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Integration Tests

Note: Simulation tests are no longer actively supported. NEAR Simulator was meant to be an in-place replacement of a blockchain environment for the purpose of testing NEAR contracts. However, simulating NEAR ledger turned out to be a much more complex endeavor than was anticipated. Eventually, the idea of workspaces was born – a library for automating workflows and writing tests for NEAR smart contracts using a real NEAR network (localnet, testnet or mainnet). Thus, NEAR Simulator is being deprecated in favor of workspaces-rs, the Rust edition of workspaces. As the two libraries have two vastly different APIs this guide was created to ease the migration process for developers.

Unit Tests vs. Integration Tests

Unit tests are great for ensuring that functionality works as expected at an insolated, functional-level. This might include checking that function get_nth_fibonacci(n: u8) works as expected, handles invalid input gracefully, etc. Unit tests in smart contracts might similarly test public functions, but can get unruly if there are several calls between accounts. As mentioned in the unit tests section, there is a VMContext object used by unit tests to mock some aspects of a transaction. One might, for instance, modify the testing context to have the predecessor_account_id of "bob.near". The limits of unit tests become obvious with certain interactions, like transferring tokens. Since "bob.near" is simply a string and not an account object, there is no way to write a unit test that confirms that Alice sent Bob 6 NEAR (Ⓝ). Furthermore, there is no way to write a unit test that executes cross-contract calls. Additionally, there is no way of profiling gas usage and the execution of the call (or set of calls) on the blockchain.

Integration tests provide the ability to have end-to-end testing that includes cross-contract calls, proper user accounts, access to state, structured execution outcomes, and more. In NEAR, we can make use of the workspaces libraries in both Rust and JavaScript for this type of testing on a locally-run blockchain or testnet.

When to Use Integration Tests

You’ll probably want to use integration tests when:

  • There are cross-contract calls.
  • There are multiple users with balance changes.
  • You’d like to gather information about gas usage and execution outcomes on-chain.
  • You want to assert the use-case execution flow of your smart contract logic works as expected.
  • You want to assert given execution patterns do not work (as expected).


Unlike unit tests (which would often live in the src/ file of the contract), integration tests in Rust are located in a separate directory at the same level as /src, called /tests (read more). Refer to this folder structure below:

├── Cargo.toml                  ⟵ contains `dependencies` for contract and `dev-dependencies` for workspaces-rs tests
├── src
│  └──                   ⟵ contract code
├── target
└── tests                       ⟵ integration test directory
   └──     ⟵ integration test file
These tests don’t have to be placed in their own /tests directory. Instead, you can place them in the /src directory which can be beneficial since then you can use the non-exported types for serialization within the test case.

A sample configuration for this project’s Cargo.toml is shown below:

name = "fungible-token-wrapper"
version = "0.0.2"
authors = ["Near Inc <[email protected]>"]
edition = "2021"

anyhow = "1.0"
near-primitives = "0.5.0"
near-sdk = "4.0.0"
near-units = "0.2.0"
serde_json = "1.0"
tokio = { version = "1.14", features = ["full"] }
workspaces = "0.4.1"

# remember to include a line for each contract
fungible-token = { path = "./ft" }
defi = { path = "./test-contract-defi" }

codegen-units = 1
# Tell `rustc` to optimize for small code size.
opt-level = "z"
lto = true
debug = false
panic = "abort"
overflow-checks = true

# remember to include a member for each contract
members = [

The file above will contain the integration tests. These can be run with the following command from the same level as the test Cargo.toml file:

cargo test --test integration-tests

Comparing an Example

Unit Test

Let’s take a look at a very simple unit test and integration test that accomplish the same thing. Normally you wouldn’t duplicate efforts like this (as integration tests are intended to be broader in scope), but it will be informative.

We’ll be using snippets from the fungible-token example from the near-sdk-rs repository to demonstrate simulation tests.

First, note this unit test that tests the functionality of the test_transfer method:

The test above sets up the testing context, instantiates the test environment through get_context(), calls the test_transfer method, and performs the storage_deposit() initialization call (to register with the fungible token contract) and the ft_transfer() fungible token transfer call.

Let’s look at how this might be written with workspaces tests. The snippet below is a bit longer as it demonstrates a couple of things worth noting.

Workspaces Test

In the test above, the compiled smart contract .wasm file (which we compiled into the /out directory) for the Fungible Token example is dev-deployed (newly created account) to the environment. The ft_contract account is created as a result from the environment which is used to create accounts. This specific file’s format has only one test entry point (main), and every test is declared with #[tokio::test]. Tests do not share state between runs.

Notice the layout within test_total_supply. .call() obtains its required gas from the account performing it. Unlike the unit test, there is no mocking being performed before the call as the context is provided by the environment initialized during init(). Every call interacts with this environment to either fetch or change state.

Pitfall: you must compile your contract before running integration tests. Because workspaces tests use the .wasm files to deploy the contracts to the network. If changes are made to the smart contract code, the smart contract wasm should be rebuilt before running these tests again.


In case you wish to preserve state between runs, you can call multiple tests within one function, passing the worker around from a workspaces::sandbox() call.


Helpful Snippets

Create an Account

You can also create a dev_account without having to deploy a contract as follows:


Create Helper Functions

Spooning – Pulling Existing State and Contracts from Mainnet/Testnet

This example showcases spooning state from a testnet contract into our local sandbox environment:

For a full example, see the examples/src/ example.

Fast Forwarding – Fast Forward to a Future Block

workspaces testing offers support for forwarding the state of the blockchain to the future. This means contracts which require time sensitive data do not need to sit and wait the same amount of time for blocks on the sandbox to be produced. We can simply just call worker.fast_forward to get us further in time:

For a full example, take a look at examples/src/

Handle Errors
Returning Err(msg) is also a viable (and arguably simpler) implementation.

Batch Transactions

let res = contract
            .args_json((, transfer_amount, Option::<String>::None, "10"))?
            .gas(300_000_000_000_000 / 2)
            .gas(300_000_000_000_000 / 2)

Inspecting Logs

    format!("Closed @{} with {}",, initial_balance.0 - transfer_amount.0)

Examining receipt outcomes:

let outcome = &res.receipt_outcomes()[5];
assert_eq!(outcome.logs[0], "The account of the sender was deleted");
assert_eq!(outcome.logs[2], format!("Account @{} burned {}",, 10));

Profiling Gas

CallExecutionDetails::total_gas_burnt includes all gas burnt by call execution, including by receipts. This is exposed as a surface level API since it is a much more commonly used concept:

println!("Burnt gas (all): {}", res.total_gas_burnt);

If you do actually want gas burnt by transaction itself you can do it like this:

println!("Burnt gas (transaction): {}", res.outcome().gas_burnt);

If you want to see the gas burnt by each receipt, you can do it like this:

for receipt in res.receipt_outcomes() {
   println!("Burnt gas (receipt): {}", receipt.gas_burnt);
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