fetch-prop

You can think of fetchProp as a strict property getter operator (e.g. o.foo or o['foo']). Unlike in vanilla JavaScript where you get undefined for non-existent properties, fetchProp will immediately signal an error.

Consider:

function addOne(wrapper) {
    const { value } = wrapper
    return value + 1
}

const ans = addOne(37)
console.log(ans) // outputs: NaN

Now with fetchProp:

function addOne(wrapper) {
    const value = fetchProp(wrapper, 'value')
    return value + 1
}

addOne(37)
// throws Uncaught MissingPropError: "foo"
//     at /home/lilsweetcaligula/my_app/index.js:6:13

Table of Contents

1. Installation
2. Fetching a prop
3. Fetching nested props
4. Assertions
5. Predicates
6. Issues
7. License

Installation

$ npm install fetch-prop -E --save

Require fetchProp in your code like so:

const { fetchProp } = require('fetch-prop')

Fetching a prop

const obj = { foo: 'bar' }
const foo = fetchProp(obj, 'foo')

Fetching nested props

const obj = { bar: { foo: 37 }, foo: 123 }
const foo = fetchProp(obj, ['bar', 'foo'])

console.log(foo) // outputs: 37

Assertions

For added safety, fetchProp allows to run assertions on the value (or its type) at runtime - for that simply pass a function as the last argument which does the assertions:

const obj = { foo: 'bar' }
const assertNumber = x => Assert(typeof x === 'number', 'must be a number')

const foo = fetchProp(obj, 'foo', assertNumber)

// throws AssertionError: "must be a number"

This works especially well with the assert-plus package (needs to be installed separately):

const Assert = require('assert-plus')
const obj = { foo: 'bar' }
const foo = fetchProp(obj, 'foo', Assert.string)

console.log(foo) // outputs: bar

You are not limited to asserting on the value's type:

const obj = { foo: -5 }
const mustBePositive = x => if (x <= 0) throw new Error('must be positive')
const foo = fetchProp(obj, 'foo', mustBePositive)

// throws Error: "must be positive"

Predicates

If the last argument to fetchProp returns false, then it is assumed that the value failed the check and, as a result, an AssertionError will be thrown. This way, you may easily use your predicates to test values:

const obj = { foo: -5 }
const isPositive = x => x > 0
const foo = fetchProp(obj, 'foo', isPositive)

// throws AssertionError: "prop \"foo\" failed the assertion"

Issues

Please feel free to submit issues and feature requests here here.

License

MIT License

Copyright (c) 2021 Efim Bogdanovsky

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.