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if-exp

Conditional (if) expressions for JavaScript

NPM

npm install if-exp

Usage

const iff = require('if-exp');

const result = iff(condition)
            .then('result 1')
        .elseIff(secondCondition)
            .then('result 2')
        .elseIff(thirdCondition)
            .then('result 3')
        .otherwise('result 4');

Motivations

Some languages implement conditionals (and other well known statements) as expressions, so we can write:

val res = if(condition){
    'result 1'
} else if(secondCondition) {
    'result 2'
} else if(thirdCondition) {
    'result 3'
} else {
    'result 4'
}

While in JavaScript, conditional expressions can be achieve using ternary operators. Multiple conditions are sometimes hard to read using ternary:

var res = condition ?  'result 1' :
    (secondCondition ? 'result 2' :
        (thirdCondition ? 'result 3' : 'result 4'));

Here is how to write the same thing with iff:

const res = iff(condition)
            .then('result 1')
        .elseIff(secondCondition)
            .then('result 2')
        .elseIff(thirdCondition)
            .then('result 3')
        .otherwise('result 4');

How to do lazy-statements evaluation in JavaScript

const res = iff(condition1)
  .then(statement1())
elseIff(condition2)
  .then(statement2())
.otherwise(statement3());

The issue with the above code, is that statement1, statement2 and statement3 functions will be executed no matter the value of condition and condition2. To only execute one or the other we can use functions:

const res = iff(condition1)
  .then(statement)
elseIff(condition2)
  .then(statement)
.otherwise(statement)
();

Now, if-exp will yield a function depending on the conditions and we directly execute it resulting in a lazy statement evaluation.

How if-exp supports lazy condition evaluation

const res = iff(conditionalFn1())
  .then(1)
elseIff(conditionalFn2())
  .then(2)
.otherwise(3);

The issue with the above code is that conditionalFn1 and conditionalFn2 will be both executed. Since if-exp v2+, you can specify a predicate function as a condition and it will be evaluated or not depending on the previous conditions:

const res = iff(conditionalFn1)
  .then(1)
elseIff(conditionalFn2)
  .then(2)
.otherwise(3);

Changelog

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I maintain this project in my free time, if it helped you please support my work via paypal or Bitcoins, thanks a lot!