What is JSON Lines?

JSON Lines is one complete JSON object per line, with no enclosing array and no commas between records. The file is not valid JSON as a whole, and that is the point.

Who uses it

Log pipelines, machine learning datasets and anything that appends records or reads them in a stream.

Reading JSON Lines

Each line is parsed on its own, so a corrupt record does not take the file with it. Keys are unioned across lines in first-seen order, so a record missing a field leaves that cell empty rather than shifting the columns.

Writing JSON Lines

Every row becomes one line. You can append to the file forever without rewriting it, and a reader can process it a line at a time without holding the whole thing in memory.

What it can hold

JSON Lines is a data interchange format. One complete JSON object per line, so a file can be read a row at a time without holding all of it in memory. A machine can read the table back out of it without guessing, which is what makes it safe to convert into and out of.

It keeps

Types. A number stays a number and a boolean stays a boolean, so whatever reads the file next does not have to guess from how a value looks.
The difference between empty and missing, which most table formats cannot express at all.
Nested values. A cell can hold a list or an object rather than only a single value.
Its own record boundaries. A reader can take one row at a time without loading the file, and a copy that was cut off part way through still yields everything before the cut.

It cannot express

A line break inside a cell. One would end the row, so it has to be replaced before writing.
Any formatting. Bold, colour, column widths and merged cells have nowhere to go.
A header row. Column names are repeated on every record instead, which is why a file in this format is larger than the table it holds.

What it looks like

Example JSON Lines

{"id":"007","name":"Halima Yusuf","role":"Analyst, data","started":"2024-03-15","hours":"38.5"}
{"id":"012","name":"Jonah \"Jo\" Pryce","role":"Engineer","started":"2025-11-02","hours":""}
{"id":"104","name":"Wei Chen","role":"Manager","started":"2023-06-30","hours":"40"}

Questions

What is the difference between JSON and JSON Lines?

JSON wraps records in an array, so a reader must load the whole document before it can use any of it, and appending means rewriting the file. JSON Lines has no wrapper, so records can be streamed and appended one at a time. Use JSON for a payload, JSON Lines for a log.

How do I open JSON Lines files?

Any text editor will show you the contents. To read it as a table, paste it into the converter on this site and it appears in an editable grid. Nothing is uploaded.

Is JSON Lines a good format for sharing data?

It depends who is receiving it. Log pipelines, machine learning datasets and anything that appends records or reads them in a stream. If the recipient is outside that group, a plain CSV travels further than anything else.