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copied!<p>It's a bit more complex.</p> <p>You'll find a very good explanation how it works here: <a href="http://www.adequatelygood.com/2010/3/Object-to-Primitive-Conversions-in-JavaScript" rel="nofollow noreferrer">http://www.adequatelygood.com/2010/3/Object-to-Primitive-Conversions-in-JavaScript</a></p> <p>But it seems the way ben cherry described did changed (also what I regonized). If you will run his example:</p> <pre><code>function population(country, pop) { return { country: country, pop: pop, toString: function () { return "[Population " + "\"" + country + "\" " + pop + "]"; }, valueOf: function () { return pop; } }; } var america_pop = population("USA", 350e6); var mexico_pop = population("Mexico", 200e6); var canada_pop = population("Canada", 200e6); alert(america_pop); // [Population "USA" 350000000 var north_america_pop = america_pop + mexico_pop + canada_pop; alert(north_america_pop); // 750000000 </code></pre> <p>This isn't true anymore, the alerts will be:<br> 350000000<br> 750000000</p> <p>alert(obj) call valueOf() not toString() anymore, execpt if you use Array.join</p> <pre><code>alert([america_pop]); // [Population "USA" 350000000 </code></pre> <p>There was already a thread about: <a href="https://stackoverflow.com/questions/2485632/valueof-vs-tostring-in-javascript">valueOf() vs. toString() in Javascript</a></p> <p>They clearify that, valueOf is always called when [hint] is a number and that's always the case, except if you join an Array. that's confusing, because if you call 'string' + 'string' this is definitely a string context, but JS will first take look at the number context, and call always the valueOf() method not toString() if it's a primitive value</p>

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