The document discusses the concept of Design by Contract (DbC) in programming, advocating for the use of assertions in production code that are removed during the release phase to enhance performance. It emphasizes the importance of preconditions, postconditions, and invariants, while suggesting that assertions should not only be used for type checking but also for validating API contracts. The author also highlights the integration of tools like power-assert and unassert for effective assertion handling in JavaScript applications.

1. unassertEncourage Design by Contract (DbC) by writing assertions
in production code, and compiling them away from release
Takuto Wada
Nov 7, 2015 @nodefest Tokyo 2015

2. Takuto Wada
Twitter: @t_wada
github: @twada

3. Do you write tests?

4. 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
I m the author of power-assert-js!
power-assert

5. Wrestling with Babel6 recently…

6. unassertEncourage Design by Contract (DbC) by writing assertions
in production code, and compiling them away from release

7. •unassert
•unassertify
•babel-plugin-unassert
•webpack-unassert-loader
unassert family

8. 'use strict';
var assert = require('assert');
function add (a, b) {
console.assert(typeof a === 'number');
assert(!isNaN(a));
assert.equal(typeof b, 'number');
assert.ok(!isNaN(b));
return a + b;
}
before
src.js

9. babel --plugins babel-plugin-unassert src.js > dest.js
browserify -t unassertify src.js > dest.js

10. 'use strict';
var assert = require('assert');
function add (a, b) {
console.assert(typeof a === 'number');
assert(!isNaN(a));
assert.equal(typeof b, 'number');
assert.ok(!isNaN(b));
return a + b;
}
after
dest.js

11. User s voice

12. https://www.mapbox.com/

13. In internal / private methods, we
check preconditions with assert,
helping us catch mistakes within the
library.
For performance, these checks are
removed from the production build
with unassertify.
CONTRIBUTING.md
where / when unassert

14. I don't think we should use this
primarily for type checking, but for
preconditions -- parts of API
contracts that cannot be statically
checked.
@jfirebaugh
why unassert

15. Flow & TypeScript are compile-time,
and preconditions could be runtime.
Mapbox GL JS is pretty minimal in
terms of transforms - envify, brfs, glify
- such that all JS is vanilla.
@tmcw
why not flow / typescript

16. preconditions?
postconditions?
invariants?

17. {P} A {Q}

18. Design by Contract by Bertrand Meyer

19. If you promise to call routine with
precondition satisfied then I, in
return, promise to deliver a final
state in which postcondition is
satisfied.
Design by Contract (DbC)

20. •redundant checks can and indeed will hurt
•simplicity becomes a crucial criterion
•complexity is the major enemy of quality
•whose responsibility it is to enforce it: the
client's, or the supplier's.
Design by Contract (DbC)

22. switch (name) {
case 'foo':
doSomething();
break;
case 'bar':
doAnother();
break;
default:
assert(false, 'Cannot be here');
}
Not only preconditions

23. add(path) {
assert(path instanceof PathElement,
`path should be PathElement but was: ${ typeName(path) }`);
this.paths.push(path);
assert.deepEqual(this.paths, sortedPathElements(this.paths),
'this.paths should always be sorted');
}
Not only preconditions

24. Not only preconditions

25. What if…
we use power-assert instead of assert?

26. import assert from 'power-assert';
class Calc {
add (a, b) {
assert(!(isNaN(a) || isNaN(b)));
assert(typeof a === 'number');
assert(typeof b === 'number');
return a + b;
}
}
const calc = new Calc();
console.log(calc.add(3, NaN));
calc.js
write assertions with power-assert

27. {
"env": {
"development": {
"plugins": [
"babel-plugin-espower"
],
},
"production": {
"plugins": [
"babel-plugin-unassert"
]
}
}
} .babelrc
unassert in prod, power-assert in dev

28. $ $(npm bin)/babel-node calc.js
AssertionError: # calc.js:5
assert(!(isNaN(a) || isNaN(b)))
| | | | | |
| | | | true NaN
| false 3 true
false
power-assert output appears!

29. user s voice

30. One more thing…

31. unassert is now babel6 ready!

32. power-assert is not babel6 ready yet
update: 2015/11/13
power-assert now
supports babel6!!

33. power-assert now works with async/await

34. # test/some_test.js:6
assert((await bigOrSmall(input)) === big)
| | | |
| | | "big"
"small" 3 false
--- [string] big
+++ [string] await bigOrSmall(input)
@@ -1,3 +1,5 @@
-big
+small
power-assert now works with async/await
by @jamestalmage

35. Encourage reliable programming
by writing assertions in
production code,
and compiling them away from
release (if you want).
to recap

36. power-assert
https://github.com/power-assert-js/power-assert
unassert
https://github.com/twada/unassert
Thank you!