---
title: testing.diff() function
description: testing.diff() produces a diff between two streams.
url: https://docs.influxdata.com/flux/v0/stdlib/testing/diff/
estimated_tokens: 2333
product: Flux
version: v0
---

# testing.diff() function

-   Flux 0.18.0+
-   View InfluxDB support

`testing.diff()` produces a diff between two streams.

The function matches tables from each stream based on group keys. For each matched table, it produces a diff. Any added or removed rows are added to the table as a row. An additional string column with the name diff is created and contains a `-` if the row was present in the `got` table and not in the `want` table or `+` if the opposite is true.

`diff()` function emits at least one row if the tables are different and no rows if the tables are the same. The exact diff produced may change. `diff()` can be used to perform in-line diffs in a query.

##### Function type signature

```js
(
    <-got: stream[A],
    want: stream[A],
    ?epsilon: B,
    ?nansEqual: C,
    ?verbose: D,
) => stream[{A with _diff: string}]
```

For more information, see [Function type signatures](/flux/v0/function-type-signatures/).

## Parameters

### got

Stream containing data to test. Default is piped-forward data (`<-`).

### want

(Required) Stream that contains data to test against.

### epsilon

Specify how far apart two float values can be, but still considered equal. Defaults to 0.000000001.

### verbose

Include detailed differences in output. Default is `false`.

### nansEqual

Consider `NaN` float values equal. Default is `false`.

## Examples

-   [Output a diff between two streams of tables](#output-a-diff-between-two-streams-of-tables)
-   [Return a diff between a stream of tables an the expected output](#return-a-diff-between-a-stream-of-tables-an-the-expected-output)

### Output a diff between two streams of tables

```js
import "sampledata"
import "testing"

want = sampledata.int()
got =
    sampledata.int()
        |> map(fn: (r) => ({r with _value: if r._value > 15 then r._value + 1 else r._value}))

testing.diff(got: got, want: want)
```

[](#view-example-input)

View example input

#### Input data

| _diff | *tag | _time | _value |
| --- | --- | --- | --- |
|  | t1 | 2021-01-01T00:00:00Z | -2 |
|  | t1 | 2021-01-01T00:00:10Z | 10 |
|  | t1 | 2021-01-01T00:00:20Z | 7 |
| - | t1 | 2021-01-01T00:00:30Z | 17 |
| + | t1 | 2021-01-01T00:00:30Z | 18 |
|  | t1 | 2021-01-01T00:00:40Z | 15 |
|  | t1 | 2021-01-01T00:00:50Z | 4 |

| _diff | *tag | _time | _value |
| --- | --- | --- | --- |
| - | t2 | 2021-01-01T00:00:00Z | 19 |
| + | t2 | 2021-01-01T00:00:00Z | 20 |
|  | t2 | 2021-01-01T00:00:10Z | 4 |
|  | t2 | 2021-01-01T00:00:20Z | -3 |
| - | t2 | 2021-01-01T00:00:30Z | 19 |
| + | t2 | 2021-01-01T00:00:30Z | 20 |
|  | t2 | 2021-01-01T00:00:40Z | 13 |
|  | t2 | 2021-01-01T00:00:50Z | 1 |

### Return a diff between a stream of tables an the expected output

```js
import "testing"

want = from(bucket: "backup-example-bucket") |> range(start: -5m)

from(bucket: "example-bucket")
    |> range(start: -5m)
    |> testing.diff(want: want)
```

[tests](/flux/v0/tags/tests/)