Writing portable code

When targeting both WebAssembly with asc and JavaScript with tsc, there are a few semantic differences to take care of.

Explicit conversions

While asc understands the meaning of

// non-portable
let someFloat: f32 = 1.5;
let someInt: i32 = <i32>someFloat;

and then inserts the correct conversion steps, tsc does not because all numeric types are just aliases of number. Hence, when targeting JavaScript with tsc, the above will result in

// js
var someFloat = 1.5;
var someInt = someFloat;

which is obviously wrong. To account for this, explicit conversions can be inserted, resulting in actually portable code. For example

// portable
let someFloat: f32 = 1.5;
let someInt: i32 = (<i32>someFloat) | 0;

will result in

// js
var someFloat = 1.5;
var someInt = 1.5 | 0;

which is correct.

Additionally, an equally named portable conversion function is present for each built-in type that transforms any other built-in type to a value of the target type:

// using a portable conversion function
let someFloat: f32 = 1.5;
let someInt: i32 = i32(someFloat);

Overflows

Likewise, again because asc knows the meaning but tsc does not, overflows must be handled explicitly:

// non-portable
let someU8: u8 = 255;
let someOtherU8: u8 = someU8 + 1;

// portable
let someU8: u8 = 255;
let someOtherU8: u8 = (someU8 + 1) & 0xff;

// using a portable conversion function
let someU8: u8 = 255;
let someOtherU8: u8 = u8(someU8 + 1);

Non portable operations

In JavaScript, all numeric values are IEEE754 doubles that cannot represent the full range of values fitting in a 64-bit integer (max. safe integer is 2^53-1). Hence i64 and u64 are not portable and not present in std/portable.d.ts. There are several ways to deal with this. One is to use an i64 polyfill like in this example.

Other than that, some generic built-in functions do not support the full range of type arguments. For example:

• load<T = u8>(offset: usize): T - and - store<T = u8>(offset: usize, value: T): void
  are limited to u8 because the type argument cannot be evaluated in JS.