Get indices of a value in an array

Collect every index where a value appears.

JavaScript Version 1.0.0 · MIT · .md

function indices({ values, value }) { return values.flatMap((item, i) => (item === value ? [i] : [])); }

Input & expected output

{
  "values": [
    "a",
    "b",
    "a",
    "c"
  ],
  "value": "a"
}

[
  0,
  2
]

The edge case

{
  "values": [
    1,
    2
  ],
  "value": 3
}

[]

Know the limits

• The original second variant dropped index 0, because filter(Boolean) removes it; flatMap keeps it.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Clone an array — Create a new array without copying the objects inside it.
• Remove duplicate values — Keep the first occurrence of each value, in its original order.
• Flatten a nested array — Turn a nested array into one sequence of values.