diff --git a/devtools/shared/tests/unit/test_sprintfjs.js b/devtools/shared/tests/unit/test_sprintfjs.js
new file mode 100644
index 000000000000..884727c3d275
--- /dev/null
+++ b/devtools/shared/tests/unit/test_sprintfjs.js
@@ -0,0 +1,113 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * This unit test checks that our string formatter works with different patterns and
+ * arguments.
+ * Initially copied from unit tests at https://github.com/alexei/sprintf.js
+ */
+
+const {sprintf} = require("devtools/shared/sprintfjs/sprintf");
+const PI = 3.141592653589793;
+
+function run_test() {
+  // Simple patterns
+  equal("%", sprintf("%%"));
+  equal("10", sprintf("%b", 2));
+  equal("A", sprintf("%c", 65));
+  equal("2", sprintf("%d", 2));
+  equal("2", sprintf("%i", 2));
+  equal("2", sprintf("%d", "2"));
+  equal("2", sprintf("%i", "2"));
+  equal("{\"foo\":\"bar\"}", sprintf("%j", {foo: "bar"}));
+  equal("[\"foo\",\"bar\"]", sprintf("%j", ["foo", "bar"]));
+  equal("2e+0", sprintf("%e", 2));
+  equal("2", sprintf("%u", 2));
+  equal("4294967294", sprintf("%u", -2));
+  equal("2.2", sprintf("%f", 2.2));
+  equal("3.141592653589793", sprintf("%g", PI));
+  equal("10", sprintf("%o", 8));
+  equal("%s", sprintf("%s", "%s"));
+  equal("ff", sprintf("%x", 255));
+  equal("FF", sprintf("%X", 255));
+  equal("Polly wants a cracker",
+    sprintf("%2$s %3$s a %1$s", "cracker", "Polly", "wants"));
+  equal("Hello world!", sprintf("Hello %(who)s!", {who: "world"}));
+  equal("true", sprintf("%t", true));
+  equal("t", sprintf("%.1t", true));
+  equal("true", sprintf("%t", "true"));
+  equal("true", sprintf("%t", 1));
+  equal("false", sprintf("%t", false));
+  equal("f", sprintf("%.1t", false));
+  equal("false", sprintf("%t", ""));
+  equal("false", sprintf("%t", 0));
+
+  equal("undefined", sprintf("%T", undefined));
+  equal("null", sprintf("%T", null));
+  equal("boolean", sprintf("%T", true));
+  equal("number", sprintf("%T", 42));
+  equal("string", sprintf("%T", "This is a string"));
+  equal("function", sprintf("%T", Math.log));
+  equal("array", sprintf("%T", [1, 2, 3]));
+  equal("object", sprintf("%T", {foo: "bar"}));
+
+  equal("regexp", sprintf("%T", /<("[^"]*"|"[^"]*"|[^"">])*>/));
+
+  equal("true", sprintf("%v", true));
+  equal("42", sprintf("%v", 42));
+  equal("This is a string", sprintf("%v", "This is a string"));
+  equal("1,2,3", sprintf("%v", [1, 2, 3]));
+  equal("[object Object]", sprintf("%v", {foo: "bar"}));
+  equal("/<(\"[^\"]*\"|'[^']*'|[^'\">])*>/",
+    sprintf("%v", /<("[^"]*"|'[^']*'|[^'">])*>/));
+
+  // sign
+  equal("2", sprintf("%d", 2));
+  equal("-2", sprintf("%d", -2));
+  equal("+2", sprintf("%+d", 2));
+  equal("-2", sprintf("%+d", -2));
+  equal("2", sprintf("%i", 2));
+  equal("-2", sprintf("%i", -2));
+  equal("+2", sprintf("%+i", 2));
+  equal("-2", sprintf("%+i", -2));
+  equal("2.2", sprintf("%f", 2.2));
+  equal("-2.2", sprintf("%f", -2.2));
+  equal("+2.2", sprintf("%+f", 2.2));
+  equal("-2.2", sprintf("%+f", -2.2));
+  equal("-2.3", sprintf("%+.1f", -2.34));
+  equal("-0.0", sprintf("%+.1f", -0.01));
+  equal("3.14159", sprintf("%.6g", PI));
+  equal("3.14", sprintf("%.3g", PI));
+  equal("3", sprintf("%.1g", PI));
+  equal("-000000123", sprintf("%+010d", -123));
+  equal("______-123", sprintf("%+'_10d", -123));
+  equal("-234.34 123.2", sprintf("%f %f", -234.34, 123.2));
+
+  // padding
+  equal("-0002", sprintf("%05d", -2));
+  equal("-0002", sprintf("%05i", -2));
+  equal("    <", sprintf("%5s", "<"));
+  equal("0000<", sprintf("%05s", "<"));
+  equal("____<", sprintf("%'_5s", "<"));
+  equal(">    ", sprintf("%-5s", ">"));
+  equal(">0000", sprintf("%0-5s", ">"));
+  equal(">____", sprintf("%'_-5s", ">"));
+  equal("xxxxxx", sprintf("%5s", "xxxxxx"));
+  equal("1234", sprintf("%02u", 1234));
+  equal(" -10.235", sprintf("%8.3f", -10.23456));
+  equal("-12.34 xxx", sprintf("%f %s", -12.34, "xxx"));
+  equal("{\n  \"foo\": \"bar\"\n}", sprintf("%2j", {foo: "bar"}));
+  equal("[\n  \"foo\",\n  \"bar\"\n]", sprintf("%2j", ["foo", "bar"]));
+
+  // precision
+  equal("2.3", sprintf("%.1f", 2.345));
+  equal("xxxxx", sprintf("%5.5s", "xxxxxx"));
+  equal("    x", sprintf("%5.1s", "xxxxxx"));
+
+  equal("foobar", sprintf("%s", function () {
+    return "foobar";
+  }));
+}
diff --git a/devtools/shared/tests/unit/xpcshell.ini b/devtools/shared/tests/unit/xpcshell.ini
index ce07e5ecf8d4..64f4312b9a82 100644
--- a/devtools/shared/tests/unit/xpcshell.ini
+++ b/devtools/shared/tests/unit/xpcshell.ini
@@ -37,6 +37,7 @@ run-if = nightly_build
 [test_require_lazy.js]
 [test_require_raw.js]
 [test_require.js]
+[test_sprintfjs.js]
 [test_stack.js]
 [test_defer.js]
 [test_executeSoon.js]