diff --git a/src/core/function.js b/src/core/function.js
index 2bfa150d6..cf5fce896 100644
--- a/src/core/function.js
+++ b/src/core/function.js
@@ -15,7 +15,6 @@
 
 import { Dict, Ref } from "./primitives.js";
 import {
-  FeatureTest,
   FormatError,
   info,
   MathClamp,
@@ -23,7 +22,6 @@ import {
   unreachable,
   warn,
 } from "../shared/util.js";
-import { PostScriptLexer, PostScriptParser } from "./ps_parser.js";
 import { BaseStream } from "./base_stream.js";
 import { buildPostScriptJsFunction } from "./postscript/js_evaluator.js";
 import { buildPostScriptWasmFunction } from "./postscript/wasm_compiler.js";
@@ -371,89 +369,20 @@ class PDFFunction {
       throw new FormatError("No range.");
     }
 
+    const psCode = fn.getString();
+
     try {
       if (factory.useWasm) {
-        const wasmFn = buildPostScriptWasmFunction(
-          fn.getString(),
-          domain,
-          range
-        );
+        const wasmFn = buildPostScriptWasmFunction(psCode, domain, range);
         if (wasmFn) {
           return wasmFn; // (src, srcOffset, dest, destOffset) → void
         }
-      } else {
-        const jsFn = buildPostScriptJsFunction(fn.getString(), domain, range);
-        if (jsFn) {
-          return jsFn; // (src, srcOffset, dest, destOffset) → void
-        }
       }
-    } catch {
-      // Fall through to the existing interpreter-based path.
-    }
+    } catch {}
 
-    warn("Unable to compile PS function, using interpreter");
-    fn.reset();
+    warn("Failed to compile PostScript function to wasm, falling back to JS");
 
-    const lexer = new PostScriptLexer(fn);
-    const parser = new PostScriptParser(lexer);
-    const code = parser.parse();
-
-    if (factory.isEvalSupported && FeatureTest.isEvalSupported) {
-      const compiled = new PostScriptCompiler().compile(code, domain, range);
-      if (compiled) {
-        // Compiled function consists of simple expressions such as addition,
-        // subtraction, Math.max, and also contains 'var' and 'return'
-        // statements. See the generation in the PostScriptCompiler below.
-        // eslint-disable-next-line no-new-func
-        return new Function("src", "srcOffset", "dest", "destOffset", compiled);
-      }
-    }
-    info("Unable to compile PS function");
-
-    const numOutputs = range.length >> 1;
-    const numInputs = domain.length >> 1;
-    const evaluator = new PostScriptEvaluator(code);
-    // Cache the values for a big speed up, the cache size is limited though
-    // since the number of possible values can be huge from a PS function.
-    const cache = Object.create(null);
-    // The MAX_CACHE_SIZE is set to ~4x the maximum number of distinct values
-    // seen in our tests.
-    const MAX_CACHE_SIZE = 2048 * 4;
-    let cache_available = MAX_CACHE_SIZE;
-    const tmpBuf = new Float32Array(numInputs);
-
-    return function constructPostScriptFn(src, srcOffset, dest, destOffset) {
-      let i, value;
-      let key = "";
-      const input = tmpBuf;
-      for (i = 0; i < numInputs; i++) {
-        value = src[srcOffset + i];
-        input[i] = value;
-        key += value + "_";
-      }
-
-      const cachedValue = cache[key];
-      if (cachedValue !== undefined) {
-        dest.set(cachedValue, destOffset);
-        return;
-      }
-
-      const output = new Float32Array(numOutputs);
-      const stack = evaluator.execute(input);
-      const stackIndex = stack.length - numOutputs;
-      for (i = 0; i < numOutputs; i++) {
-        output[i] = MathClamp(
-          stack[stackIndex + i],
-          range[i * 2],
-          range[i * 2 + 1]
-        );
-      }
-      if (cache_available > 0) {
-        cache_available--;
-        cache[key] = output;
-      }
-      dest.set(output, destOffset);
-    };
+    return buildPostScriptJsFunction(psCode, domain, range);
   }
 }
 
diff --git a/src/core/postscript/js_evaluator.js b/src/core/postscript/js_evaluator.js
index 8bcd77c20..220455fc8 100644
--- a/src/core/postscript/js_evaluator.js
+++ b/src/core/postscript/js_evaluator.js
@@ -528,23 +528,311 @@ function compilePostScriptToIR(source, domain, range) {
 }
 
 /**
- * Same calling convention as the Wasm wrapper:
- *   fn(src, srcOffset, dest, destOffset)
+ * Direct stack-based interpreter for a parsed PsProgram.
+ * Used when PSStackToTree fails to optimize the AST.
+ */
+class PSStackBasedInterpreter {
+  // Safe: JS is single-threaded.
+  static #stack = new Float64Array(100);
+
+  static #sp = 0;
+
+  static #push(v) {
+    if (this.#sp < this.#stack.length) {
+      this.#stack[this.#sp++] = v;
+    }
+  }
+
+  static #execOp(op) {
+    const stack = this.#stack;
+    switch (op) {
+      case TOKEN.true:
+        this.#push(1);
+        break;
+      case TOKEN.false:
+        this.#push(0);
+        break;
+      case TOKEN.abs:
+        stack[this.#sp - 1] = Math.abs(stack[this.#sp - 1]);
+        break;
+      case TOKEN.neg:
+        stack[this.#sp - 1] = -stack[this.#sp - 1];
+        break;
+      case TOKEN.ceiling:
+        stack[this.#sp - 1] = Math.ceil(stack[this.#sp - 1]);
+        break;
+      case TOKEN.floor:
+        stack[this.#sp - 1] = Math.floor(stack[this.#sp - 1]);
+        break;
+      case TOKEN.round:
+        stack[this.#sp - 1] = Math.floor(stack[this.#sp - 1] + 0.5);
+        break;
+      case TOKEN.truncate:
+        stack[this.#sp - 1] = Math.trunc(stack[this.#sp - 1]);
+        break;
+      case TOKEN.sqrt:
+        stack[this.#sp - 1] = Math.sqrt(stack[this.#sp - 1]);
+        break;
+      case TOKEN.sin:
+        stack[this.#sp - 1] = Math.sin(
+          (stack[this.#sp - 1] % 360) * _DEG_TO_RAD
+        );
+        break;
+      case TOKEN.cos:
+        stack[this.#sp - 1] = Math.cos(
+          (stack[this.#sp - 1] % 360) * _DEG_TO_RAD
+        );
+        break;
+      case TOKEN.ln:
+        stack[this.#sp - 1] = Math.log(stack[this.#sp - 1]);
+        break;
+      case TOKEN.log:
+        stack[this.#sp - 1] = Math.log10(stack[this.#sp - 1]);
+        break;
+      case TOKEN.cvi:
+        stack[this.#sp - 1] = Math.trunc(stack[this.#sp - 1]) | 0;
+        break;
+      case TOKEN.cvr:
+        break; // values are already f64
+      case TOKEN.not: {
+        const v = stack[this.#sp - 1];
+        stack[this.#sp - 1] = v === 0 || v === 1 ? 1 - v : ~(v | 0);
+        break;
+      }
+      case TOKEN.add: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] += b;
+        break;
+      }
+      case TOKEN.sub: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] -= b;
+        break;
+      }
+      case TOKEN.mul: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] *= b;
+        break;
+      }
+      case TOKEN.div: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = b !== 0 ? stack[this.#sp - 1] / b : 0;
+        break;
+      }
+      case TOKEN.idiv: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = b !== 0 ? Math.trunc(stack[this.#sp - 1] / b) : 0;
+        break;
+      }
+      case TOKEN.mod: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = b !== 0 ? stack[this.#sp - 1] % b : 0;
+        break;
+      }
+      case TOKEN.exp: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] **= b;
+        break;
+      }
+      case TOKEN.atan: {
+        // Stack: [..., dy, dx] — dx on top.
+        const dx = stack[--this.#sp];
+        const deg = Math.atan2(stack[this.#sp - 1], dx) * _RAD_TO_DEG;
+        stack[this.#sp - 1] = deg < 0 ? deg + 360 : deg;
+        break;
+      }
+      case TOKEN.eq: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = stack[this.#sp - 1] === b ? 1 : 0;
+        break;
+      }
+      case TOKEN.ne: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = stack[this.#sp - 1] !== b ? 1 : 0;
+        break;
+      }
+      case TOKEN.gt: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = stack[this.#sp - 1] > b ? 1 : 0;
+        break;
+      }
+      case TOKEN.ge: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = stack[this.#sp - 1] >= b ? 1 : 0;
+        break;
+      }
+      case TOKEN.lt: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = stack[this.#sp - 1] < b ? 1 : 0;
+        break;
+      }
+      case TOKEN.le: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = stack[this.#sp - 1] <= b ? 1 : 0;
+        break;
+      }
+      case TOKEN.and: {
+        const b = stack[--this.#sp] | 0;
+        stack[this.#sp - 1] = (stack[this.#sp - 1] | 0) & b;
+        break;
+      }
+      case TOKEN.or: {
+        const b = stack[--this.#sp] | 0;
+        stack[this.#sp - 1] = stack[this.#sp - 1] | 0 | b;
+        break;
+      }
+      case TOKEN.xor: {
+        const b = stack[--this.#sp] | 0;
+        stack[this.#sp - 1] = (stack[this.#sp - 1] | 0) ^ b;
+        break;
+      }
+      case TOKEN.bitshift: {
+        const amt = stack[--this.#sp] | 0;
+        const v = stack[this.#sp - 1] | 0;
+        stack[this.#sp - 1] = amt > 0 ? v << amt : v >> -amt;
+        break;
+      }
+      case TOKEN.min: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = Math.min(stack[this.#sp - 1], b);
+        break;
+      }
+      case TOKEN.max: {
+        const b = stack[--this.#sp];
+        stack[this.#sp - 1] = Math.max(stack[this.#sp - 1], b);
+        break;
+      }
+      case TOKEN.dup:
+        this.#push(stack[this.#sp - 1]);
+        break;
+      case TOKEN.exch: {
+        const a = stack[--this.#sp];
+        const b = stack[--this.#sp];
+        this.#push(a);
+        this.#push(b);
+        break;
+      }
+      case TOKEN.pop:
+        this.#sp--;
+        break;
+      case TOKEN.copy: {
+        const n = Math.trunc(stack[--this.#sp]);
+        const base = this.#sp - n;
+        for (let k = 0; k < n; k++) {
+          this.#push(stack[base + k]);
+        }
+        break;
+      }
+      case TOKEN.index: {
+        const i = Math.trunc(stack[--this.#sp]);
+        this.#push(stack[this.#sp - 1 - i]);
+        break;
+      }
+      case TOKEN.roll: {
+        // Rotate top n elements by j positions toward the top.
+        const j = Math.trunc(stack[--this.#sp]);
+        const n = Math.trunc(stack[--this.#sp]);
+        if (n > 1 && j !== 0) {
+          const mod = ((j % n) + n) % n;
+          if (mod !== 0) {
+            const base = this.#sp - n;
+            const sub = stack.slice(base, this.#sp);
+            for (let k = 0; k < n; k++) {
+              stack[base + k] = sub[(k - mod + n) % n];
+            }
+          }
+        }
+        break;
+      }
+    }
+  }
+
+  static #execBlock(instructions) {
+    for (const instr of instructions) {
+      switch (instr.type) {
+        case PS_NODE.number:
+          this.#push(instr.value);
+          break;
+        case PS_NODE.operator:
+          this.#execOp(instr.op);
+          break;
+        case PS_NODE.if:
+          if (this.#stack[--this.#sp] !== 0) {
+            this.#execBlock(instr.then.instructions);
+          }
+          break;
+        case PS_NODE.ifelse:
+          if (this.#stack[--this.#sp] !== 0) {
+            this.#execBlock(instr.then.instructions);
+          } else {
+            this.#execBlock(instr.otherwise.instructions);
+          }
+          break;
+      }
+    }
+  }
+
+  /**
+   * @param {import("./ast.js").PsProgram} program
+   * @param {number[]} domain  – flat [min0,max0, …]
+   * @param {number[]} range   – flat [min0,max0, …]
+   * @returns {Function}  – `(src, srcOffset, dest, destOffset) => void`
+   */
+  static build(program, domain, range) {
+    const nIn = domain.length >> 1;
+    const nOut = range.length >> 1;
+    const { instructions } = program.body;
+    return (src, srcOffset, dest, destOffset) => {
+      this.#sp = 0;
+      for (let i = 0; i < nIn; i++) {
+        this.#push(src[srcOffset + i]);
+      }
+      this.#execBlock(instructions);
+      // Outputs: first at bottom, last at top.
+      const base = this.#sp - nOut;
+      for (let i = 0; i < nOut; i++) {
+        const v = base + i >= 0 ? this.#stack[base + i] : 0;
+        dest[destOffset + i] = Math.max(
+          range[i * 2],
+          Math.min(range[i * 2 + 1], v)
+        );
+      }
+    };
+  }
+}
+
+/**
+ * Tries PSStackToTree-optimized IR first; falls back to direct interpreter.
  *
  * @param {string}   source
  * @param {number[]} domain  – flat [min0,max0, …]
  * @param {number[]} range   – flat [min0,max0, …]
- * @returns {Function|null}
+ * @returns {Function}  – `(src, srcOffset, dest, destOffset) => void`
  */
 function buildPostScriptJsFunction(source, domain, range) {
-  const ir = compilePostScriptToIR(source, domain, range);
-  if (!ir) {
-    return null;
+  const program = parsePostScriptFunction(source);
+  const ir = new PsJsCompiler(domain, range).compile(program);
+  if (ir) {
+    return (src, srcOffset, dest, destOffset) => {
+      PsJsCompiler.execute(ir, src, srcOffset, dest, destOffset);
+    };
   }
-
-  return (src, srcOffset, dest, destOffset) => {
-    PsJsCompiler.execute(ir, src, srcOffset, dest, destOffset);
-  };
+  // Fall back to direct interpreter.
+  return PSStackBasedInterpreter.build(program, domain, range);
 }
 
-export { buildPostScriptJsFunction, compilePostScriptToIR };
+/**
+ * @param {import("./ast.js").PsProgram} program
+ * @param {number[]} domain  – flat [min0,max0, …]
+ * @param {number[]} range   – flat [min0,max0, …]
+ * @returns {Function}  – `(src, srcOffset, dest, destOffset) => void`
+ */
+function buildPostScriptProgramFunction(program, domain, range) {
+  return PSStackBasedInterpreter.build(program, domain, range);
+}
+
+export {
+  buildPostScriptJsFunction,
+  buildPostScriptProgramFunction,
+  compilePostScriptToIR,
+};
diff --git a/test/unit/postscript_spec.js b/test/unit/postscript_spec.js
index 569a3ad23..5ad7d69e9 100644
--- a/test/unit/postscript_spec.js
+++ b/test/unit/postscript_spec.js
@@ -13,6 +13,10 @@
  * limitations under the License.
  */
 
+import {
+  buildPostScriptJsFunction,
+  buildPostScriptProgramFunction,
+} from "../../src/core/postscript/js_evaluator.js";
 import {
   buildPostScriptWasmFunction,
   compilePostScriptToWasm,
@@ -35,7 +39,6 @@ import {
   PsTernaryNode,
   PsUnaryNode,
 } from "../../src/core/postscript/ast.js";
-import { buildPostScriptJsFunction } from "../../src/core/postscript/js_evaluator.js";
 
 // Precision argument for toBeCloseTo() in trigonometric tests.
 const TRIGONOMETRY_EPS = 1e-10;
@@ -192,25 +195,41 @@ describe("PostScript Type 4 lexer, parser, and Wasm compiler", function () {
   describe("PostScript Type 4 Wasm compiler", function () {
     /**
      * Compile and instantiate a PostScript Type 4 function, then call it.
-     * Returns null if compilation returns null (unsupported program).
+     * Returns null if Wasm compilation returns null (unsupported program).
      * For single-output functions returns a scalar; for multi-output an Array.
+     *
+     * Validates three implementations against each other:
+     *   - Wasm compiler (PSStackToTree → Wasm binary)
+     *   - JS IR compiler (PSStackToTree → flat IR interpreted in JS)
+     *   - Direct program interpreter (raw PsProgram stack-machine interpreter)
      */
     function compileAndRun(src, domain, range, args) {
       const wasmFn = buildPostScriptWasmFunction(src, domain, range);
+      // jsFn now always returns a function: PSStackToTree IR when possible,
+      // direct program interpreter otherwise.
       const jsFn = buildPostScriptJsFunction(src, domain, range);
+      // Direct interpreter: always available, never uses PSStackToTree.
+      const interpFn = buildPostScriptProgramFunction(
+        parsePostScriptFunction(src),
+        domain,
+        range
+      );
+
       if (!wasmFn) {
-        expect(jsFn).toBeNull();
         return null;
       }
-      expect(jsFn).not.toBeNull();
+
       const nOut = range.length >> 1;
       const srcBuf = new Float64Array(args);
       const wasmDest = new Float64Array(nOut);
       const jsDest = new Float64Array(nOut);
+      const interpDest = new Float64Array(nOut);
       wasmFn(srcBuf, 0, wasmDest, 0);
       jsFn(srcBuf, 0, jsDest, 0);
+      interpFn(srcBuf, 0, interpDest, 0);
       for (let i = 0; i < nOut; i++) {
         expect(jsDest[i]).toBeCloseTo(wasmDest[i], 10);
+        expect(interpDest[i]).toBeCloseTo(wasmDest[i], 10);
       }
       return nOut === 1 ? wasmDest[0] : Array.from(wasmDest);
     }