String.prototype.concat() - JavaScript | MDN (original) (raw)

Baseline

Widely available

The concat() method of String values concatenates the string arguments to this string and returns a new string.

Try it

const str1 = "Hello";
const str2 = "World";

console.log(str1.concat(" ", str2));
// Expected output: "Hello World"

console.log(str2.concat(", ", str1));
// Expected output: "World, Hello"

Syntax

concat(str1)
concat(str1, str2)
concat(str1, str2, /* …, */ strN)

Parameters

str1, …, strN

One or more strings to concatenate to str. Though technically permitted, calling String.prototype.concat() with no arguments is a useless operation, because it does not result in observable copying (like Array.prototype.concat()), since strings are immutable. It should only happen if you are spreading an array of strings as arguments, and that array happens to be empty.

Return value

A new string containing the combined text of the strings provided.

Description

The concat() function concatenates the string arguments to the calling string and returns a new string.

If the arguments are not of the type string, they are converted to string values before concatenating.

The concat() method is very similar to the addition/string concatenation operators (+, +=), except that concat() coerces its arguments directly to strings, while addition coerces its operands to primitives first. For more information, see the reference page for the + operator.

Examples

Using concat()

The following example combines strings into a new string.

const hello = "Hello, ";
console.log(hello.concat("Kevin", ". Have a nice day."));
// Hello, Kevin. Have a nice day.

const greetList = ["Hello", " ", "Venkat", "!"];
"".concat(...greetList); // "Hello Venkat!"

"".concat({}); // "[object Object]"
"".concat([]); // ""
"".concat(null); // "null"
"".concat(true); // "true"
"".concat(4, 5); // "45"

Specifications

Browser compatibility

See also