assert(Boolean(function () {}));