JS: Number Constructor

By Xah Lee. Date: 2017-03-25. Last updated: 2026-02-02.

new Number(arg)

return a object that represent the number arg.

// Number called as constructor, returns object
console.assert((typeof (new Number(3))) === "object");

Number(arg)

return a Primitive Value of the number arg

// Number called as function, return primitive
console.assert((typeof Number(3)) === "number");

Special Results

Argument Type	Result
`undefined`	Return `NaN`.
`null`	Return +0.
`true` or `false`	Return `1` if argument is `true`. Return `+0` if argument is `false`.
Number	Return argument (no conversion).
String	Return a number or `NaN`.
Symbol	Throw a TypeError exception.
Object	Return a number or `NaN`

Example. Special Literal Values of Number to Primitive

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

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

/* test convert number literal value to number primitive. */
console.assert(Number(Infinity) === Infinity);
console.assert(Number(-Infinity) === -Infinity);
console.assert(Number.isNaN(Number(NaN)));

Example. Convert String to Number Primitive

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

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

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

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

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

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