JavaScript - array join vs string concatenation performance

String concatenation using += operator is faster. Below I've prepared a performance test.

Performance test

// ONLINE-RUNNER:browser;

const test = (repeats, description, func) => {
    const t1 = Date.now();
    for (let i = 0; i < repeats; ++i) {
        func();
    }
    const t2 = Date.now();
    const dt = t2 - t1;
    console.log(`${description}: totoal time is ${dt} ms`);
};


// Tested algorithms:

const joinString = (array) => array.join('');

const concatString = (array) => {
    var result = '';
    for (var i = 0; i < array.length; ++i) {
        result += array[i];
    }
    return result;
};


// Input data:

const array = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'];


// Performance tests:

const repeats = 10000; // set number of repeats depending on cases complexity

test(repeats, 'Using join()', () => joinString(array));
test(repeats, 'Using += operator', () => concatString(array));

See also

1. JavaScript - test algorithm performance