Runtime Types

Morph's C++ runtime provides JavaScript-compatible types that handle the dynamic nature of JSX expressions.

JsValue

The universal value type. Every expression in JSX resolves to a JsValue which can hold:

Type	C++	Example
`undefined`	`JsUndefined`	`morphState()` with no arg
`null`	`JsNull`	`null`
`boolean`	`JsBoolean`	`true`, `false`
`number`	`JsNumber`	`42`, `3.14`
`string`	`JsString`	`"hello"`
`array`	`JsArray`	`[1, 2, 3]`
`object`	`JsObject`	`{ key: "value" }`

typeof

typeof works like JavaScript:

typeof(undefined)  → "undefined"
typeof(null)       → "object"
typeof(42)         → "number"
typeof("hello")    → "string"
typeof(true)       → "boolean"

Truthiness

Same rules as JavaScript: 0, "", null, undefined, false are falsy. Everything else is truthy.

Equality

== and != follow JavaScript coercion rules. === and !== check type + value.

JsNumber

A numeric type that handles integers, doubles, and big-number strings:

JsNumber(42)         // int
JsNumber(3.14)       // double
JsNumber("12345678901234567890")  // big string

Arithmetic operators (+, -, *, /) work with proper coercion. fmt_double() renders numbers cleanly — 8 not 8.0, 2.5 not 2.500000, "Error" not nan/inf.

JsString

String type with JS-compatible methods:

Method	Description
`toUpperCase()`	Convert to uppercase
`toLowerCase()`	Convert to lowercase
`trim()`	Remove whitespace
`charAt(i)`	Get character at index
`indexOf(s)`	Find substring position
`substring(start, end)`	Extract substring
`slice(start, end)`	Extract substring
`replace(old, new)`	Replace first occurrence
`split(sep)`	Split into array

String concatenation with + works: "count: " + 42 produces "count: 42".

JsArray

Array backed by shared_ptr<vector<JsValue>>:

Method	Description
`push(value)`	Append to end
`pop()`	Remove from end
`index(i)`	Access by index

JsObject

Object backed by shared_ptr<map<string, JsValue>>:

Method	Description
`has(key)`	Check if key exists
`keys()`	Get all keys
`index(key)`	Access by key (also `obj["key"]`)

JsBoolean

Wrapper around bool with truthiness semantics.