Distinguishing types between int and double #30543

Search Terms

int double, integer, float, double

Suggestion

This proposal introduces new number types int and double or float. Goal is to make a difference between integers and floating point numbers since some JavaScript engines does such kind of difference in the back-end.

Use Cases

As we know about V8 engine, it uses pointer tagging to distinguish between different types such as Smi (small integer), HeapObject, WeakHeapObject etc.

Consider this example:

const o1 = {};
o1.x = 3;
o1.y = 4;

const o2 = {};
o2.x = 5;
o2.y = 6;

For both o1 and o2 object the engine will create same shapes. See image below:

However, after changing o2.x to some double value, say that o2.x = 5.5; then V8 will use create new subtree from that shape and the old subtree in this shape will be marked as deprecated and will be garbage-collected. Reason for this is because there is a difference how V8 stores Smi and Double values in the memory.

Here is second image after changing x to 5.5:

Read more here: https://ponyfoo.com/articles/javascript-performance-pitfalls-v8#smi-to-double-migrations

Examples

Some code examples from the proposal:

class Point {
   constructor(private x: int, private y: int) {}
   
   getX(): int { this.x = x; }
   getY(): int { this.y = y; }
 }

let point = new Point(12, 5);
point.x = 1.5; // Error: double is not assignable to integer

This proposal does not require a breaking change, since number types are also okay, if one considers some possible performance pitfalls.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.