如何在 ECMAScript 类中调用方法

Question

 class Percentage { constructor(percent) { this.percent = percent; } toString() { return `${this.percent}%`; } valueOf(){ return this.percent / 100; } } let fivePercent = new Percentage(5); console.log(`${fivePercent} of 50 is ${50*fivePercent}`);

I am getting consoled with valid output. But Unable to understand how the "toString" and "valueOf" methods are called here?

Any one help me to understand?

Answer 1

When you use console.log, or perform any operation that implicitly converts an object to a string, javascript will call toString() on that object, if that method is defined.

So this expression:

`${fivePercent}`

Tells javascript to get the value of fivePercent.toString() .

Likewise, when you evaluate this expression:

50 * fivePercent

You're telling javascript to convert fivePercent to a numerical value, then multiply it by 50. If valueOf() is defined on the object, it will call that method to get the value; otherwise, the result will be NaN.

Answer 2

JavaScript calls the valueOf method to convert an object to a primitive value. It automatically invokes it when encountering an object where a primitive value is expected.

You can read more about it in the below link
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/valueOf