diff --git a/dom/bindings/nsScriptError.cpp b/dom/bindings/nsScriptError.cpp
index b99edb83d0a3..d401000ced54 100644
--- a/dom/bindings/nsScriptError.cpp
+++ b/dom/bindings/nsScriptError.cpp
@@ -257,7 +257,7 @@ ToStringHelper(const char* aSeverity, const nsString& aMessage,
     static const char format2[] =
         "[%s: \"%s\"]";
 
-    char* temp;
+    UniqueChars temp;
     char* tempMessage = nullptr;
     char* tempSourceName = nullptr;
     char* tempSourceLine = nullptr;
@@ -301,8 +301,7 @@ ToStringHelper(const char* aSeverity, const nsString& aMessage,
     if (!temp)
         return NS_ERROR_OUT_OF_MEMORY;
 
-    aResult.Assign(temp);
-    JS_smprintf_free(temp);
+    aResult.Assign(temp.get());
     return NS_OK;
 }
 
diff --git a/js/src/builtin/Intl.cpp b/js/src/builtin/Intl.cpp
index 1db8fd5db152..60795cc6e6e6 100644
--- a/js/src/builtin/Intl.cpp
+++ b/js/src/builtin/Intl.cpp
@@ -1308,7 +1308,7 @@ NewUCollator(JSContext* cx, Handle<CollatorObject*> collator)
         memcpy(newLocale + index, insert, insertLen);
         memcpy(newLocale + index + insertLen, oldLocale + index, localeLen - index + 1); // '\0'
         locale.clear();
-        locale.initBytes(newLocale);
+        locale.initBytes(JS::UniqueChars(newLocale));
     } else {
         MOZ_ASSERT(StringEqualsAscii(usage, "sort"));
     }
diff --git a/js/src/jit/BacktrackingAllocator.cpp b/js/src/jit/BacktrackingAllocator.cpp
index ea8f4498f92c..bf26b3293d6e 100644
--- a/js/src/jit/BacktrackingAllocator.cpp
+++ b/js/src/jit/BacktrackingAllocator.cpp
@@ -2313,21 +2313,21 @@ LiveRange::toString() const
 {
     AutoEnterOOMUnsafeRegion oomUnsafe;
 
-    char* buf = JS_smprintf("v%u [%u,%u)", hasVreg() ? vreg() : 0, from().bits(), to().bits());
+    UniqueChars buf = JS_smprintf("v%u [%u,%u)", hasVreg() ? vreg() : 0, from().bits(), to().bits());
 
     if (buf && bundle() && !bundle()->allocation().isBogus())
-        buf = JS_sprintf_append(buf, " %s", bundle()->allocation().toString().get());
+        buf = JS_sprintf_append(Move(buf), " %s", bundle()->allocation().toString().get());
 
     if (buf && hasDefinition())
-        buf = JS_sprintf_append(buf, " (def)");
+        buf = JS_sprintf_append(Move(buf), " (def)");
 
     for (UsePositionIterator iter = usesBegin(); buf && iter; iter++)
-        buf = JS_sprintf_append(buf, " %s@%u", iter->use()->toString().get(), iter->pos.bits());
+        buf = JS_sprintf_append(Move(buf), " %s@%u", iter->use()->toString().get(), iter->pos.bits());
 
     if (!buf)
         oomUnsafe.crash("LiveRange::toString()");
 
-    return UniqueChars(buf);
+    return buf;
 }
 
 UniqueChars
@@ -2336,10 +2336,10 @@ LiveBundle::toString() const
     AutoEnterOOMUnsafeRegion oomUnsafe;
 
     // Suppress -Wformat warning.
-    char *buf = JS_smprintf("%s", "");
+    UniqueChars buf = JS_smprintf("%s", "");
 
     for (LiveRange::BundleLinkIterator iter = rangesBegin(); buf && iter; iter++) {
-        buf = JS_sprintf_append(buf, "%s %s",
+        buf = JS_sprintf_append(Move(buf), "%s %s",
                                 (iter == rangesBegin()) ? "" : " ##",
                                 LiveRange::get(*iter)->toString().get());
     }
@@ -2347,7 +2347,7 @@ LiveBundle::toString() const
     if (!buf)
         oomUnsafe.crash("LiveBundle::toString()");
 
-    return UniqueChars(buf);
+    return buf;
 }
 
 #endif // JS_JITSPEW
diff --git a/js/src/jit/Ion.cpp b/js/src/jit/Ion.cpp
index f89d05f6283e..76e13d1e0d81 100644
--- a/js/src/jit/Ion.cpp
+++ b/js/src/jit/Ion.cpp
@@ -3325,12 +3325,11 @@ jit::Invalidate(JSContext* cx, JSScript* script, bool resetUses, bool cancelOffT
             filename = "<unknown>";
 
         // Construct the descriptive string.
-        char* buf = JS_smprintf("Invalidate %s:%" PRIuSIZE, filename, script->lineno());
+        UniqueChars buf = JS_smprintf("Invalidate %s:%" PRIuSIZE, filename, script->lineno());
 
         // Ignore the event on allocation failure.
         if (buf) {
-            cx->runtime()->geckoProfiler().markEvent(buf);
-            JS_smprintf_free(buf);
+            cx->runtime()->geckoProfiler().markEvent(buf.get());
         }
     }
 
diff --git a/js/src/jit/LIR.cpp b/js/src/jit/LIR.cpp
index c83841809bf1..ad8895c2d51d 100644
--- a/js/src/jit/LIR.cpp
+++ b/js/src/jit/LIR.cpp
@@ -376,26 +376,26 @@ LDefinition::toString() const
 {
     AutoEnterOOMUnsafeRegion oomUnsafe;
 
-    char* buf;
+    UniqueChars buf;
     if (isBogusTemp()) {
         buf = JS_smprintf("bogus");
     } else {
         buf = JS_smprintf("v%u<%s>", virtualRegister(), typeName(type()));
         if (buf) {
             if (policy() == LDefinition::FIXED)
-                buf = JS_sprintf_append(buf, ":%s", output()->toString().get());
+                buf = JS_sprintf_append(Move(buf), ":%s", output()->toString().get());
             else if (policy() == LDefinition::MUST_REUSE_INPUT)
-                buf = JS_sprintf_append(buf, ":tied(%u)", getReusedInput());
+                buf = JS_sprintf_append(Move(buf), ":tied(%u)", getReusedInput());
         }
     }
 
     if (!buf)
         oomUnsafe.crash("LDefinition::toString()");
 
-    return UniqueChars(buf);
+    return buf;
 }
 
-static char*
+static UniqueChars
 PrintUse(const LUse* use)
 {
     switch (use->policy()) {
@@ -420,7 +420,7 @@ LAllocation::toString() const
 {
     AutoEnterOOMUnsafeRegion oomUnsafe;
 
-    char* buf;
+    UniqueChars buf;
     if (isBogus()) {
         buf = JS_smprintf("bogus");
     } else {
@@ -452,7 +452,7 @@ LAllocation::toString() const
     if (!buf)
         oomUnsafe.crash("LAllocation::toString()");
 
-    return UniqueChars(buf);
+    return buf;
 }
 
 void
diff --git a/js/src/jit/MacroAssembler.cpp b/js/src/jit/MacroAssembler.cpp
index d91f0d2d93e3..38fa67ff9cdc 100644
--- a/js/src/jit/MacroAssembler.cpp
+++ b/js/src/jit/MacroAssembler.cpp
@@ -1733,11 +1733,10 @@ MacroAssembler::printf(const char* output)
 static void
 Printf1_(const char* output, uintptr_t value) {
     AutoEnterOOMUnsafeRegion oomUnsafe;
-    char* line = JS_sprintf_append(nullptr, output, value);
+    js::UniqueChars line = JS_sprintf_append(nullptr, output, value);
     if (!line)
         oomUnsafe.crash("OOM at masm.printf");
-    fprintf(stderr, "%s", line);
-    js_free(line);
+    fprintf(stderr, "%s", line.get());
 }
 
 void
diff --git a/js/src/jsapi-tests/testParseJSON.cpp b/js/src/jsapi-tests/testParseJSON.cpp
index e82c6791143c..aab1c4ee2826 100644
--- a/js/src/jsapi-tests/testParseJSON.cpp
+++ b/js/src/jsapi-tests/testParseJSON.cpp
@@ -306,9 +306,8 @@ Error(JSContext* cx, const char (&input)[N], uint32_t expectedLine,
     CHECK(report.init(cx, exn, js::ErrorReport::WithSideEffects));
     CHECK(report.report()->errorNumber == JSMSG_JSON_BAD_PARSE);
 
-    const char* lineAndColumnASCII = JS_smprintf("line %d column %d", expectedLine, expectedColumn);
-    CHECK(strstr(report.toStringResult().c_str(), lineAndColumnASCII) != nullptr);
-    js_free((void*)lineAndColumnASCII);
+    UniqueChars lineAndColumnASCII = JS_smprintf("line %d column %d", expectedLine, expectedColumn);
+    CHECK(strstr(report.toStringResult().c_str(), lineAndColumnASCII.get()) != nullptr);
 
     /* We do not execute JS, so there should be no exception thrown. */
     CHECK(!JS_IsExceptionPending(cx));
diff --git a/js/src/jsapi-tests/testPrintf.cpp b/js/src/jsapi-tests/testPrintf.cpp
index ca7d1d49bb8d..f9645a2babe0 100644
--- a/js/src/jsapi-tests/testPrintf.cpp
+++ b/js/src/jsapi-tests/testPrintf.cpp
@@ -22,13 +22,10 @@ print_one (const char *expect, const char *fmt, ...)
     va_list ap;
 
     va_start(ap, fmt);
-    char *output = JS_vsmprintf (fmt, ap);
+    JS::UniqueChars output = JS_vsmprintf (fmt, ap);
     va_end(ap);
 
-    bool result = output && !strcmp(output, expect);
-    JS_smprintf_free(output);
-
-    return result;
+    return output && !strcmp(output.get(), expect);
 }
 
 static const char *
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index b2d1c1982a74..438680274e67 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -5073,9 +5073,9 @@ class MOZ_RAII JSAutoByteString
     }
 
     /* Take ownership of the given byte array. */
-    void initBytes(char* bytes) {
+    void initBytes(JS::UniqueChars&& bytes) {
         MOZ_ASSERT(!mBytes);
-        mBytes = bytes;
+        mBytes = bytes.release();
     }
 
     char* encodeLatin1(JSContext* cx, JSString* str) {
diff --git a/js/src/jscntxt.cpp b/js/src/jscntxt.cpp
index 923fa8d00d10..74ae103c206e 100644
--- a/js/src/jscntxt.cpp
+++ b/js/src/jscntxt.cpp
@@ -541,14 +541,13 @@ static bool
 PrintSingleError(JSContext* cx, FILE* file, JS::ConstUTF8CharsZ toStringResult,
                  T* report, PrintErrorKind kind)
 {
-    UniquePtr<char> prefix;
+    UniqueChars prefix;
     if (report->filename)
-        prefix.reset(JS_smprintf("%s:", report->filename));
+        prefix = JS_smprintf("%s:", report->filename);
 
     if (report->lineno) {
-        UniquePtr<char> tmp(JS_smprintf("%s%u:%u ", prefix ? prefix.get() : "", report->lineno,
-                                        report->column));
-        prefix = Move(tmp);
+        prefix = JS_smprintf("%s%u:%u ", prefix ? prefix.get() : "", report->lineno,
+                                        report->column);
     }
 
     if (kind != PrintErrorKind::Error) {
@@ -567,8 +566,7 @@ PrintSingleError(JSContext* cx, FILE* file, JS::ConstUTF8CharsZ toStringResult,
             break;
         }
 
-        UniquePtr<char> tmp(JS_smprintf("%s%s: ", prefix ? prefix.get() : "", kindPrefix));
-        prefix = Move(tmp);
+        prefix = JS_smprintf("%s%s: ", prefix ? prefix.get() : "", kindPrefix);
     }
 
     const char* message = toStringResult ? toStringResult.c_str() : report->message().c_str();
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index 058de0d8ee9c..b378f882e1e3 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -791,7 +791,7 @@ sprintf_append(JSContext* cx, char* buf, const char* fmt, ...)
     va_list ap;
 
     va_start(ap, fmt);
-    char* result = JS_vsprintf_append(buf, fmt, ap);
+    char* result = JS_vsprintf_append(UniqueChars(buf), fmt, ap).release();
     va_end(ap);
 
     if (!result) {
@@ -1046,7 +1046,7 @@ JS::FormatStackDump(JSContext* cx, char* buf, bool showArgs, bool showLocals, bo
     }
 
     if (!num)
-        buf = JS_sprintf_append(buf, "JavaScript stack is empty\n");
+        buf = JS_sprintf_append(UniqueChars(buf), "JavaScript stack is empty\n").release();
 
     return buf;
 }
diff --git a/js/src/jsopcode.cpp b/js/src/jsopcode.cpp
index 0d819c20257b..da1cf9ae3752 100644
--- a/js/src/jsopcode.cpp
+++ b/js/src/jsopcode.cpp
@@ -1184,22 +1184,22 @@ ToDisassemblySource(JSContext* cx, HandleValue v, JSAutoByteString* bytes)
         char* nbytes = QuoteString(&sprinter, v.toString(), '"');
         if (!nbytes)
             return false;
-        nbytes = JS_sprintf_append(nullptr, "%s", nbytes);
+        UniqueChars copy = JS_smprintf("%s", nbytes);
         if (!nbytes) {
             ReportOutOfMemory(cx);
             return false;
         }
-        bytes->initBytes(nbytes);
+        bytes->initBytes(Move(copy));
         return true;
     }
 
     if (JS::CurrentThreadIsHeapBusy() || !cx->isAllocAllowed()) {
-        char* source = JS_sprintf_append(nullptr, "<value>");
+        UniqueChars source = JS_smprintf("<value>");
         if (!source) {
             ReportOutOfMemory(cx);
             return false;
         }
-        bytes->initBytes(source);
+        bytes->initBytes(Move(source));
         return true;
     }
 
@@ -1228,7 +1228,7 @@ ToDisassemblySource(JSContext* cx, HandleValue v, JSAutoByteString* bytes)
 static bool
 ToDisassemblySource(JSContext* cx, HandleScope scope, JSAutoByteString* bytes)
 {
-    char* source = JS_sprintf_append(nullptr, "%s {", ScopeKindString(scope->kind()));
+    UniqueChars source = JS_smprintf("%s {", ScopeKindString(scope->kind()));
     if (!source) {
         ReportOutOfMemory(cx);
         return false;
@@ -1239,7 +1239,7 @@ ToDisassemblySource(JSContext* cx, HandleScope scope, JSAutoByteString* bytes)
         if (!AtomToPrintableString(cx, bi.name(), &nameBytes))
             return false;
 
-        source = JS_sprintf_append(source, "%s: ", nameBytes.ptr());
+        source = JS_sprintf_append(Move(source), "%s: ", nameBytes.ptr());
         if (!source) {
             ReportOutOfMemory(cx);
             return false;
@@ -1248,27 +1248,27 @@ ToDisassemblySource(JSContext* cx, HandleScope scope, JSAutoByteString* bytes)
         BindingLocation loc = bi.location();
         switch (loc.kind()) {
           case BindingLocation::Kind::Global:
-            source = JS_sprintf_append(source, "global");
+            source = JS_sprintf_append(Move(source), "global");
             break;
 
           case BindingLocation::Kind::Frame:
-            source = JS_sprintf_append(source, "frame slot %u", loc.slot());
+            source = JS_sprintf_append(Move(source), "frame slot %u", loc.slot());
             break;
 
           case BindingLocation::Kind::Environment:
-            source = JS_sprintf_append(source, "env slot %u", loc.slot());
+            source = JS_sprintf_append(Move(source), "env slot %u", loc.slot());
             break;
 
           case BindingLocation::Kind::Argument:
-            source = JS_sprintf_append(source, "arg slot %u", loc.slot());
+            source = JS_sprintf_append(Move(source), "arg slot %u", loc.slot());
             break;
 
           case BindingLocation::Kind::NamedLambdaCallee:
-            source = JS_sprintf_append(source, "named lambda callee");
+            source = JS_sprintf_append(Move(source), "named lambda callee");
             break;
 
           case BindingLocation::Kind::Import:
-            source = JS_sprintf_append(source, "import");
+            source = JS_sprintf_append(Move(source), "import");
             break;
         }
 
@@ -1278,7 +1278,7 @@ ToDisassemblySource(JSContext* cx, HandleScope scope, JSAutoByteString* bytes)
         }
 
         if (!bi.isLast()) {
-            source = JS_sprintf_append(source, ", ");
+            source = JS_sprintf_append(Move(source), ", ");
             if (!source) {
                 ReportOutOfMemory(cx);
                 return false;
@@ -1286,13 +1286,13 @@ ToDisassemblySource(JSContext* cx, HandleScope scope, JSAutoByteString* bytes)
         }
     }
 
-    source = JS_sprintf_append(source, "}");
+    source = JS_sprintf_append(Move(source), "}");
     if (!source) {
         ReportOutOfMemory(cx);
         return false;
     }
 
-    bytes->initBytes(source);
+    bytes->initBytes(Move(source));
     return true;
 }
 
diff --git a/js/src/jsprf.cpp b/js/src/jsprf.cpp
index 8c21b828b9b1..d5b2418f64a4 100644
--- a/js/src/jsprf.cpp
+++ b/js/src/jsprf.cpp
@@ -20,13 +20,13 @@ using namespace js;
 
 typedef mozilla::SmprintfPolicyPointer<js::SystemAllocPolicy> JSSmprintfPointer;
 
-JS_PUBLIC_API(char*) JS_smprintf(const char* fmt, ...)
+JS_PUBLIC_API(JS::UniqueChars) JS_smprintf(const char* fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
     JSSmprintfPointer result = mozilla::Vsmprintf<js::SystemAllocPolicy>(fmt, ap);
     va_end(ap);
-    return result.release();
+    return JS::UniqueChars(result.release());
 }
 
 JS_PUBLIC_API(void) JS_smprintf_free(char* mem)
@@ -34,23 +34,26 @@ JS_PUBLIC_API(void) JS_smprintf_free(char* mem)
     mozilla::SmprintfFree<js::SystemAllocPolicy>(mem);
 }
 
-JS_PUBLIC_API(char*) JS_sprintf_append(char* last, const char* fmt, ...)
+JS_PUBLIC_API(JS::UniqueChars) JS_sprintf_append(JS::UniqueChars&& last, const char* fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
+    JSSmprintfPointer lastPtr(last.release());
     JSSmprintfPointer result =
-        mozilla::VsmprintfAppend<js::SystemAllocPolicy>(JSSmprintfPointer(last), fmt, ap);
+        mozilla::VsmprintfAppend<js::SystemAllocPolicy>(Move(lastPtr), fmt, ap);
     va_end(ap);
-    return result.release();
+    return JS::UniqueChars(result.release());
 }
 
-JS_PUBLIC_API(char*) JS_vsmprintf(const char* fmt, va_list ap)
+JS_PUBLIC_API(JS::UniqueChars) JS_vsmprintf(const char* fmt, va_list ap)
 {
-    return mozilla::Vsmprintf<js::SystemAllocPolicy>(fmt, ap).release();
+    return JS::UniqueChars(mozilla::Vsmprintf<js::SystemAllocPolicy>(fmt, ap).release());
 }
 
-JS_PUBLIC_API(char*) JS_vsprintf_append(char* last, const char* fmt, va_list ap)
+JS_PUBLIC_API(JS::UniqueChars) JS_vsprintf_append(JS::UniqueChars&& last,
+                                                  const char* fmt, va_list ap)
 {
-    return mozilla::VsmprintfAppend<js::SystemAllocPolicy>(JSSmprintfPointer(last),
-                                                           fmt, ap).release();
+    JSSmprintfPointer lastPtr(last.release());
+    return JS::UniqueChars(mozilla::VsmprintfAppend<js::SystemAllocPolicy>(Move(lastPtr),
+                                                                           fmt, ap).release());
 }
diff --git a/js/src/jsprf.h b/js/src/jsprf.h
index 26595885fc24..cda5e5a93223 100644
--- a/js/src/jsprf.h
+++ b/js/src/jsprf.h
@@ -12,19 +12,22 @@
 #include <stdarg.h>
 
 #include "jstypes.h"
+#include "js/Utility.h"
 
 /* Wrappers for mozilla::Smprintf and friends that are used throughout
    JS.  */
 
-extern JS_PUBLIC_API(char*) JS_smprintf(const char* fmt, ...)
+extern JS_PUBLIC_API(JS::UniqueChars) JS_smprintf(const char* fmt, ...)
     MOZ_FORMAT_PRINTF(1, 2);
 
 extern JS_PUBLIC_API(void) JS_smprintf_free(char* mem);
 
-extern JS_PUBLIC_API(char*) JS_sprintf_append(char* last, const char* fmt, ...)
+extern JS_PUBLIC_API(JS::UniqueChars) JS_sprintf_append(JS::UniqueChars&& last,
+                                                        const char* fmt, ...)
      MOZ_FORMAT_PRINTF(2, 3);
 
-extern JS_PUBLIC_API(char*) JS_vsmprintf(const char* fmt, va_list ap);
-extern JS_PUBLIC_API(char*) JS_vsprintf_append(char* last, const char* fmt, va_list ap);
+extern JS_PUBLIC_API(JS::UniqueChars) JS_vsmprintf(const char* fmt, va_list ap);
+extern JS_PUBLIC_API(JS::UniqueChars) JS_vsprintf_append(JS::UniqueChars&& last,
+                                                         const char* fmt, va_list ap);
 
 #endif /* jsprf_h */
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 9c376118e324..440fc92d7e62 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -1363,22 +1363,22 @@ Options(JSContext* cx, unsigned argc, Value* vp)
         }
     }
 
-    char* names = strdup("");
+    UniqueChars names = DuplicateString("");
     bool found = false;
     if (names && oldContextOptions.extraWarnings()) {
-        names = JS_sprintf_append(names, "%s%s", found ? "," : "", "strict");
+        names = JS_sprintf_append(Move(names), "%s%s", found ? "," : "", "strict");
         found = true;
     }
     if (names && oldContextOptions.werror()) {
-        names = JS_sprintf_append(names, "%s%s", found ? "," : "", "werror");
+        names = JS_sprintf_append(Move(names), "%s%s", found ? "," : "", "werror");
         found = true;
     }
     if (names && oldContextOptions.throwOnAsmJSValidationFailure()) {
-        names = JS_sprintf_append(names, "%s%s", found ? "," : "", "throw_on_asmjs_validation_failure");
+        names = JS_sprintf_append(Move(names), "%s%s", found ? "," : "", "throw_on_asmjs_validation_failure");
         found = true;
     }
     if (names && oldContextOptions.strictMode()) {
-        names = JS_sprintf_append(names, "%s%s", found ? "," : "", "strict_mode");
+        names = JS_sprintf_append(Move(names), "%s%s", found ? "," : "", "strict_mode");
         found = true;
     }
     if (!names) {
@@ -1386,8 +1386,7 @@ Options(JSContext* cx, unsigned argc, Value* vp)
         return false;
     }
 
-    JSString* str = JS_NewStringCopyZ(cx, names);
-    free(names);
+    JSString* str = JS_NewStringCopyZ(cx, names.get());
     if (!str)
         return false;
     args.rval().setString(str);
@@ -4944,10 +4943,10 @@ NestedShell(JSContext* cx, unsigned argc, Value* vp)
         // As a special case, if the caller passes "--js-cache", replace that
         // with "--js-cache=$(jsCacheDir)"
         if (!strcmp(argv.back(), "--js-cache") && jsCacheDir) {
-            char* newArg = JS_smprintf("--js-cache=%s", jsCacheDir);
+            UniqueChars newArg = JS_smprintf("--js-cache=%s", jsCacheDir);
             if (!newArg)
                 return false;
-            argv.replaceBack(newArg);
+            argv.replaceBack(newArg.release());
         }
     }
 
@@ -8198,12 +8197,12 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
     jsCacheDir = op.getStringOption("js-cache");
     if (jsCacheDir) {
         if (!op.getBoolOption("no-js-cache-per-process"))
-            jsCacheDir = JS_smprintf("%s/%u", jsCacheDir, (unsigned)getpid());
+            jsCacheDir = JS_smprintf("%s/%u", jsCacheDir, (unsigned)getpid()).release();
         else
             jsCacheDir = JS_strdup(cx, jsCacheDir);
         if (!jsCacheDir)
             return false;
-        jsCacheAsmJSPath = JS_smprintf("%s/asmjs.cache", jsCacheDir);
+        jsCacheAsmJSPath = JS_smprintf("%s/asmjs.cache", jsCacheDir).release();
     }
 
 #ifdef DEBUG
diff --git a/js/src/vm/SelfHosting.cpp b/js/src/vm/SelfHosting.cpp
index 8388d686824a..2d84c3c1271f 100644
--- a/js/src/vm/SelfHosting.cpp
+++ b/js/src/vm/SelfHosting.cpp
@@ -291,7 +291,7 @@ ThrowErrorWithType(JSContext* cx, JSExnType type, const CallArgs& args)
             UniqueChars bytes = DecompileValueGenerator(cx, JSDVG_SEARCH_STACK, val, nullptr);
             if (!bytes)
                 return;
-            errorArgs[i - 1].initBytes(bytes.release());
+            errorArgs[i - 1].initBytes(Move(bytes));
         }
         if (!errorArgs[i - 1])
             return;
diff --git a/js/src/wasm/AsmJS.cpp b/js/src/wasm/AsmJS.cpp
index ba22ea2d5963..b586f94e60c1 100644
--- a/js/src/wasm/AsmJS.cpp
+++ b/js/src/wasm/AsmJS.cpp
@@ -2273,7 +2273,7 @@ class MOZ_STACK_CLASS ModuleValidator
         MOZ_ASSERT(errorOffset_ == UINT32_MAX);
         MOZ_ASSERT(fmt);
         errorOffset_ = offset;
-        errorString_.reset(JS_vsmprintf(fmt, ap));
+        errorString_ = JS_vsmprintf(fmt, ap);
         return false;
     }
 
@@ -8588,7 +8588,7 @@ LookupAsmJSModuleInCache(JSContext* cx, AsmJSParser& parser, bool* loadedFromCac
 
     int64_t after = PRMJ_Now();
     int ms = (after - before) / PRMJ_USEC_PER_MSEC;
-    *compilationTimeReport = UniqueChars(JS_smprintf("loaded from cache in %dms", ms));
+    *compilationTimeReport = JS_smprintf("loaded from cache in %dms", ms);
     if (!*compilationTimeReport)
         return false;
 
@@ -8689,7 +8689,7 @@ BuildConsoleMessage(JSContext* cx, unsigned time, JS::AsmJSCacheResult cacheResu
         break;
     }
 
-    return UniqueChars(JS_smprintf("total compilation time %dms; %s", time, cacheString));
+    return JS_smprintf("total compilation time %dms; %s", time, cacheString);
 #else
     return DuplicateString("");
 #endif
diff --git a/js/src/wasm/WasmTextToBinary.cpp b/js/src/wasm/WasmTextToBinary.cpp
index a2eed6197fad..27dc511da2cc 100644
--- a/js/src/wasm/WasmTextToBinary.cpp
+++ b/js/src/wasm/WasmTextToBinary.cpp
@@ -556,11 +556,11 @@ class WasmTokenStream
     {}
     void generateError(WasmToken token, UniqueChars* error) {
         unsigned column = token.begin() - lineStart_ + 1;
-        error->reset(JS_smprintf("parsing wasm text at %u:%u", line_, column));
+        *error = JS_smprintf("parsing wasm text at %u:%u", line_, column);
     }
     void generateError(WasmToken token, const char* msg, UniqueChars* error) {
         unsigned column = token.begin() - lineStart_ + 1;
-        error->reset(JS_smprintf("parsing wasm text at %u:%u: %s", line_, column, msg));
+        *error = JS_smprintf("parsing wasm text at %u:%u: %s", line_, column, msg);
     }
     WasmToken peek() {
         if (!lookaheadDepth_) {
@@ -3410,7 +3410,7 @@ class Resolver
     bool failResolveLabel(const char* kind, AstName name) {
         TwoByteChars chars(name.begin(), name.length());
         UniqueChars utf8Chars(CharsToNewUTF8CharsZ(nullptr, chars).c_str());
-        error_->reset(JS_smprintf("%s label '%s' not found", kind, utf8Chars.get()));
+        *error_ = JS_smprintf("%s label '%s' not found", kind, utf8Chars.get());
         return false;
     }
 
@@ -3494,7 +3494,7 @@ class Resolver
     }
 
     bool fail(const char* message) {
-        error_->reset(JS_smprintf("%s", message));
+        *error_ = JS_smprintf("%s", message);
         return false;
     }
 };
diff --git a/js/xpconnect/loader/mozJSComponentLoader.cpp b/js/xpconnect/loader/mozJSComponentLoader.cpp
index 9a41b937cebc..516c7f39bbac 100644
--- a/js/xpconnect/loader/mozJSComponentLoader.cpp
+++ b/js/xpconnect/loader/mozJSComponentLoader.cpp
@@ -139,11 +139,11 @@ public:
     explicit JSCLContextHelper(JSContext* aCx);
     ~JSCLContextHelper();
 
-    void reportErrorAfterPop(char* buf);
+    void reportErrorAfterPop(UniqueChars&& buf);
 
 private:
     JSContext* mContext;
-    char*      mBuf;
+    UniqueChars mBuf;
 
     // prevent copying and assignment
     JSCLContextHelper(const JSCLContextHelper&) = delete;
@@ -161,14 +161,13 @@ ReportOnCallerUTF8(JSContext* callerContext,
     va_list ap;
     va_start(ap, format);
 
-    char* buf = JS_vsmprintf(format, ap);
+    UniqueChars buf = JS_vsmprintf(format, ap);
     if (!buf) {
         va_end(ap);
         return NS_ERROR_OUT_OF_MEMORY;
     }
 
-    JS_ReportErrorUTF8(callerContext, "%s", buf);
-    JS_smprintf_free(buf);
+    JS_ReportErrorUTF8(callerContext, "%s", buf.get());
 
     va_end(ap);
     return NS_OK;
@@ -182,13 +181,13 @@ ReportOnCallerUTF8(JSCLContextHelper& helper,
     va_list ap;
     va_start(ap, format);
 
-    char* buf = JS_vsmprintf(format, ap);
+    UniqueChars buf = JS_vsmprintf(format, ap);
     if (!buf) {
         va_end(ap);
         return NS_ERROR_OUT_OF_MEMORY;
     }
 
-    helper.reportErrorAfterPop(buf);
+    helper.reportErrorAfterPop(Move(buf));
     va_end(ap);
     return NS_OK;
 }
@@ -1351,14 +1350,13 @@ JSCLContextHelper::JSCLContextHelper(JSContext* aCx)
 JSCLContextHelper::~JSCLContextHelper()
 {
     if (mBuf) {
-        JS_ReportErrorUTF8(mContext, "%s", mBuf);
-        JS_smprintf_free(mBuf);
+        JS_ReportErrorUTF8(mContext, "%s", mBuf.get());
     }
 }
 
 void
-JSCLContextHelper::reportErrorAfterPop(char* buf)
+JSCLContextHelper::reportErrorAfterPop(UniqueChars&& buf)
 {
     MOZ_ASSERT(!mBuf, "Already called reportErrorAfterPop");
-    mBuf = buf;
+    mBuf = Move(buf);
 }
diff --git a/js/xpconnect/src/XPCShellImpl.cpp b/js/xpconnect/src/XPCShellImpl.cpp
index 980c54027b00..da280750c1c9 100644
--- a/js/xpconnect/src/XPCShellImpl.cpp
+++ b/js/xpconnect/src/XPCShellImpl.cpp
@@ -520,31 +520,30 @@ Options(JSContext* cx, unsigned argc, Value* vp)
         }
     }
 
-    char* names = nullptr;
+    UniqueChars names;
     if (oldContextOptions.extraWarnings()) {
-        names = JS_sprintf_append(names, "%s", "strict");
+        names = JS_sprintf_append(Move(names), "%s", "strict");
         if (!names) {
             JS_ReportOutOfMemory(cx);
             return false;
         }
     }
     if (oldContextOptions.werror()) {
-        names = JS_sprintf_append(names, "%s%s", names ? "," : "", "werror");
+        names = JS_sprintf_append(Move(names), "%s%s", names ? "," : "", "werror");
         if (!names) {
             JS_ReportOutOfMemory(cx);
             return false;
         }
     }
     if (names && oldContextOptions.strictMode()) {
-        names = JS_sprintf_append(names, "%s%s", names ? "," : "", "strict_mode");
+        names = JS_sprintf_append(Move(names), "%s%s", names ? "," : "", "strict_mode");
         if (!names) {
             JS_ReportOutOfMemory(cx);
             return false;
         }
     }
 
-    str = JS_NewStringCopyZ(cx, names);
-    free(names);
+    str = JS_NewStringCopyZ(cx, names.get());
     if (!str)
         return false;
 
@@ -717,7 +716,7 @@ env_setProperty(JSContext* cx, HandleObject obj, HandleId id, MutableHandleValue
         return false;
 #if defined XP_WIN || defined HPUX || defined OSF1 || defined SCO
     {
-        char* waste = JS_smprintf("%s=%s", name.ptr(), value.ptr());
+        char* waste = JS_smprintf("%s=%s", name.ptr(), value.ptr()).release();
         if (!waste) {
             JS_ReportOutOfMemory(cx);
             return false;
diff --git a/js/xpconnect/src/XPCThrower.cpp b/js/xpconnect/src/XPCThrower.cpp
index 4a37e1554287..8916fe32be04 100644
--- a/js/xpconnect/src/XPCThrower.cpp
+++ b/js/xpconnect/src/XPCThrower.cpp
@@ -116,9 +116,9 @@ XPCThrower::ThrowBadResult(nsresult rv, nsresult result, XPCCallContext& ccx)
         format = "";
 
     if (nsXPCException::NameAndFormatForNSResult(result, &name, nullptr) && name)
-        sz = JS_smprintf("%s 0x%x (%s)", format, (unsigned) result, name);
+        sz = JS_smprintf("%s 0x%x (%s)", format, (unsigned) result, name).release();
     else
-        sz = JS_smprintf("%s 0x%x", format, (unsigned) result);
+        sz = JS_smprintf("%s 0x%x", format, (unsigned) result).release();
     NS_ENSURE_TRUE_VOID(sz);
 
     if (sz && sVerbose)
@@ -140,7 +140,7 @@ XPCThrower::ThrowBadParam(nsresult rv, unsigned paramNum, XPCCallContext& ccx)
     if (!nsXPCException::NameAndFormatForNSResult(rv, nullptr, &format))
         format = "";
 
-    sz = JS_smprintf("%s arg %d", format, paramNum);
+    sz = JS_smprintf("%s arg %d", format, paramNum).release();
     NS_ENSURE_TRUE_VOID(sz);
 
     if (sz && sVerbose)
@@ -168,7 +168,7 @@ XPCThrower::Verbosify(XPCCallContext& ccx,
         if (!name) {
             name = "";
         }
-        sz = JS_smprintf("%s [%s.%s]", *psz, iface->GetNameString(), name);
+        sz = JS_smprintf("%s [%s.%s]", *psz, iface->GetNameString(), name).release();
     }
 
     if (sz) {
diff --git a/js/xpconnect/src/XPCWrappedJSClass.cpp b/js/xpconnect/src/XPCWrappedJSClass.cpp
index e5922a73879b..7ac8718588b8 100644
--- a/js/xpconnect/src/XPCWrappedJSClass.cpp
+++ b/js/xpconnect/src/XPCWrappedJSClass.cpp
@@ -1220,16 +1220,14 @@ pre_call_clean_up:
                     NS_ERROR_XPC_JSOBJECT_HAS_NO_FUNCTION_NAMED;
             static const char format[] = "%s \"%s\"";
             const char * msg;
-            char* sz = nullptr;
+            UniqueChars sz;
 
             if (nsXPCException::NameAndFormatForNSResult(code, nullptr, &msg) && msg)
                 sz = JS_smprintf(format, msg, name);
 
-            XPCConvert::ConstructException(code, sz, GetInterfaceName(), name,
+            XPCConvert::ConstructException(code, sz.get(), GetInterfaceName(), name,
                                            nullptr, getter_AddRefs(syntheticException),
                                            nullptr, nullptr);
-            if (sz)
-                JS_smprintf_free(sz);
             success = false;
         }
     }
diff --git a/js/xpconnect/src/XPCWrappedNative.cpp b/js/xpconnect/src/XPCWrappedNative.cpp
index 97ee9bc5875e..e9a0a9b95170 100644
--- a/js/xpconnect/src/XPCWrappedNative.cpp
+++ b/js/xpconnect/src/XPCWrappedNative.cpp
@@ -2141,15 +2141,15 @@ XPCWrappedNative::ToString(XPCWrappedNativeTearOff* to /* = nullptr */ ) const
 #  define PARAM_ADDR(w)
 #endif
 
-    char* sz = nullptr;
-    char* name = nullptr;
+    UniqueChars sz;
+    UniqueChars name;
 
     nsCOMPtr<nsIXPCScriptable> scr = GetScriptable();
     if (scr)
         name = JS_smprintf("%s", scr->GetJSClass()->name);
     if (to) {
         const char* fmt = name ? " (%s)" : "%s";
-        name = JS_sprintf_append(name, fmt,
+        name = JS_sprintf_append(Move(name), fmt,
                                  to->GetInterface()->GetNameString());
     } else if (!name) {
         XPCNativeSet* set = GetSet();
@@ -2158,15 +2158,15 @@ XPCWrappedNative::ToString(XPCWrappedNativeTearOff* to /* = nullptr */ ) const
         uint16_t count = set->GetInterfaceCount();
 
         if (count == 1)
-            name = JS_sprintf_append(name, "%s", array[0]->GetNameString());
+            name = JS_sprintf_append(Move(name), "%s", array[0]->GetNameString());
         else if (count == 2 && array[0] == isupp) {
-            name = JS_sprintf_append(name, "%s", array[1]->GetNameString());
+            name = JS_sprintf_append(Move(name), "%s", array[1]->GetNameString());
         } else {
             for (uint16_t i = 0; i < count; i++) {
                 const char* fmt = (i == 0) ?
                                     "(%s" : (i == count-1) ?
                                         ", %s)" : ", %s";
-                name = JS_sprintf_append(name, fmt,
+                name = JS_sprintf_append(Move(name), fmt,
                                          array[i]->GetNameString());
             }
         }
@@ -2180,12 +2180,9 @@ XPCWrappedNative::ToString(XPCWrappedNativeTearOff* to /* = nullptr */ ) const
     if (scr) {
         fmt = "[object %s" FMT_ADDR FMT_STR(" (native") FMT_ADDR FMT_STR(")") "]";
     }
-    sz = JS_smprintf(fmt, name PARAM_ADDR(this) PARAM_ADDR(mIdentity.get()));
+    sz = JS_smprintf(fmt, name.get() PARAM_ADDR(this) PARAM_ADDR(mIdentity.get()));
 
-    JS_smprintf_free(name);
-
-
-    return sz;
+    return sz.release();
 
 #undef FMT_ADDR
 #undef PARAM_ADDR
diff --git a/js/xpconnect/src/XPCWrappedNativeJSOps.cpp b/js/xpconnect/src/XPCWrappedNativeJSOps.cpp
index 405fdd95c1aa..7ee51f0f1ae6 100644
--- a/js/xpconnect/src/XPCWrappedNativeJSOps.cpp
+++ b/js/xpconnect/src/XPCWrappedNativeJSOps.cpp
@@ -44,11 +44,11 @@ static bool Throw(nsresult errNum, JSContext* cx)
 static bool
 ToStringGuts(XPCCallContext& ccx)
 {
-    char* sz;
+    UniqueChars sz;
     XPCWrappedNative* wrapper = ccx.GetWrapper();
 
     if (wrapper)
-        sz = wrapper->ToString(ccx.GetTearOff());
+        sz.reset(wrapper->ToString(ccx.GetTearOff()));
     else
         sz = JS_smprintf("[xpconnect wrapped native prototype]");
 
@@ -57,8 +57,7 @@ ToStringGuts(XPCCallContext& ccx)
         return false;
     }
 
-    JSString* str = JS_NewStringCopyZ(ccx, sz);
-    JS_smprintf_free(sz);
+    JSString* str = JS_NewStringCopyZ(ccx, sz.get());
     if (!str)
         return false;