Significance of valueOf() in javascript

问题

I have noticed the snippet_1 in the implementation of one of the Node API's. Snippet_2 has been written by me. I don't feel much difference between them. Is there really any significance of using valueOf() function. And also, we can notice a property called as valueOf which would return [Function: valueOf]

Snippet_1

Buffer.from = function from(value, encodingOrOffset, length) {
    const valueOf = value.valueOf && value.valueOf();
      if (valueOf !== null && valueOf !== undefined && valueOf !== value)
        return Buffer.from(valueOf, encodingOrOffset, length);

}

Snippet_2

Buffer.from = function from(value, encodingOrOffset, length) {
  if (value !== null && value !== undefined)
    return Buffer.from(value, encodingOrOffset, length);

}

回答1:

If you have an object wrapper and you want to get the underlying primitive value out, you should use the valueOf() method. This is called Unboxing.

All normal objects have the built-in Object.prototype as the top of the prototype chain (like the global scope in scope look-up), where property resolution will stop if not found anywhere prior in the chain. toString(), valueOf(), and several other common utilities exist on this Object.prototype object, explaining how all objects in the language are able to access them.

Assume the following example:

var i = 2;

Number.prototype.valueOf = function() {
    return i++;
};

var a = new Number( 42 );

if (a == 2 && a == 3) {
    console.log( "Yep, this happened." );
}

Will this behave the same way with your snipper? The answer is no.

来源：https://stackoverflow.com/questions/46236011/significance-of-valueof-in-javascript

标签

javascript

node.js

value-of