diff --git a/README.md b/README.md
index 22da1495..ecf8f8af 100644
--- a/README.md
+++ b/README.md
@@ -67,7 +67,7 @@ see [optimizations](#optimizations) for opts implemented/supported.
 - supports i32, i64, and f64 for valtypes
 - `NaN` and `Infinity` (f64 only)
 - `isNaN` and `isFinite` (f64 only)
-- most of `Number` (`MAX_VALUE`, `MIN_VALUE`, `MAX_SAFE_INTEGER`, `MIN_SAFE_INTEGER`, `POSITIVE_INFINITY`, `NEGATIVE_INFINITY`, `EPSILON`, `NaN`, `isNaN`, `isFinite`, `isInteger`) (some f64 only)
+- most of `Number` (`MAX_VALUE`, `MIN_VALUE`, `MAX_SAFE_INTEGER`, `MIN_SAFE_INTEGER`, `POSITIVE_INFINITY`, `NEGATIVE_INFINITY`, `EPSILON`, `NaN`, `isNaN`, `isFinite`, `isInteger`, `isSafeInteger`) (some f64 only)
 - some `Math` funcs (`Math.sqrt`, `Math.abs`, `Math.floor`, `Math.sign`, `Math.round`, `Math.trunc`) (f64 only)
 
 ## soon todo
diff --git a/compiler/builtins.js b/compiler/builtins.js
index 15509583..69f3bd68 100644
--- a/compiler/builtins.js
+++ b/compiler/builtins.js
@@ -1,4 +1,4 @@
-import { Opcodes, Valtype } from "./wasmSpec.js";
+import { Blocktype, Opcodes, Valtype } from "./wasmSpec.js";
 import { number, i32x4 } from "./embedding.js";
 // import parse from "./parse.js";
 
@@ -203,6 +203,28 @@ export const BuiltinFuncs = function() {
     ]
   };
 
+  this.__Number_isSafeInteger = {
+    floatOnly: true,
+    params: [ valtypeBinary ],
+    locals: [],
+    returns: [ valtypeBinary ],
+    wasm: [
+      [ Opcodes.local_get, 0 ],
+      [ Opcodes.local_get, 0 ],
+      [ Opcodes.f64_trunc ],
+      [ Opcodes.f64_ne ],
+      [ Opcodes.if, Blocktype.void ],
+      ...number(0),
+      [ Opcodes.return ],
+      [ Opcodes.end ],
+      [ Opcodes.local_get, 0 ],
+      [ Opcodes.f64_abs ],
+      ...number(9007199254740991),
+      [ Opcodes.f64_le ],
+      Opcodes.i32_from
+    ]
+  };
+
   this.__Math_sqrt = {
     floatOnly: true,
     params: [ valtypeBinary ],
diff --git a/test/float_issafeinteger.js b/test/float_issafeinteger.js
new file mode 100644
index 00000000..e0b66996
--- /dev/null
+++ b/test/float_issafeinteger.js
@@ -0,0 +1,11 @@
+// "1\n1\n1\n1\n1\n0\n0\n"
+
+console.log(Number.isSafeInteger(0));
+console.log(Number.isSafeInteger(1));
+console.log(Number.isSafeInteger(-1));
+
+console.log(Number.isSafeInteger(Number.MAX_SAFE_INTEGER));
+console.log(Number.isSafeInteger(Number.MIN_SAFE_INTEGER));
+
+console.log(Number.isSafeInteger(Number.MAX_SAFE_INTEGER + 1));
+console.log(Number.isSafeInteger(Number.MIN_SAFE_INTEGER - 1));
\ No newline at end of file