diff --git a/src/bin_wasm.rs b/src/bin_wasm.rs
index 242c235f..4a2212c4 100644
--- a/src/bin_wasm.rs
+++ b/src/bin_wasm.rs
@@ -3,15 +3,25 @@
 use organic::parser::parse_with_settings;
 use organic::settings::GlobalSettings;
 use wasm::ParseResult;
+use wasm::ToWasm;
+use wasm::ToWasmContext;
 use wasm_bindgen::prelude::wasm_bindgen;
 
+mod error;
 mod wasm;
 
 #[wasm_bindgen]
 pub fn parse_org(org_contents: &str) -> wasm_bindgen::JsValue {
     let global_settings = GlobalSettings::default();
-    let rust_parsed = parse_with_settings(org_contents, &global_settings)
-        .map(|document| ParseResult::Success((org_contents, document).into()))
-        .unwrap_or_else(|err| ParseResult::Error(format!("{:?}", err)));
+    let to_wasm_context = ToWasmContext::new(org_contents);
+    let rust_parsed = match parse_with_settings(org_contents, &global_settings)
+        .map(|document| document.to_wasm(to_wasm_context))
+        .map(|wasm_document| match wasm_document {
+            Ok(wasm_document) => ParseResult::Success(wasm_document),
+            Err(err) => ParseResult::Error(format!("{:?}", err)),
+        }) {
+        Ok(wasm_document) => wasm_document,
+        Err(err) => ParseResult::Error(format!("{:?}", err)),
+    };
     serde_wasm_bindgen::to_value(&rust_parsed).unwrap()
 }
diff --git a/src/wasm/document.rs b/src/wasm/document.rs
index abd73e28..9e80c88a 100644
--- a/src/wasm/document.rs
+++ b/src/wasm/document.rs
@@ -3,16 +3,27 @@ use serde::Deserialize;
 use serde::Serialize;
 
 use super::macros::to_wasm;
+use super::standard_properties::WasmStandardProperties;
+use super::to_wasm::ToWasm;
+use crate::wasm::to_wasm::ToWasmStandardProperties;
 
 #[derive(Serialize, Deserialize)]
 #[serde(tag = "ast_node")]
 #[serde(rename = "org-data")]
 pub(crate) struct WasmDocument {
+    standard_properties: WasmStandardProperties,
     children: Vec<()>,
 }
 
-to_wasm!(WasmDocument, Document<'s>, original, document, {
-    WasmDocument {
-        children: Vec::new(),
+to_wasm!(
+    WasmDocument,
+    Document<'s>,
+    wasm_context,
+    standard_properties,
+    {
+        Ok(WasmDocument {
+            standard_properties,
+            children: Vec::new(),
+        })
     }
-});
+);
diff --git a/src/wasm/macros.rs b/src/wasm/macros.rs
index 8d02399b..76f5c272 100644
--- a/src/wasm/macros.rs
+++ b/src/wasm/macros.rs
@@ -2,10 +2,15 @@
 ///
 /// This exists to make changing the type signature easier.
 macro_rules! to_wasm {
-    ($ostruct:ty, $istruct:ty, $original:ident, $document:ident, $fnbody:tt) => {
-        impl<'s> From<(&'s str, $istruct)> for $ostruct {
-            fn from(value: (&'s str, $istruct)) -> $ostruct {
-                let ($original, $document) = value;
+    ($ostruct:ty, $istruct:ty, $wasm_context:ident, $standard_properties:ident, $fnbody:tt) => {
+        impl<'s> ToWasm for $istruct {
+            type Output = $ostruct;
+
+            fn to_wasm(
+                &self,
+                $wasm_context: crate::wasm::to_wasm::ToWasmContext<'_>,
+            ) -> Result<Self::Output, crate::error::CustomError> {
+                let $standard_properties = self.to_wasm_standard_properties($wasm_context)?;
                 $fnbody
             }
         }
diff --git a/src/wasm/mod.rs b/src/wasm/mod.rs
index ad43a2dc..d0e568ff 100644
--- a/src/wasm/mod.rs
+++ b/src/wasm/mod.rs
@@ -2,5 +2,8 @@ mod document;
 mod macros;
 mod parse_result;
 mod standard_properties;
+mod to_wasm;
 
 pub(crate) use parse_result::ParseResult;
+pub(crate) use to_wasm::ToWasm;
+pub(crate) use to_wasm::ToWasmContext;
diff --git a/src/wasm/standard_properties.rs b/src/wasm/standard_properties.rs
index f0b162bc..92db1e7f 100644
--- a/src/wasm/standard_properties.rs
+++ b/src/wasm/standard_properties.rs
@@ -1,19 +1,71 @@
 use organic::types::PostBlank;
+use organic::types::StandardProperties;
 use serde::Deserialize;
 use serde::Serialize;
 
-use super::macros::to_wasm;
+use super::to_wasm::ToWasmContext;
+use super::to_wasm::ToWasmStandardProperties;
 
 #[derive(Serialize, Deserialize)]
 pub(crate) struct WasmStandardProperties {
-    begin: Option<usize>,
-    end: Option<usize>,
+    begin: usize,
+    end: usize,
     contents_begin: Option<usize>,
     contents_end: Option<usize>,
-    post_blank: Option<PostBlank>,
+    post_blank: PostBlank,
 }
 
-to_wasm!(WasmStandardProperties, Document<'s>, original, document, {
-    // foo
-    todo!()
-});
+impl<'s, SP: StandardProperties<'s>> ToWasmStandardProperties for SP {
+    type Output = WasmStandardProperties;
+
+    fn to_wasm_standard_properties(
+        &self,
+        wasm_context: ToWasmContext<'_>,
+    ) -> Result<Self::Output, crate::error::CustomError> {
+        let (begin, end) = get_char_indices(wasm_context.full_document, self.get_source());
+        let (contents_begin, contents_end) = match self.get_contents() {
+            Some(contents) => {
+                let (begin, end) = get_char_indices(wasm_context.full_document, contents);
+                (Some(begin), Some(end))
+            }
+            None => (None, None),
+        };
+
+        let post_blank = self.get_post_blank();
+        Ok(WasmStandardProperties {
+            begin,
+            end,
+            contents_begin,
+            contents_end,
+            post_blank,
+        })
+    }
+}
+
+fn get_char_indices(original_document: &str, subset: &str) -> (usize, usize) {
+    let (begin_byte, end_byte) = get_rust_byte_offsets(original_document, subset);
+    // Add 1 to make this 1-based to match emacs.
+    let begin_char = original_document[..begin_byte].chars().count() + 1;
+    // Also 1-based:
+    let end_char = begin_char + original_document[begin_byte..end_byte].chars().count();
+    (begin_char, end_char)
+}
+
+/// Get the byte offset into source that the string slice exists at.
+///
+/// These offsets are zero-based.
+fn get_rust_byte_offsets(original_document: &str, subset: &str) -> (usize, usize) {
+    debug_assert!(is_slice_of(original_document, subset));
+    let offset = subset.as_ptr() as usize - original_document.as_ptr() as usize;
+    let end = offset + subset.len();
+    (offset, end)
+}
+
+/// Check if the child string slice is a slice of the parent string slice.
+fn is_slice_of(parent: &str, child: &str) -> bool {
+    let parent_start = parent.as_ptr() as usize;
+    let parent_end = parent_start + parent.len();
+    let child_start = child.as_ptr() as usize;
+    let child_end = child_start + child.len();
+    child_start >= parent_start && child_end <= parent_end
+}
diff --git a/src/wasm/to_wasm.rs b/src/wasm/to_wasm.rs
new file mode 100644
index 00000000..cd3f6924
--- /dev/null
+++ b/src/wasm/to_wasm.rs
@@ -0,0 +1,27 @@
+use crate::error::CustomError;
+
+pub(crate) trait ToWasm {
+    type Output;
+
+    fn to_wasm(&self, full_document: ToWasmContext<'_>) -> Result<Self::Output, CustomError>;
+}
+
+pub(crate) trait ToWasmStandardProperties {
+    type Output;
+
+    fn to_wasm_standard_properties(
+        &self,
+        wasm_context: ToWasmContext<'_>,
+    ) -> Result<Self::Output, CustomError>;
+}
+
+#[derive(Debug, Clone)]
+pub(crate) struct ToWasmContext<'s> {
+    pub(crate) full_document: &'s str,
+}
+
+impl<'s> ToWasmContext<'s> {
+    pub(crate) fn new(full_document: &'s str) -> ToWasmContext<'s> {
+        ToWasmContext { full_document }
+    }
+}