136445 2014-09-01 23:33:13 -0700 Assignment to a property on a primitive in strict mode should not throw 2014-09-02 19:40:43 -0700 1 1 1 Unclassified WebKit JavaScriptCore 528+ (Nightly build) Unspecified Unspecified RESOLVED INVALID P2 Normal --- 1 ljharb webkit-unassigned ggaren mathias oliver oldest_to_newest 1032569 0 ljharb 2014-09-01 23:33:13 -0700 `(function () { "use strict"; 3.0.a = true; }())` `(function () { "use strict"; 'foo'.a = true; }())` `(function () { "use strict"; true.a = true; }())` None of these throw exceptions in Chrome, Firefox, Opera - but in Safari 7.1, these all throw a TypeError "Attempted to assign to readonly property.". According to the ES5 spec ( https://es5.github.io/#C ) and ES6 spec ( https://people.mozilla.org/~jorendorff/es6-draft.html#sec-strict-mode-of-ecmascript ), Safari is in error here. In addition, one of the TC39 spec maintainers confirmed via Twitter that this is a Safari bug ( https://twitter.com/awbjs/status/506687103506587648 ) Steps to Reproduce: Run any of the JS functions in the console, or any other JS mechanism: (function () { "use strict"; 3.0.a = true; }()) (function () { "use strict"; 'foo'.a = true; }()) (function () { "use strict"; true.a = true; }()) 1032682 1 ljharb 2014-09-02 10:10:41 -0700 Sorry for the noise - turns out Safari is the only browser doing this correctly. https://twitter.com/awbjs/status/506845194025594881 https://code.google.com/p/v8/issues/detail?id=3088 and https://bugzilla.mozilla.org/show_bug.cgi?id=779682