JavaScript - what do square brackets around expression mean?

Square brackets can be used to:

• create a new array,
• destructure arrays.

1. Creating new array

// ONLINE-RUNNER:browser;

const array1 = new Array('a', 'b');

const array2 = ['a', 'b']; // equal to the syntax above

console.log(array1); // [ 'a', 'b' ]
console.log(array2); // [ 'a', 'b' ]

2. Destructuring arrays (ES6)

Square brackets ([]) on the left side of the expression are used to "unpack" values from arrays.

// ONLINE-RUNNER:browser;

const letters = ['a', 'b', 'c'];

const [first] = letters;

console.log(first); // 'a'

going further:

// ONLINE-RUNNER:browser;

const letters = ['a', 'b', 'c'];

const [first, second] = letters;

console.log(first);  // 'a'
console.log(second); // 'b'

References

1. Destructuring assignment - JavaScript | MDN
2. Array() constructor - JavaScript | MDN

In your case, there's no difference if you use square brackets or not because if it's an array it will be converted to a string.

However, when you remove the brackets it will take less time because it doesn't have to build a new array and convert it.

// ONLINE-RUNNER:browser;

const x = 5;

const text = 'value: ' + [x];

console.log(text);        // 'value: 5'