Lessons · JavaScript · join (pieces to text)
Gluing a list into text
arr.join(sep) makes one string with sep between the items. split does the reverse.
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What it is for
A comma-separated line for a file, a path from segments, tags shown with a dot between them: join is how a list becomes something a person reads.
How to think about it
Choose the separator to match the format: ',' for CSV-like output, '' to run items together, '\n' for lines. Convert items to the text you want first; join calls String on each.
Worked example
console.log([1, 2, 3].join("-"));1-2-3.console.log(["a", "b"].join(""));ab: an empty separator runs them together.console.log([1, 2].join());1,2: the default separator is a comma.
console.log("1-2-3".split("-"));[ '1', '2', '3' ]: split is the inverse.Your turn
One CSV line from the fields.
const line = fields.(",");Solve one with the tests running
The trap
join turns null and undefined into empty strings: [null, undefined].join('-') is '-'. Filter them out first if they should not vanish.
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