---
title: testing.diff() function
description: >
  `testing.diff()` produces a diff between two streams.
menu:
  flux_v0_ref:
    name: testing.diff
    parent: testing
    identifier: testing/diff
weight: 101
flux/v0/tags: [tests]
introduced: 0.18.0
---

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`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}]
```

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

## Parameters

### got

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



### want
({{< req >}})
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)

```

{{< expand-wrapper >}}
{{% expand "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       |

{{% /expand %}}
{{< /expand-wrapper >}}

### 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)

```