`Concat`

Two strings (or more) can be combined into a single string—this is called concatenation. In Node.js we use the plus operator or the concat method.

For performance, modern JavaScript runtimes handle concatenation in complex ways. They use "ropes" to avoid copying characters. There is no "string builder" type in JavaScript.

First example

Here we see 2 strings—left and right. We combine them with the plus operator and then we test the concat method. The syntax for concat() is somewhat more complex.

Tip For concat, we call the method on a string. One or more arguments can be passed to concat().

var left = "cat and ";
var right = "bird";

// Use plus to concatenate strings.
var plusResult = left + right;
console.log(plusResult);

// Use concat method.
var concatResult = left.concat(right);
console.log(concatResult);cat and bird
cat and bird

Three strings

The plus operator and the concat method can handle more than two strings at once. We can combine 3 or more strings—here we combine 3 strings.

Detail We can pass two strings as arguments to concat(). This is the same thing as using two plus signs to concatenate 3 strings.

var one = "a";
var two = "b";
var three = "c";

// Concat 3 strings at once.
var all = one + two + three;
var allConcat = one.concat(two, three);

// Write results.
console.log(all);
console.log(allConcat);abc
abc

Benchmark

String concatenation is fast in Node, but how does it compare to using an array? Here we compare similar programs that use concat and push.

Version 1 This loop uses string concat to create many large strings with a single character repeated in them.

Version 2 This loop calls push() on an array to build up a buffer of 1 character repeated 10000 times.

Result The array is faster. If an array can be used instead of concat(), this is likely a performance gain.

// Version 1: append data to a string with concat.
var x1 = performance.now();
for (var v = 0; v < 1000; v++) {
    var letters = "";

    for (var i = 0; i < 10000; i++) {
        letters += "a";
    }
}

// Version 2: append data to an array with push.
var x2 = performance.now();
for (var v = 0; v < 1000; v++) {
    var letters = [];

    for (var i = 0; i < 10000; i++) {
        letters.push("a");
    }
}

var x3 = performance.now();
console.log(x2 - x1);
console.log(x3 - x2);45.46 ms   string concat
38.73 ms   array push

An analysis

The plus sign and concat() are equivalent in JavaScript engines. Internally a rope optimization is used to avoid copying—this makes many concatenations or appends faster.

String concatenation is a common task in Node. With the powerful rope optimizations used, we can concatenate strings without too much performance loss.