diff --git a/src/core/document.js b/src/core/document.js
index 412c80caf..e6d61acfb 100644
--- a/src/core/document.js
+++ b/src/core/document.js
@@ -60,6 +60,7 @@ import {
   RefSet,
   RefSetCache,
 } from "./primitives.js";
+import { FunctionType, PDFFunctionFactory } from "./function.js";
 import { getXfaFontDict, getXfaFontName } from "./xfa_fonts.js";
 import { NullStream, Stream } from "./stream.js";
 import { BaseStream } from "./base_stream.js";
@@ -73,7 +74,6 @@ import { LocalColorSpaceCache } from "./image_utils.js";
 import { ObjectLoader } from "./object_loader.js";
 import { OperatorList } from "./operator_list.js";
 import { PartialEvaluator } from "./evaluator.js";
-import { PDFFunctionFactory } from "./function.js";
 import { PDFImage } from "./image.js";
 import { StreamsSequenceStream } from "./decode_stream.js";
 import { StructTreePage } from "./struct_tree.js";
@@ -2131,7 +2131,7 @@ class PDFDocument {
         return obj;
       }
 
-      if (dict.get("FunctionType") === 4) {
+      if (dict.get("FunctionType") === FunctionType.POSTSCRIPT_CALCULATOR) {
         const source = value.getString();
         value.reset();
         const domain = dict.get("Domain") ?? [];
diff --git a/src/core/function.js b/src/core/function.js
index 5336f1f76..2fbc81423 100644
--- a/src/core/function.js
+++ b/src/core/function.js
@@ -22,6 +22,13 @@ import { isNumberArray } from "./core_utils.js";
 import { LocalFunctionCache } from "./image_utils.js";
 import { MathClamp } from "../shared/math_clamp.js";
 
+const FunctionType = {
+  SAMPLED: 0,
+  EXPONENTIAL_INTERPOLATION: 2,
+  STITCHING: 3,
+  POSTSCRIPT_CALCULATOR: 4,
+};
+
 class PDFFunctionFactory {
   static #useWasm = true;
 
@@ -126,18 +133,16 @@ class PDFFunction {
     const typeNum = dict.get("FunctionType");
 
     switch (typeNum) {
-      case 0:
+      case FunctionType.SAMPLED:
         return this.constructSampled(factory, fn, dict);
-      case 1:
-        break;
-      case 2:
+      case FunctionType.EXPONENTIAL_INTERPOLATION:
         return this.constructInterpolated(factory, dict);
-      case 3:
+      case FunctionType.STITCHING:
         return this.constructStiched(factory, dict);
-      case 4:
+      case FunctionType.POSTSCRIPT_CALCULATOR:
         return this.constructPostScript(factory, fn, dict);
     }
-    throw new FormatError("Unknown type of function");
+    throw new FormatError(`Unknown function type: ${typeNum}`);
   }
 
   static parseArray(factory, fnObj) {
@@ -391,4 +396,4 @@ function isPDFFunction(v) {
   return fnDict.has("FunctionType");
 }
 
-export { isPDFFunction, PDFFunctionFactory };
+export { FunctionType, isPDFFunction, PDFFunctionFactory };
diff --git a/test/unit/colorspace_spec.js b/test/unit/colorspace_spec.js
index 79a2ef222..4c4129ca0 100644
--- a/test/unit/colorspace_spec.js
+++ b/test/unit/colorspace_spec.js
@@ -14,6 +14,7 @@
  */
 
 import { Dict, Name, Ref } from "../../src/core/primitives.js";
+import { FunctionType, PDFFunctionFactory } from "../../src/core/function.js";
 import {
   GlobalColorSpaceCache,
   LocalColorSpaceCache,
@@ -21,7 +22,6 @@ import {
 import { Stream, StringStream } from "../../src/core/stream.js";
 import { ColorSpace } from "../../src/core/colorspace.js";
 import { ColorSpaceUtils } from "../../src/core/colorspace_utils.js";
-import { PDFFunctionFactory } from "../../src/core/function.js";
 import { XRefMock } from "./test_utils.js";
 
 describe("colorspace", function () {
@@ -836,7 +836,7 @@ describe("colorspace", function () {
 
     it("should handle the case when cs is an array", function () {
       const fnDict = new Dict();
-      fnDict.set("FunctionType", 4);
+      fnDict.set("FunctionType", FunctionType.POSTSCRIPT_CALCULATOR);
       fnDict.set("Domain", [0.0, 1.0]);
       fnDict.set("Range", [0.0, 1.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0]);
       fnDict.set("Length", 58);