为什么我不能覆盖字符串实例的迭代器而不是 String.prototype？

Question

This code allows to use the spread operator to split the words of the sentence, just as expected:

 String.prototype[Symbol.iterator] = function* () { yield* this.split(' '); } const result = [...'The quick brown fox jumped over the lazy dog.']; console.log(result);

I wanted to try overriding the Symbol.iterator at instance level to avoid mutating String.prototype , here is my attempt:

 const sentence = 'The quick brown fox jumped over the lazy dog.'; sentence[Symbol.iterator] = function* () { yield* this.split(' '); } const result = [...sentence]; console.log(result); // Not the expected result console.log(sentence[Symbol.iterator]); // Native String iterator

The native String.prototype[Symbol.iterator] doesn't look overridable at instance level, and I don't get why. My guess is that sentence is a primitive and not really a String instance. Then how do I achieve this?

Answer 1

Because string primitives are not string objects. Instead, when you use methods on them (or access properties on them), it's as though they were converted to object first. (In loose mode, an object is actually created, but the object is thrown away when the method returns unless something keeps a reference to it.) That means that this code modifies a temporary object that's never saved (not the string primitive; string primitives are immutable):

sentence[Symbol.iterator] = function* () {
  yield* this.split(' ');
}

Since it's never saved, later when you use the iterator, you get the default one.

You can create a string object instead:

 const sentence = new String('The quick brown fox jumped over the lazy dog.'); // −−−−−−−−−−−−−−^^^^^^^^^^^−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−^ sentence[Symbol.iterator] = function* () { yield* this.split(' '); }; const result = [...sentence]; console.log(result);

You could define your own words method on strings:

 Object.defineProperty(String.prototype, "words", { *value() { yield* this.split(" "); }, configurable: true, writable: true, }); const sentence = "The quick brown fox jumped over the lazy dog."; const result = [...sentence.words()]; console.log(result);

I strongly discourage modifying built-in prototype in a library or similar, but if you're aware of the dangers of future naming conflicts and such, you can do it on your own page or app.

Or as VLAZ points out , just write a function you pass the string into. :-)

Answer 2

You cannot change the "instance" because a primitive is not one. For example "hello" is only a string primitive that does not have any properties of its own. Same with other primitives like 42 .

Calling any object method or using propertirs on a primitive will convert the value to an object for just that operation: ("hello").split(" ") or (42).toFixed(2) . The primitives themselves are immutable.

Without polluting any prototypes or changing any objects, you can simply create a reusable generator function and spread its result:

 const sentence = 'The quick brown fox jumped over the lazy dog.'; function* toWords(str) { yield* str.split(' '); } const result = [...toWords(sentence)]; console.log(result);