Nukem's Possible Manifestation
Learn about our RFC process, Open RFC meetings & more.Join in the discussion! »


1.0.3 • Public • Published


Very efficient module for extracting a subset of lines from a string.

Similar to str.slice(beginIndex, endIndex) except that the indexes are lines instead of bytes.

Build status js-standard-style


npm install slice-lines --save


const sliceLines = require('slice-lines')
const text = 'first line\n2nd line\r\nlast line'
sliceLines(text, 1)    // => "2nd line\r\nlast line"
sliceLines(text, 1, 2) // => "2nd line"
sliceLines(text, 0, 2) // => "first line\n2nd line"
sliceLines(text, -1))  // => "last line"


lines = sliceLines(str, beginIndex[, endIndex])

Returns a string containing the requested lines.


  • str - The string to extract lines from
  • beginIndex - The zero-based line index at which to begin extraction. If negative, it is treated as totalLines + beginIndex where totalLines is the total number of lines in str (for example, if beginIndex is -3 it is treated as totalLines - 3). If beginIndex is greater than or equal to the total number of lines in the string, sliceLines() returns an empty string
  • endIndex - Optional. The zero-based line index before which to end extraction. The line at this index will not be included. If endIndex is omitted, sliceLines() extracts to the end of the string. If negative, it is treated as totalLines + endIndex where totalLines is the total number of lines in str (for example, if endIndex is -3 it is treated as totalLines - 3)


The classic way of extracting a specific line from a text in JavaScript is using str.split() with either \n as the separator or the even slower regex /\r?\n/ if support for Windows line-breaks are required.

That approach requires a complete traversal of the entire text + contruction of new objects and strings for each line in the text.

As seen below sliceLines() can be several orders of magnitudes faster and scales linear with the amount of lines in the text.

Example extracting a line from a text with 500,000 lines

# sliceLines(str, 0, 1) // V8 unoptimized
ok ~1.19 ms (0 s + 1189464 ns)

# sliceLines(str, 0, 1) // V8 optimized
ok ~258 μs (0 s + 258238 ns)

# sliceLines(str, 10000, 10001)
ok ~406 μs (0 s + 406363 ns)

# sliceLines(str, -1) // last line relative
ok ~26 ms (0 s + 25749953 ns)

# sliceLines(str, 513828) // last line by index
ok ~15 ms (0 s + 14732301 ns)

# str.split('\n')[10000]
ok ~120 ms (0 s + 119909150 ns)

# str.split(/\r?\n/)[10000]
ok ~175 ms (0 s + 174632829 ns)




npm i slice-lines

DownloadsWeekly Downloads






Last publish


  • avatar