Array.prototype.toString()
The toString()
method returns a string representing the
specified array and its elements.
Try it
Syntax
js
toString()
Return value
A string representing the elements of the array.
Description
The Array
object overrides the toString
method of Object
. The toString
method of arrays calls join()
internally, which joins the array and returns one string containing each array element separated by commas. If the join
method is unavailable or is not a function, Object.prototype.toString
is used instead, returning [object Array]
.
js
const arr = [];
arr.join = 1; // re-assign `join` with a non-function
console.log(arr.toString()); // [object Array]
console.log(Array.prototype.toString.call({ join: () => 1 })); // 1
JavaScript calls the toString
method automatically when an array is to be represented as a text value or when an array is referred to in a string concatenation.
Array.prototype.toString
recursively converts each element, including other arrays, to strings. Because the string returned by Array.prototype.toString
does not have delimiters, nested arrays look like they are flattened.
js
const matrix = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9],
];
console.log(matrix.toString()); // 1,2,3,4,5,6,7,8,9
When an array is cyclic (it contains an element that is itself), browsers avoid infinite recursion by ignoring the cyclic reference.
js
const arr = [];
arr.push(1, [3, arr, 4], 2);
console.log(arr.toString()); // 1,3,,4,2
Examples
Using toString()
js
const array1 = [1, 2, "a", "1a"];
console.log(array1.toString()); // "1,2,a,1a"
Using toString() on sparse arrays
Following the behavior of join()
, toString()
treats empty slots the same as undefined
and produces an extra separator:
js
console.log([1, , 3].toString()); // '1,,3'
Calling toString() on non-array objects
toString()
is generic. It expects this
to have a join()
method; or, failing that, uses Object.prototype.toString()
instead.
js
console.log(Array.prototype.toString.call({ join: () => 1 }));
// 1; a number
console.log(Array.prototype.toString.call({ join: () => undefined }));
// undefined
console.log(Array.prototype.toString.call({ join: "not function" }));
// "[object Object]"
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.tostring |
Browser compatibility
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