Number() 构造函数

¥Syntax

new Number(value)
Number(value)

注意：Number() 可以与 new 一起调用，也可以不与 new 一起调用，但效果不同。参见 返回值。

¥Note: Number() can be called with or without new, but with different effects. See Return value.

¥Parameters

value

正在创建的对象的数值。

¥Return value

当 Number() 作为函数调用时（不带 new），它返回 value 强制转换为数字原语。具体来说，BigInts 值会转换为数字而不是抛出。如果 value 不存在，则变为 0。

¥When Number() is called as a function (without new), it returns value coerced to a number primitive. Specially, BigInts values are converted to numbers instead of throwing. If value is absent, it becomes 0.

当 Number() 作为构造函数调用时（带有 new），它使用上面的强制过程并返回封装 Number 对象，该对象不是基元。

¥When Number() is called as a constructor (with new), it uses the coercion process above and returns a wrapping Number object, which is not a primitive.

警告：你应该很少发现自己使用 Number 作为构造函数。

¥Warning: You should rarely find yourself using Number as a constructor.

¥Examples

¥Creating Number objects

const a = new Number("123"); // a === 123 is false
const b = Number("123"); // b === 123 is true
a instanceof Number; // is true
b instanceof Number; // is false
typeof a; // "object"
typeof b; // "number"

¥Using Number() to convert a BigInt to a number

Number() 是 BigInt 可以转换为数字而不抛出异常的唯一情况，因为它非常明确。

¥Number() is the only case where a BigInt can be converted to a number without throwing, because it's very explicit.

+1n; // TypeError: Cannot convert a BigInt value to a number
0 + 1n; // TypeError: Cannot mix BigInt and other types, use explicit conversions

Number(1n); // 1

请注意，如果 BigInt 太大而无法达到 安全代表，这可能会导致精度损失。

¥Note that this may result in loss of precision, if the BigInt is too large to be safely represented.

BigInt(Number(2n ** 54n + 1n)) === 2n ** 54n + 1n; // false

¥See also

• core-js 中现代 Number 行为的 Polyfill（支持二进制和八进制文字）
• NaN
• Math
• BigInt