diff --git a/CHANGELOG.md b/CHANGELOG.md
index 847fea187b2..62e12de53c4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -230,3 +230,6 @@
 - Fixed a bug where the formatter would remove the `@deprecated` attribute from
   constants.
   ([Surya Rose](https://github.com/GearsDatapacks))
+
+- Fix invalid JavaScript codegen in cases where underscores follow a decimal.
+  ([Patrick Dewey](https://github.com/ptdewey))
diff --git a/compiler-core/src/javascript/decision.rs b/compiler-core/src/javascript/decision.rs
index 3e36fe73e24..54ec77d4421 100644
--- a/compiler-core/src/javascript/decision.rs
+++ b/compiler-core/src/javascript/decision.rs
@@ -1,6 +1,6 @@
 use super::{
     INDENT, bit_array_segment_int_value_to_bytes,
-    expression::{self, Generator, Ordering, float},
+    expression::{self, Generator, Ordering, float, float_from_value},
 };
 use crate::{
     ast::{AssignmentKind, Endianness, SrcSpan, TypedClause, TypedExpr, TypedPattern},
@@ -1024,7 +1024,7 @@ impl<'generator, 'module, 'a> Variables<'generator, 'module, 'a> {
             RuntimeCheck::String { value: expected } => docvec![value, equality, string(expected)],
             RuntimeCheck::Float {
                 float_value: expected,
-            } => docvec![value, equality, expected.value()],
+            } => docvec![value, equality, float_from_value(expected.value())],
             RuntimeCheck::Int {
                 int_value: expected,
             } => docvec![value, equality, expected.clone()],
diff --git a/compiler-core/src/javascript/expression.rs b/compiler-core/src/javascript/expression.rs
index 33cbb490620..3365f0f12e1 100644
--- a/compiler-core/src/javascript/expression.rs
+++ b/compiler-core/src/javascript/expression.rs
@@ -288,7 +288,7 @@ impl<'module, 'a> Generator<'module, 'a> {
             TypedExpr::String { value, .. } => string(value),
 
             TypedExpr::Int { value, .. } => int(value),
-            TypedExpr::Float { value, .. } => float(value),
+            TypedExpr::Float { float_value, .. } => float_from_value(float_value.value()),
 
             TypedExpr::List { elements, tail, .. } => {
                 self.not_in_tail_position(Some(Ordering::Strict), |this| match tail {
@@ -2353,6 +2353,22 @@ pub fn float(value: &str) -> Document<'_> {
     out.to_doc()
 }
 
+pub fn float_from_value(value: f64) -> Document<'static> {
+    if value.is_infinite() {
+        if value.is_sign_positive() {
+            "Infinity".to_doc()
+        } else {
+            "-Infinity".to_doc()
+        }
+    } else if value.is_nan() {
+        // NOTE: this case is probably unnecessary, as this function is only
+        // invoked with `LiteralFloatValue` values, which cannot be nan.
+        "NaN".to_doc()
+    } else {
+        value.to_doc()
+    }
+}
+
 /// The context where the constant expression is used, it might be inside a
 /// function call, or in the definition of another constant.
 ///
diff --git a/compiler-core/src/javascript/tests/numbers.rs b/compiler-core/src/javascript/tests/numbers.rs
index f3a92291787..eed9f293105 100644
--- a/compiler-core/src/javascript/tests/numbers.rs
+++ b/compiler-core/src/javascript/tests/numbers.rs
@@ -431,6 +431,56 @@ pub fn main() {
     );
 }
 
+#[test]
+fn underscore_after_decimal_point() {
+    assert_js!(
+        "
+pub fn main() {
+  0._1
+}
+"
+    );
+}
+
+#[test]
+fn underscore_after_decimal_point_case_statement() {
+    assert_js!(
+        "
+pub fn main(x) {
+  case x {
+    0._1 -> \"bar\"
+    _ -> \"foo\"
+  }
+}
+"
+    );
+}
+
+#[test]
+fn inf_float_case_statement() {
+    assert_js!(
+        "
+pub fn main(x) {
+  case x {
+  100.001e123_456_789 -> \"bar\"
+    _ -> \"foo\"
+  }
+}
+"
+    );
+}
+
+#[test]
+fn division_inf_by_inf_float() {
+    assert_js!(
+        "
+pub fn main(x) {
+  -100.001e123_456_789 /. 100.001e123_456_789
+}
+"
+    );
+}
+
 #[test]
 fn division_by_zero_float() {
     assert_js!(
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__division_inf_by_inf_float.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__division_inf_by_inf_float.snap
new file mode 100644
index 00000000000..9b33a2135e1
--- /dev/null
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__division_inf_by_inf_float.snap
@@ -0,0 +1,15 @@
+---
+source: compiler-core/src/javascript/tests/numbers.rs
+expression: "\npub fn main(x) {\n  -100.001e123_456_789 /. 100.001e123_456_789\n}\n"
+---
+----- SOURCE CODE
+
+pub fn main(x) {
+  -100.001e123_456_789 /. 100.001e123_456_789
+}
+
+
+----- COMPILED JAVASCRIPT
+export function main(x) {
+  return -Infinity / Infinity;
+}
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_literals.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_literals.snap
index 0590a670682..68b31ce7ebb 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_literals.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_literals.snap
@@ -1,8 +1,6 @@
 ---
 source: compiler-core/src/javascript/tests/numbers.rs
-assertion_line: 23
 expression: "\npub fn go() {\n  1.5\n  2.0\n  -0.1\n  1.\n}\n"
-snapshot_kind: text
 ---
 ----- SOURCE CODE
 
@@ -19,5 +17,5 @@ export function go() {
   1.5;
   2.0;
   -0.1;
-  return 1.;
+  return 1.0;
 }
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_scientific_literals.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_scientific_literals.snap
index 7601d5188cd..d062626b3c5 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_scientific_literals.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__float_scientific_literals.snap
@@ -1,8 +1,6 @@
 ---
 source: compiler-core/src/javascript/tests/numbers.rs
-assertion_line: 37
 expression: "\npub fn go() {\n  0.01e-1\n  0.01e-0\n  -10.01e-1\n  -10.01e-0\n  100.001e523\n  -100.001e-523\n  100.001e123_456_789\n  -100.001e-123_456_789\n}\n"
-snapshot_kind: text
 ---
 ----- SOURCE CODE
 
@@ -20,12 +18,12 @@ pub fn go() {
 
 ----- COMPILED JAVASCRIPT
 export function go() {
-  0.01e-1;
-  0.01e-0;
-  -10.01e-1;
-  -10.01e-0;
-  100.001e523;
-  -100.001e-523;
-  100.001e123_456_789;
-  return -100.001e-123_456_789;
+  0.001;
+  0.01;
+  -1.001;
+  -10.01;
+  Infinity;
+  -0.0;
+  Infinity;
+  return -0.0;
 }
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__inf_float_case_statement.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__inf_float_case_statement.snap
new file mode 100644
index 00000000000..c7d51378682
--- /dev/null
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__inf_float_case_statement.snap
@@ -0,0 +1,22 @@
+---
+source: compiler-core/src/javascript/tests/numbers.rs
+expression: "\npub fn main(x) {\n  case x {\n  100.001e123_456_789 -> \"bar\"\n    _ -> \"foo\"\n  }\n}\n"
+---
+----- SOURCE CODE
+
+pub fn main(x) {
+  case x {
+  100.001e123_456_789 -> "bar"
+    _ -> "foo"
+  }
+}
+
+
+----- COMPILED JAVASCRIPT
+export function main(x) {
+  if (x === Infinity) {
+    return "bar";
+  } else {
+    return "foo";
+  }
+}
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__preceeding_zeros_float.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__preceeding_zeros_float.snap
index 5fd9820bfa4..36f6db6f3fe 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__preceeding_zeros_float.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__preceeding_zeros_float.snap
@@ -1,8 +1,6 @@
 ---
 source: compiler-core/src/javascript/tests/numbers.rs
-assertion_line: 264
 expression: "\npub fn main() {\n  09_179.1\n}\n"
-snapshot_kind: text
 ---
 ----- SOURCE CODE
 
@@ -13,5 +11,5 @@ pub fn main() {
 
 ----- COMPILED JAVASCRIPT
 export function main() {
-  return 9_179.1;
+  return 9179.1;
 }
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__underscore_after_decimal_point.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__underscore_after_decimal_point.snap
new file mode 100644
index 00000000000..070c78d43fa
--- /dev/null
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__underscore_after_decimal_point.snap
@@ -0,0 +1,15 @@
+---
+source: compiler-core/src/javascript/tests/numbers.rs
+expression: "\npub fn main() {\n  0._1\n}\n"
+---
+----- SOURCE CODE
+
+pub fn main() {
+  0._1
+}
+
+
+----- COMPILED JAVASCRIPT
+export function main() {
+  return 0.1;
+}
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__underscore_after_decimal_point_case_statement.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__underscore_after_decimal_point_case_statement.snap
new file mode 100644
index 00000000000..35f7f1163e9
--- /dev/null
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__underscore_after_decimal_point_case_statement.snap
@@ -0,0 +1,22 @@
+---
+source: compiler-core/src/javascript/tests/numbers.rs
+expression: "\npub fn main(x) {\n  case x {\n    0._1 -> \"bar\"\n    _ -> \"foo\"\n  }\n}\n"
+---
+----- SOURCE CODE
+
+pub fn main(x) {
+  case x {
+    0._1 -> "bar"
+    _ -> "foo"
+  }
+}
+
+
+----- COMPILED JAVASCRIPT
+export function main(x) {
+  if (x === 0.1) {
+    return "bar";
+  } else {
+    return "foo";
+  }
+}
diff --git a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__wide_float_div.snap b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__wide_float_div.snap
index cca7044fb6a..2ad43271b94 100644
--- a/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__wide_float_div.snap
+++ b/compiler-core/src/javascript/tests/snapshots/gleam_core__javascript__tests__numbers__wide_float_div.snap
@@ -11,5 +11,5 @@ pub fn go() {
 
 ----- COMPILED JAVASCRIPT
 export function go() {
-  return 111111111111111111111111111111. / 22222222222222222222222222222222222.;
+  return 1.111111111111111e29 / 2.222222222222222e34;
 }