babel-plugin-chai-assert-async
Transforms assert.async.*
calls into test-until
expressions that resolve when the assertion passes, preserving error messages for failed assertions.
Installation
Assuming you're already using Babel, install the Babel plugin and the peer dependency with your package manager of choice:
$ npm install @atom/babel-plugin-chai-assert-async test-until
Usage
.babelrc
Via .babelrc
{
"plugins": ["@atom/babel-plugin-chai-assert-async"]
}
Via CLI
$ babel --plugins @atom/babel-plugin-chai-assert-async script.js
Via Node API
require("babel-core").transform("code", {
plugins: ["@atom/babel-plugin-chai-assert-async"]
});
Details
During tests, it's sometime's useful to test values that are set asynchronously, or to wait for a condition to be true before continuing the test. Here's a convoluted example:
function setValueAsync(obj) {
setTimeout(() => obj.val = 42, 100)
}
describe('setValueAsync', function() {
it('sets a value asynchronously', function() {
const obj = {val: 0}
setValueAsync(obj)
assert.equal(obj.val, 42) // will fail
})
})
This test will fail because obj.val
does not equal 42
at the time the assertion is run.
We can use a library like test-until
to ensure the test doesn't continue until obj.val
does equal 42
. Here's an example with Mocha, async
/await
import until from 'test-until'
// ....
it('sets a value asynchronously', function() {
const obj = {val: 0}
setValueAsync(obj)
await until(() => obj.val === 42)
})
However, in converting from the assert.equal
version of this test to the await until
version, we lost some information. In particular, it's more difficult to know exactly what we're testing, and if the until
expression times out, we get a very generic error message like "timed out waiting for something to happen" instead of something useful like "expected obj.val to equal 42".
This transform allows you to convert something like assert.equal(obj.val, 42)
into an equivalent until
expression by writing assert.async.equal(obj.val, 42)
:
function setValueAsync(obj) {
setTimeout(() => obj.val = 42, 100)
}
describe('setValueAsync', function() {
it('sets a value asynchronously', async function() {
const obj = {val: 0}
setValueAsync(obj)
await assert.async.equal(obj.val, 42)
})
})
In this case, if the async assertion fails, the original error message like "expected obj.val to equal 42" will be preserved.
If you want to pass a timeout to test-until
, you can specify it as an argument to async
:
await assert.async(100).equal(obj.val, 42)
Examples
Basic
In
async function test() {
await assert.async.equal(thing, other);
}
Out
"use strict";
import until from 'test-until';
async function test() {
await until(async function (fail) {
try {
assert.equal(thing, other);
return true;
} catch (err) {
return fail(err);
}
});
}
With explicit timeout
In
async function test() {
await assert.async(500).equal(thing, other);
}
Out
"use strict";
import until from 'test-until';
async function test() {
await until(async function (fail) {
try {
assert.equal(thing, other);
return true;
} catch (err) {
return fail(err);
}
}, 500);
}