JS: Number Constructor

By Xah Lee. Date: . Last updated: .
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 TypeResult
undefinedReturn NaN.
nullReturn +0.
true or falseReturn 1 if argument is true. Return +0 if argument is false.
NumberReturn argument (no conversion).
StringReturn a number or NaN.
SymbolThrow a TypeError exception.
ObjectReturn 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)) === true);
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")));

JavaScript. Number