JS: Number Constructor

new Number(arg)

return a object that represent the number arg.

// Number called as constructor, returns object
(typeof (new Number(3))) === "object"

Number(arg)

return a Primitive Value of the number arg

// Number called as function, return primitive
(typeof Number(3)) === "number"

Special Results

Example: Special Literal Values of Number to Primitive

/* test convert number literal value to number primitive. */

console.log(
  Number(Infinity) === Infinity,
  Number(-Infinity) === -Infinity,
  Number.isNaN(Number(NaN)),
);

/* test convert undefined and null to number primitive. */
console.log(
  Number.isNaN(Number(undefined)),
  Number(null) === 0,
);

/* test convert boolean value to number primitive. */
console.log(
  Number(true) === 1,
  Number(false) === 0,
);

Example: Convert String to Number Primitive

console.log(Number("0.03") === 0.03);

// extra zero before or after is ok
console.log(
  Number("00.03") === 0.03,
  Number("00.030") === 0.03,
);

// extra space in front or back are ok
console.log(
  Number("  0.03  ") === 0.03,
  Number("  +0.03  ") === 0.03,
  Number("  -0.03  ") === -0.03,
);

// cannot have space after sign
console.log(Number.isNaN(Number("- 0.03 ")));

// cannot have space between digits
console.log(Number.isNaN(Number("1 000")));

// lowline is ok
console.log(Number.isNaN(Number("1_000")));