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Mistake: Lab 5.2 - Prototypal Inheritance
dmsheiko
Posts: 28
Hi,
There are some mistakes in code that verifies the prototype chain.
Here is the code:
assert(Object.getOwnPropertyNames(felixProto).length, 1) assert(Object.getOwnPropertyNames(felixProtoProto).length, 1) assert(Object.getOwnPropertyNames(felixProtoProto).length, 1) assert(typeof felixProto.meow, 'function') assert(typeof felixProtoProto.purr, 'function') assert(typeof felixProtoProtoProto.hiss, 'function')
- the 3rd line should likely check felixProtoProtoProto rather than felixProtoProto (which is already checked in previous line)
- there should be assert.equal rather than assert.
- in case of class syntax, Object.getOwnPropertyNames() may return 'constructor' in addition to 'meow', 'purr' or 'hiss', and I believe this is acceptable. So instead of checking the number of property names I would check the content of the array.
So, I would rewrite the checks this way:
// prototype checks, do not remove const felixProto = Object.getPrototypeOf(felix) const felixProtoProto = Object.getPrototypeOf(felixProto) const felixProtoProtoProto = Object.getPrototypeOf(felixProtoProto) function checkOwnPropertyNames(proto, right, ...wrongs) { const ownPropertyNames = Object.getOwnPropertyNames(proto) assert(ownPropertyNames.includes(right), `${ownPropertyNames} should contain ${right}`) for (const wrong of wrongs) { assert(!ownPropertyNames.includes(wrong), `${ownPropertyNames} should NOT contain ${wrong}`) } } checkOwnPropertyNames(felixProto, 'meow', 'purr', 'hiss') checkOwnPropertyNames(felixProtoProto, 'purr', 'hiss', 'meow') checkOwnPropertyNames(felixProtoProtoProto, 'hiss', 'meow', 'purr') assert.equal(typeof felixProto.meow, 'function') assert.equal(typeof felixProtoProto.purr, 'function') assert.equal(typeof felixProtoProtoProto.hiss, 'function') console.log('prototype checks passed!')
Regards,
Dmytro
249ymzn0nkk5-LFW211Labs06.28.2023.pdf
h08po53bto1k-labs-june-2023.zip
1
Comments
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thanks @dmsheiko great catches and feedback, this being fixed, I went with:
const assert = require('assert') // TODO: // implement a way to create a prototype chain // of leopard -> lynx -> cat // leopard prototype must have ONLY a hiss method // lynx prototype must have ONLY a purr method // cat prototype must have ONLY a meow method const felix = null //TODO replace null with instantiation of a cat felix.meow() // prints Felix the cat: meow felix.purr() // prints Felix the cat: prrr felix.hiss() // prints Felix the cat: hsss // prototype checks, do not remove const felixProto = Object.getPrototypeOf(felix) const felixProtoProto = Object.getPrototypeOf(felixProto) const felixProtoProtoProto = Object.getPrototypeOf(felixProtoProto) const protoPropNames = Object.getOwnPropertyNames(felixProto) const protoProtoPropNames = Object.getOwnPropertyNames(felixProtoProto) const protoProtoProtoPropNames = Object.getOwnPropertyNames(felixProtoProtoProto) assert(protoPropNames.includes('meow')) assert(protoProtoPropNames.includes('purr')) assert(protoProtoProtoPropNames.includes('hiss')) assert(protoPropNames.length <= (protoPropNames.includes('constructor') ? 2 : 1)) assert(protoProtoPropNames.length <= (protoProtoPropNames.includes('constructor') ? 2 : 1)) assert(protoProtoProtoPropNames.length <= (protoProtoProtoPropNames.includes('constructor') ? 2 : 1)) assert.equal(typeof felixProto.meow, 'function') assert.equal(typeof felixProtoProto.purr, 'function') assert.equal(typeof felixProtoProtoProto.hiss, 'function') console.log('prototype checks passed!')
This way failure information is most rich (i.e. first you know if you haven't put a method in a correct place, then you know if there are too many properties anywhere (allowing for constructor), then you know if you've not made it a method. I also prefer flatness and minimum indirection for these scripts. Thanks again!
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