Number-Interval Predicate for JavaScript

ECMAScript Stage-0 Proposal. J. S. Choi, 2021.

Description

Checking whether one number is between two others is a frequent task in any programming language. However, JavaScript’s comparison operators only supports checking the order between two values at a time, resulting in code that looks like x <= y && y < z. A function that allows checking whether y is in between x and z would be useful for many developers.

We therefore propose exploring the addition of a number-interval predicate to the JavaScript language. If this proposal is approved for Stage 1, then we would explore various directions for the API’s design. We would also assemble as many real-world use cases as possible and shape our design to fulfill them.

Description

There are several possible ways to do this, all of which would be static functions on the Math constructor.

Option A: isIncreasing

We could have a variadic function checking for ascending monotonicity, with inclusive limits:

// 0 ≤ 2 ≤ 3 ≤ 6:
Math.isIncreasing(0, 2, 3, 6);

…or the same, except with exclusive limits:

// 0 < 2 < 3 < 6:
Math.isIncreasing(0, 2, 3, 6);

…or with an extra boolean argument for configuration:

// 0 ≤ 2 ≤ 3 ≤ 6:
Math.isIncreasing([ 0, 2, 3, 6 ], true);
// 0 < 2 < 3 < 6:
Math.isIncreasing([ 0, 2, 3, 6 ], false);

…or we could separate inclusive and exclusive limits into separate functions:

// 0 ≤ 2 ≤ 3 ≤ 6:
Math.isIncreasing(0, 2, 3, 6);
// 0 < 2 < 3 < 6:
Math.isStrictlyIncreasing(0, 2, 3, 6);

Option B: isBetween

We could have a function that always uses an inclusive minimum and exclusive maximum:

// 0 ≤ 2 < 6:
Math.isBetween(2, 0, 6);

…or the same except it would always use an inclusive minimum and inclusive maximum:

// 0 ≤ 2 ≤ 6:
Math.isBetween(2, 0, 6);

We could have a function that also takes two boolean arguments indicating their respective limits’ inclusivity, like Google Sheets’ ISBETWEEN:

// 0 ≤ 2 < 6:
Math.isBetween(2, 0, 6, true, false);
// 0 ≤ 2 ≤ 6:
Math.isBetween(2, 0, 6, true, true);
// 0 < 2 < 6:
Math.isBetween(2, 0, 6, false, false);
// 0 < 2 ≤ 6:
Math.isBetween(2, 0, 6, false, true);

(The boolean arguments might be required, or they might be optional with their default values being both true, or false and true, or something else.

Or we could have four functions that correspond to the four possible configurations of inclusivity, forcing the user to be explicit:

// 0 ≤ 2 < 6:
Math.isBetweenIE(2, 0, 6);
// 0 ≤ 2 ≤ 6:
Math.isBetweenII(2, 0, 6);
// 0 < 2 < 6:
Math.isBetweenEE(2, 0, 6);
// 0 < 2 ≤ 6:
Math.isBetweenEI(2, 0, 6);