1. power-assert
mechanism and philosophy
Takuto Wada (a.k.a @t_wada)
Nov 15, 2014 @ Nodefest Tokyo 2014

2. Takuto Wada
@t_wada
github: twada

3. agenda
•what is power-assert?
•mechanism
•design philosophy

4. state of JavaScript testing
testing
framework
remote
test runner
environment
assertion
library
mocking
library
plenty of alternatives...

5. 33 Methods!?
too much to learn...

6. Something is wrong
with arms race among
assertion libraries...
What if the `assert` is
informative enough?

7. power-assert
https://github.com/twada/power-assert

8. 1. npm install --save-dev power-assert intelli-espower-loader
2. add "directories" to package.json
"directories": {
"test": "test"
},
3. change
var assert = require('assert');
to
var assert = require('power-assert');
4. mocha --require intelli-espower-loader
Just 4 steps

9. assert(typeof item.id === 'strong')
| | | |
| | | false
| | "foo"
| Item{id:"foo",name:"bar"}
"string"
--- [string] 'strong'
+++ [string] typeof item.id
@@ -1,6 +1,6 @@
str
-o
+i
ng
You get this
work unobtrusively, fail noisily

10. assert(actual)
assert.deepEqual(actual,expected)
assert.notDeepEqual(actual,expected)
You don t need to memorize
assertion APIs any more.
Just write simple assert

11. arms race among assertion APIs enrich
test vocabulary...resulted in overkill
power-assert is a
Renaissance of code
No more assertion war

12. So, why don t you run with tests?

13. agenda
•what is power-assert?
•mechanism
•design philosophy

14. var assert = require('power-assert');
function Item(id, name) {
this.id = id;
this.name = name;
}
describe('Item', function () {
it('has string id', function () {
var item = new Item(1000, 'bar');
assert(assert._expr(assert._capt(assert._capt(typeof
assert._capt(assert._capt(item, 'arguments/0/left/argument/object').id,
'arguments/0/left/argument'), 'arguments/0/left') === 'string', 'arguments/0'), {
content: 'assert(typeof item.id === 'string')',
filepath: 'test/item_test.js',
line: 11
}));
});
});
var assert = require('power-assert');
function Item (id, name) {
this.id = id;
this.name = name;
}
describe('Item', function () {
it('has string id', function () {
var item = new Item(1000, 'bar');
assert(typeof item.id === 'string');
});
});
But How?
test code
transpiled to

15. https://speakerdeck.com/michaelficarra/spidermonkey-parser-api-a-standard-for-structured-js-representations
new C(1 + a)

16. https://speakerdeck.com/constellation/escodegen-and-esmangle-using-mozilla-javascript-ast-as-an-ir

17. https://speakerdeck.com/constellation/escodegen-and-esmangle-using-mozilla-javascript-ast-as-an-ir

18. 3 essential AST tools
•esprima
•escodegen
•estraverse

19. espoweresprima
code
AST
escodegen
AST
AST
AST
code
code
code
espower-source
grunt-espower / gulp-espower / espowerify /
espower-loader / espower-coffee / espower-cli

20. agenda
•what is power-assert?
•mechanism
•design philosophy

21. The UNIX Philosophy
https://github.com/substack/nodefest-2012

22. Make each program do one thing well
https://github.com/substack/nodefest-2012

23. Rule of Composition:
Design programs to be
connected with other programs.
Rule of Modularity:
Write simple parts connected
by clean interfaces.

24. substack pattern
module.exports = function(arg) {
// do one thing well
};

25. First version (4)
•power-assert
•empower
•espower
•grunt-espower
Currently (15)
• power-assert
• empower
• power-assert-formatter
• espower
• espower-source
• type-name
• stringifier
• espurify
• escallmatch
• grunt-espower
• gulp-espower
• espowerify
• espower-loader
• espower-coffee
• espower-cli
modularize

26. Your sensibilities about simplicity
being equal to ease of use are wrong.
Easy is not simple
Simple: objective
Easy: relative

27. Easy
•power-assert
•grunt-espower
•gulp-espower
•espowerify
•espower-loader
•espower-cli
•espower-coffee
Simple
•espower
•espower-source
•empower
separate `simple` from `easy`

28. wrap-up
•No more assertion war
•Use JS to modify JS
•Separate simple from easy
•Make things small and
composable

29. power-assert
https://github.com/twada/power-assert
Thank you!