coderaiserMock Import 
Mocking of Node.js EcmaScript Modules, similar to mock-require.
Install
npm i mock-import -D
Run
Loaders used to get things working, so you need to run tests with:
node --import mock-import/register test.js
mock-import uses transformSource hook, which replaces on the fly all imports with constants declaration:
const {readFile} = global.__mockImportCache.get('fs/promises');
mockImport adds new entry into Map, stopAll clears all mocks and reImport imports file again with new mocks applied.
Supported Declarations
/* ✅ */
import fs from 'node:fs/promises';
/* ✅ */
import {readFile} from 'node:fs/promises';
/* ✅ */
import * as fs1 from 'node:fs/promises';
/* ✅ */
const {writeFile} = await import('fs/promses');
/* ✅ */
export * as fs2 from 'fs/promises';
/* ✅ */
export {readFile as readFile1} from 'fs/promises';
Unsupported Declarations
/* ❌ */
export * from 'fs/promises';
// doesn't have syntax equivalent
How mock-import works?
As was said before, loaders used to get things working. This is experimental technology,
but most likely it wan't change. If it will mock-import will be adapted according to node.js API.
loader hookintercepts intoimportprocess and getpathnameof imported file;if
pathnameinreImportsit is processed with 🐊Putout code transformer, changes allimportcalls to access to__mockImportsCachewhich is a Map filled with data set bymockImportcall. And appendssourcemapat the end, sonodecan generate correct codecoverage.
-import glob from 'glob';
+const glob = global.__mockImportCache.get('./glob.js');
- if
traceCachecontainspathnameit calls are traced with estrace;
Code like this
const f = () => {};
will be changed to
const f = () => {
try {
__estrace.enter('<anonymous:1>', 'trace.js:1', arguments);
} finally {
__estrace.exit('<anonymous:1>', 'trace.js:1');
}
};
Straight after loading and passed to traceImport stack will be filled with data this way:
__estrace.enter = (name, url, args) => stack.push([name, url, Array.from(args)]);
And when the work is done stack will contain all function calls.
traceCachecontains somepathscurrent file will be checked for traced imports and change them to form${path}?count=${count}tore-importthem;
Environment variables
mock-import supports a couple env variables that extend functionality:
MOCK_IMPORT_NESTED- transform eachimportstatement so mock of module work in nested imports as well (slowdown tests a bit)
API
mockImport(name, mock)
name: string- module name;mock: object- mock data;
Mock import of a module.
stopAll()
Stop all mocks.
reImport(name)
name: string- name of a module
Fresh import of a module.
traceImport(name, {stack})
name: stringname of a modulestack: [fn, url, args];
Add tracing of a module.
reTrace(name)
name: string- name of traced module
Apply tracing.
enableNestedImports()
Enable nested imports, can slowdown tests;
disableNestedImports()
Disable nested imports, use when you do not need nested imports support;
Example
Let's suppose you have cat.js:
import {readFile} from 'node:fs/promises';
export default async function cat() {
const readme = await readFile('./README.md', 'utf8');
return readme;
}
You can test it with 📼Supertape:
import {createMockImport} from 'mock-import';
import {test, stub} from 'supertape';
const {
mockImport,
reImport,
stopAll,
} = createMockImport(import.meta.url);
test('cat: should call readFile', async (t) => {
const readFile = stub();
mockImport('fs/promises', {
readFile,
});
const cat = await reImport('./cat.js');
await cat();
stopAll();
t.calledWith(readFile, ['./README.md', 'utf8']);
t.end();
});
Now let's trace it:
import {createMockImport} from 'mock-import';
import {test, stub} from 'supertape';
const {
mockImport,
reImport,
stopAll,
} = createMockImport(import.meta.url);
test('cat: should call readFile', async (t) => {
const stack = [];
traceImport('fs/promises', {
stack,
});
const cat = await reImport('./cat.js');
await cat();
stopAll();
const expected = [
['parse', 'parser.js:3', [
'const a = 5',
]],
['tokenize', 'tokenizer.js:1', [
'parser call',
'const a = 5',
]],
];
t.deepEqual(stack, expected);
t.end();
});
License
MIT