JavaScript Array valueOf() Method
Array.prototype.valueOf() does not return a comma-separated copy of the array's elements. It returns the array object itself. If an old example prints comma-separated text, that text appears because the array is converted to a string afterward.
const scripts = ["PHP", "ASP", "JavaScript", "HTML"];
const result = scripts.valueOf();
console.log(result === scripts); // true
console.log(result); // the same array
What Array valueOf() Actually Returns Top ↑
const values = [1, 2, 3];
const result = values.valueOf();
console.log(result === values); // true
result.push(4);
console.log(values); // [1, 2, 3, 4]
Because result is the same array object, mutating one reference changes what the other reference observes.
Why document.write(array.valueOf()) Looked Like Array Elements Top ↑
The original page used document.write(scripts.valueOf()) and saw comma-separated values. The important correction is that valueOf() returned the array, and then the writing/string-conversion step converted that array to text.
const scripts = ["PHP", "ASP", "JavaScript", "HTML"];
const sameArray = scripts.valueOf();
console.log(String(sameArray)); // PHP,ASP,JavaScript,HTML
console.log(sameArray === scripts); // true
For explicit text conversion, use toString() or join().
valueOf() with Nested Arrays Top ↑
const data = ["PHP", ["Apache", "IIS"]];
const result = data.valueOf();
console.log(result === data); // true
console.log(String(result)); // PHP,Apache,IIS
Again, the comma-separated output comes from string conversion, not from valueOf() itself.
When Do You Need Array valueOf()? Top ↑
In ordinary application code, you rarely need to call array valueOf() directly because the default method simply returns the object itself. It is more useful as part of understanding JavaScript's object-to-primitive conversion process than as an array-display method.
Use the Method That Matches Your Goal Top ↑
| Goal | Use |
|---|---|
| Get the same array object | array.valueOf() (usually unnecessary) |
| Comma-separated text | array.toString() |
| Custom separator | array.join(separator) |
| Preserve JSON-compatible structure | JSON.stringify(array) |
| Copy the top-level array | [...array], Array.from(array), or array.slice() |
Common valueOf() Misconceptions Top ↑
- Saying
valueOf()returns all array elements as primitive values. It returns the array object. - Using
valueOf()to copy an array. The returned value is the same object. - Attributing comma-separated display to
valueOf()instead of the later string conversion.
Empty an Array Display Array Elements
Subhendu Mohapatra
Author & Instructor at plus2net
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