CODEWITHSUNDEEP
javascriptfunctional-programmingpure-function
Stretch0
Stretch0

Reputation: 9251

Creating a composed functions with pure functions

I am trying to get my head around both composed functions and pure functions.

I have an object with a mixture of data. On some values I need to:

  1. remove the value's units
  2. parse string to integer
  3. Convert to value decimal

I have written three functions attempting to make them pure in the sense that they only do one thing but strictly speaking, they are mutating state. I'm not sure how to avoid mutating state though and if this technically makes it not a pure function?

My three "pure" functions are:

function parseValue(val) {
    return typeof val === 'number' ? val : parseInt(val)
}

function stripUnits(val) {
    return typeof val === 'string' ? val.match(/\d+/)[0] : val
}

function convertToDecimal(val) {
    return val / 100
}

I am then trying to compose these functions into one function with the help of lodash compose() 

function prepValue(val) {
    return compose(stripUnits, parseValue, convertToDecimal)
}

When I try run this console.log("prepValue", prepValue(weather.current.temperature)) I get the following in the terminal:

prepValue function (){var n=arguments,e=n[0];if(o&&1==n.length&&of(e))return o.plant(e).value();for(var u=0,n=r?t[u].apply(this,n):e;++u<r;)n=t[u].call(this,n);return n}

So the main things is,

  1. How can I make my three functions "pure"
  2. How can I compose these functions into 1

Upvotes: 1

Views: 104

Answers (2)

rafaelcastrocouto
rafaelcastrocouto

Reputation: 12161

You need to "create the function composition" before calling it.

function parseValue(val) { console.log(val)
    return typeof val === 'number' ? val : parseInt(val)
}

function stripUnits(val) { console.log(val)
    return typeof val === 'string' ? val.match(/\d+/)[0] : val
}

function convertToDecimal(val) {console.log(val)
    return val / 100
}

function prepValue(val) {
    return _.compose(stripUnits, parseValue, convertToDecimal)(val);
}

console.log("prepValue", prepValue('001232'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.5/lodash.fp.js"></script>

Upvotes: 1

Jamiec
Jamiec

Reputation: 136094

The only mistake you made is not calling the resulting composed method with val as the argument:

function prepValue(val) {
    return compose(stripUnits, parseValue, convertToDecimal)(val);
}

Upvotes: 2

Related Questions