--- a/js/src/frontend/TokenStream.cpp
+++ b/js/src/frontend/TokenStream.cpp
@@ -2432,17 +2432,21 @@ TokenStreamSpecific<Unit, AnyCharsAccess
     if (isIntegerUnit(unit)) {
       continue;
     }
     if (unit != '_') {
       break;
     }
     unit = getCodeUnit();
     if (!isIntegerUnit(unit)) {
-      error(JSMSG_MISSING_DIGIT_AFTER_SEPARATOR);
+      if (unit == '_') {
+        error(JSMSG_NUMBER_MULTIPLE_ADJACENT_UNDERSCORES);
+      } else {
+        error(JSMSG_NUMBER_END_WITH_UNDERSCORE);
+      }
       return false;
     }
   }
 
   *nextUnit = unit;
   return true;
 }
 

--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -286,17 +286,18 @@ MSG_DEF(JSMSG_LINE_BREAK_AFTER_THROW,  0
 MSG_DEF(JSMSG_LINE_BREAK_BEFORE_ARROW, 0, JSEXN_SYNTAXERR, "no line break is allowed before '=>'")
 MSG_DEF(JSMSG_MALFORMED_ESCAPE,        1, JSEXN_SYNTAXERR, "malformed {0} character escape sequence")
 MSG_DEF(JSMSG_MISSING_BINARY_DIGITS,   0, JSEXN_SYNTAXERR, "missing binary digits after '0b'")
 MSG_DEF(JSMSG_MISSING_EXPONENT,        0, JSEXN_SYNTAXERR, "missing exponent")
 MSG_DEF(JSMSG_MISSING_EXPR_AFTER_THROW,0, JSEXN_SYNTAXERR, "throw statement is missing an expression")
 MSG_DEF(JSMSG_MISSING_FORMAL,          0, JSEXN_SYNTAXERR, "missing formal parameter")
 MSG_DEF(JSMSG_MISSING_HEXDIGITS,       0, JSEXN_SYNTAXERR, "missing hexadecimal digits after '0x'")
 MSG_DEF(JSMSG_MISSING_OCTAL_DIGITS,    0, JSEXN_SYNTAXERR, "missing octal digits after '0o'")
-MSG_DEF(JSMSG_MISSING_DIGIT_AFTER_SEPARATOR, 0, JSEXN_SYNTAXERR, "missing digit after '_' numeric separator")
+MSG_DEF(JSMSG_NUMBER_END_WITH_UNDERSCORE, 0, JSEXN_SYNTAXERR, "underscore can appear only between digits, not after the last digit in a number")
+MSG_DEF(JSMSG_NUMBER_MULTIPLE_ADJACENT_UNDERSCORES, 0, JSEXN_SYNTAXERR, "number cannot contain multiple adjacent underscores")
 MSG_DEF(JSMSG_MODULE_SPEC_AFTER_FROM,  0, JSEXN_SYNTAXERR, "missing module specifier after 'from' keyword")
 MSG_DEF(JSMSG_NAME_AFTER_DOT,          0, JSEXN_SYNTAXERR, "missing name after . operator")
 MSG_DEF(JSMSG_NAMED_IMPORTS_OR_NAMESPACE_IMPORT, 0, JSEXN_SYNTAXERR, "expected named imports or namespace import after comma")
 MSG_DEF(JSMSG_NO_BINDING_NAME,        0, JSEXN_SYNTAXERR, "missing binding name")
 MSG_DEF(JSMSG_NO_EXPORT_NAME,          0, JSEXN_SYNTAXERR, "missing export name")
 MSG_DEF(JSMSG_NO_IMPORT_NAME,          0, JSEXN_SYNTAXERR, "missing import name")
 MSG_DEF(JSMSG_NO_VARIABLE_NAME,        0, JSEXN_SYNTAXERR, "missing variable name")
 MSG_DEF(JSMSG_OF_AFTER_FOR_NAME,       0, JSEXN_SYNTAXERR, "missing 'of' after for")

new file mode 100644
--- /dev/null
+++ b/js/src/tests/non262/Number/numericSeparator.js
@@ -0,0 +1,9 @@
+// Any copyright is dedicated to the Public Domain.
+// http://creativecommons.org/licenses/publicdomain/
+
+assertThrowsInstanceOf(function() { eval('let a = 100_00_;'); }, SyntaxError);
+assertThrowsInstanceOf(() => eval("let b = 10__;"), SyntaxError);
+
+if (typeof reportCompare === "function")
+  reportCompare(true, true);
+