JavaScript: Number Constructor

By Xah Lee. Date: . Last updated: .
Number(arg)
Convert arg to a number Primitive Value , return it .
// Number called as function, return a number primitive value
console.log((typeof Number(3)) === "number");
new Number(arg)
Convert arg to a number primitive value, return a number object that represent the number.
// Number called as constructor, returns a number object
console.log((typeof (new Number(3))) === "object");

Convert to Number

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

Special literal values of number.

// test converting special literal value to number primitive.

// no change
console.log(Number(Infinity) === Infinity);
console.log(Number(-Infinity) === -Infinity);
console.log(Number.isNaN(Number(NaN)));

// changed
console.log(Number.isNaN(Number(undefined)));
console.log(Number(null) === 0);
console.log(Number(true) === 1);
console.log(Number(false) === 0);
// all true

Convert string to number.

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

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

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

// all true

JavaScript Number

BUY
ΣJS
JavaScript in Depth

JS Obj Reference

Number

prototype